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System Description
Wireless Power Transfer
Volume I: Low Power
Part 1: Interface Definition
Version 1.0
July 2010
 
System Description
Wireless Power Transfer
Volume I: Low Power
Part 1: Interface Definition
Version 1.0
July 2010

COPYRIGHT
This System Description Wireless Power Transfer is published by the Wireless Power Consortium, and has
been prepared by the Wireless Power Consortium in close co-operation with ConvenientPower Ltd.,
Fulton Innovation LLC, National Semiconductor Corporation, Nokia Corporation, Olympus Imaging
Corporation, Research In Motion, Limited, Royal Philips Electronics, Sanyo Electric Co. Ltd., Shenzhen Sang
Fei Consumer Communications Co. Ltd., and Texas Instruments Inc.. All rights are reserved. Reproduction
in whole or in part is prohibited without express and prior written permission of the Wireless Power
Consortium.
DISCLAIMER
The information contained herein is believed to be accurate as of the date of publication. However, neither
the Wireless Power Consortium, nor ConvenientPower Ltd., nor Fulton Innovation LLC, nor National
Semiconductor Corporation, nor Nokia Corporation, nor Olympus Imaging Corporation, nor Research In
Motion Limited, nor Royal Philips Electronics, nor Sanyo Electric Co. Ltd., nor Shenzhen Sang Fei
Consumer Communications Co. Ltd., nor Texas Instruments Inc. will be liable for any damages, including
indirect or consequential, from use of this System Description Wireless Power Transfer or reliance on the
accuracy of this document.
CLASSIFICATION
The information contained in this document is marked as confidential.
NOTICE
For any further explanation of the contents of this document, or in case of any perceived inconsistency or
ambiguity of interpretation, or for any information regarding the associated patent license program,
please contact: info@wirelesspowerconsortium.com.

Table of Contents
1 General ................................................................................................................... 1
1.1 Scope ............................................................................................................................................................................................ 1
1.2 Main features ............................................................................................................................................................................ 1
1.3 Conformance and references ............................................................................................................................................ 1
1.4 Definitions ................................................................................................................................................................................. 2
1.5 Acronyms ................................................................................................................................................................................... 3
1.6 Symbols ....................................................................................................................................................................................... 3
1.7 Conventions .............................................................................................................................................................................. 4
1.7.1 Cross references ........................................................................................................................................................... 4
1.7.2 Informative text ............................................................................................................................................................ 4
1.7.3 Terms in capitals .......................................................................................................................................................... 4
1.7.4 Notation of numbers ................................................................................................................................................... 4
1.7.5 Units of physical quantities ..................................................................................................................................... 4
1.7.6 Bit ordering in a byte .................................................................................................................................................. 4
1.7.7 Byte numbering ............................................................................................................................................................ 5
1.7.8 Multiple-bit Fields........................................................................................................................................................ 5
1.8 Operators ................................................................................................................................................................................... 5
1.8.1 Exclusive-OR .................................................................................................................................................................. 5
1.8.2 Concatenation ................................................................................................................................................................ 5
2 System Overview (Informative) .................................................................... 7
3 Basic Power Transmitter Designs ............................................................. 11
3.1 Introduction ........................................................................................................................................................................... 11
3.2 Power Transmitter designs that are based on a single Primary Coil ............................................................ 11
3.2.1 Power Transmitter design A1 .............................................................................................................................. 11
3.2.2 Power Transmitter design A2 .............................................................................................................................. 16
3.3 Power Transmitter designs that are based on an array of Primary Coils ................................................... 20
3.3.1 Power Transmitter design B1 .............................................................................................................................. 20
3.3.2 Power Transmitter design B2 .............................................................................................................................. 26
4 Power Receiver Design Requirements ..................................................... 29
4.1 Introduction ........................................................................................................................................................................... 29
4.2 Power Receiver design requirements ......................................................................................................................... 30
4.2.1 Mechanical requirements ...................................................................................................................................... 30
4.2.2 Electrical requirements .......................................................................................................................................... 31
5 System Control ................................................................................................. 35
5.1 Introduction ........................................................................................................................................................................... 35
5.2 Power Transmitter perspective .................................................................................................................................... 38
5.2.1 Ping phase .................................................................................................................................................................... 38
5.2.2 Identification & configuration phase ................................................................................................................. 39
5.2.3 Power transfer phase .............................................................................................................................................. 42
5.3 Power Receiver perspective ........................................................................................................................................... 46
5.3.1 Selection phase........................................................................................................................................................... 46
5.3.2 Ping phase .................................................................................................................................................................... 47
5.3.3 Identification & configuration phase ................................................................................................................. 47
5.3.4 Power transfer phase .............................................................................................................................................. 48
6 Communications Interface ........................................................................... 51
6.1 Introduction ........................................................................................................................................................................... 51
6.2 Physical and data link layers .......................................................................................................................................... 51
6.2.1 Modulation scheme .................................................................................................................................................. 51
6.2.2 Bit encoding scheme ................................................................................................................................................ 52
6.2.3 Byte encoding scheme ............................................................................................................................................ 52
6.2.4 Packet structure......................................................................................................................................................... 52
6.3 Logical layer ........................................................................................................................................................................... 55
6.3.1 Signal Strength Packet (0x01) .............................................................................................................................. 55
6.3.2 End Power Transfer Packet (0x02) ................................................................................................................... 56
6.3.3 Control Error Packet (0x03) ................................................................................................................................ 57
6.3.4 Rectified Power Packet (0x04) ............................................................................................................................ 57
6.3.5 Charge Status Packet (0x05) ................................................................................................................................ 57
6.3.6 Power Control Hold-off Packet (0x06) ............................................................................................................. 57
6.3.7 Configuration Packet (0x51) ................................................................................................................................ 58
6.3.8 Identification Packet (0x71) ................................................................................................................................ 58
6.3.9 Extended Identification Packet (0x81) ............................................................................................................ 60
Annex A Example Power Receiver Designs (Informative) .................... 61
A.1 Power Receiver example 1 .............................................................................................................................................. 61
A.1.1 Mechanical details .................................................................................................................................................... 61
A.1.2 Electrical details ........................................................................................................................................................ 62
A.2 Power Receiver example 2 .............................................................................................................................................. 64
A.2.1 Mechanical details .................................................................................................................................................... 64
A.2.2 Electrical details ........................................................................................................................................................ 65
Annex B Object Detection (Informative) ..................................................... 67
B.1 Resonance shift .................................................................................................................................................................... 67
B.2 Capacitance change ............................................................................................................................................................ 68
Annex C Power Receiver Localization (Informative) ............................. 69
C.1 Guided Positioning .............................................................................................................................................................. 69
C.2 Primary Coil array based Free Positioning ............................................................................................................... 69
C.2.1 A single Power Receiver covering multiple Primary Cells ...................................................................... 69
C.2.2 Two Power Receivers covering two adjacent Primary Cells .................................................................. 70
C.2.3 Two Power Receivers covering a single Primary Cell ............................................................................... 70
C.3 Moving Primary Coil based Free Positioning ........................................................................................................... 71
Annex D Metal Object Detection (Informative) ......................................... 73
 List of Figures
Figure 1-1: Bit positions in a byte .................................................................................................................................................... 5
Figure 1-2: Example of multiple-bit field ...................................................................................................................................... 5
Figure 2-1: Basic system overview .................................................................................................................................................. 8
Figure 3-1: Functional block diagram of Power Transmitter design A1 ....................................................................... 11
Figure 3-2: Primary Coil of Power Transmitter design A1.................................................................................................. 12
Figure 3-3: Primary Coil assembly of Power Transmitter design A1 ............................................................................. 13
Figure 3-4: Electrical diagram (outline) of Power Transmitter design A1 .................................................................. 14
Figure 3-5: Functional block diagram of Power Transmitter design A2 ....................................................................... 16
Figure 3-6: Primary Coil of Power Transmitter design A2.................................................................................................. 17
Figure 3-7: Primary Coil assembly of Power Transmitter design A2 ............................................................................. 18
Figure 3-8: Electrical diagram (outline) of Power Transmitter design A2 .................................................................. 19
Figure 3-9: Functional block diagram of Power Transmitter design B1 ....................................................................... 20
Figure 3-10: Primary Coil array of Power Transmitter design B1 .................................................................................. 21
Figure 3-11: Primary Coil array assembly of Power Transmitter design B1 .............................................................. 22
Figure 3-12: Electrical diagram (outline) of Power Transmitter design B1 ............................................................... 23
Figure 3-13: Multiple type B1 Power Transmitters sharing a multiplexer and Primary Coil array ................. 25
Figure 3-14: Primary Coil array of Power Transmitter design B2 .................................................................................. 27
Figure 4-1: Example functional block diagram of a Power Receiver .............................................................................. 29
Figure 4-2: Secondary Coil assembly ........................................................................................................................................... 30
Figure 4-3: Dual resonant circuit of a Power Receiver ......................................................................................................... 31
Figure 4-4: Characterization of resonant frequencies .......................................................................................................... 32
Figure 5-1: Power transfer phases ................................................................................................................................................ 35
Figure 5-2: Power transfer control loop ..................................................................................................................................... 37
Figure 5-3: Power Transmitter timing in the ping phase .................................................................................................... 39
Figure 5-4: Power Transmitter timing in the identification & configuration phase ................................................. 41
Figure 5-5: Power Transmitter timing in the power transfer phase ............................................................................... 43
Figure 5-6: PID control algorithm ................................................................................................................................................. 44
Figure 5-7: Power Receiver timing in the selection phase ................................................................................................... 47
Figure 5-8: Power Receiver timing in the ping phase............................................................................................................ 47
Figure 5-9: Power Receiver timing in the identification & configuration phase ........................................................ 48
Figure 5-10: Power Receiver timing in the power transfer phase ................................................................................... 49
Figure 6-1: Amplitude modulation of the Power Signal ....................................................................................................... 51
Figure 6-2: Example of the differential bi-phase encoding ................................................................................................. 52
Figure 6-3: Example of the asynchronous serial format ...................................................................................................... 52
Figure 6-4: Packet format ................................................................................................................................................................. 53
Figure A-1: Secondary Coil of Power Receiver example 1 ................................................................................................... 61
Figure A-2: Secondary Coil and Shielding assembly of Power Receiver example 1 ................................................ 62
Figure A-3: Electrical details of Power Receiver example 1 ............................................................................................... 62
Figure A-4: Li-ion battery charging profile ............................................................................................................................... 63
Figure A-5: Secondary Coil of Power Receiver example 2 ................................................................................................... 64
Figure A-6: Secondary Coil and Shielding assembly of Power Receiver example 2 ................................................ 65
Figure A-7: Electrical details of Power Receiver example 2 ............................................................................................... 66
Figure B-1: Analog ping based on a resonance shift .............................................................................................................. 67
Figure C-1: Single Power Receiver covering multiple Primary Cells .............................................................................. 70
Figure C-2: Two Power Receivers covering two adjacent Primary Cells ...................................................................... 70
Figure C-3: Two Power Receivers covering a single Primary Cell ................................................................................... 71
Figure C-4: Detection Coil ................................................................................................................................................................. 72
List of Tables
Table 3-1: Primary Coil parameters of Power Transmitter design A1 .......................................................................... 12
Table 3-2: PID parameters for Operating Frequency control ............................................................................................ 14
Table 3-3: Operating Frequency dependent scaling factor ................................................................................................. 14
Table 3-4: PID parameters for duty cycle control ................................................................................................................... 15
Table 3-5: Primary Coil parameters of Power Transmitter design A2 .......................................................................... 17
Table 3-6: PID parameters for voltage control ......................................................................................................................... 19
Table 3-7: Primary Coil array parameters of Power Transmitter design B1 .............................................................. 22
Table 3-8: PID parameters for voltage control ......................................................................................................................... 24
Table 3-9: Primary Coil array parameters of Power Transmitter design B2 .............................................................. 26
Table 5-1: Power Transmitter timing in the ping phase ...................................................................................................... 38
Table 5-2: Power Transmitter timing in the identification & configuration phase ................................................... 41
Table 5-3: Power control hold-off time ....................................................................................................................................... 41
Table 5-4: Power Transmitter timing in the power transfer phase ................................................................................. 44
Table 5-5: Power Receiver timing in any phase ....................................................................................................................... 46
Table 5-6: Power Receiver timing in the selection phase..................................................................................................... 47
Table 5-7: Power Receiver timing in the identification & configuration phase .......................................................... 48
Table 5-8: Power Receiver timing in the power transfer phase ........................................................................................ 49
Table 6-1: Amplitude modulation of the Power Signal ......................................................................................................... 52
Table 6-2: Message size ...................................................................................................................................................................... 53
Table 6-3: Packet types ...................................................................................................................................................................... 54
Table 6-4: Signal Strength ................................................................................................................................................................. 55
Table 6-5: End Power Transfer ...................................................................................................................................................... 56
Table 6-6: End Power Transfer values ........................................................................................................................................ 56
Table 6-7: Control Error .................................................................................................................................................................... 57
Table 6-8: Rectified Power ............................................................................................................................................................... 57
Table 6-9: Charge Status .................................................................................................................................................................... 57
Table 6-10: Power control hold-off .............................................................................................................................................. 58
Table 6-11: Configuration ................................................................................................................................................................. 58
Table 6-12: Identification .................................................................................................................................................................. 59
Table 6-13: Extended Identification ............................................................................................................................................. 60
Table A-1: Secondary Coil parameters of Power Receiver example 1 ........................................................................... 61
Table A-2: Parameters of the Secondary Coil of Power Receiver example 2 .............................................................. 64
Table B-1: Analog ping based on a resonance shift ................................................................................................................ 67
1 General
1.1 Scope
Volume I of the System Description Wireless Power Transfer consists of the following documents:
. Part 1, Interface Definition.
. Part 2, Performance Requirements.
. Part 3, Compliance Testing.

This document defines the interface between a Power Transmitter and a Power Receiver.
1.2 Main features
. A method of contactless power transfer from a Base Station to a Mobile Device, which is based on
near field magnetic induction between coils.
. Transfer of around 5 W of power, using an appropriate Secondary Coil (having a typical outer
dimension of around 40 mm).
. Operation at frequencies in the 110…205 kHz range.
. Support for two methods of placing the Mobile Device on the surface of the Base Station:
o Guided Positioning helps a user to properly place the Mobile Device on the surface of a
Base Station that provides power through a single or a few fixed locations of that surface.
o Free Positioning enables arbitrary placement of the Mobile Device on the surface of a
Base Station that can provide power through any location of that surface.

. A simple communications protocol enabling the Mobile Device to take full control of the power
transfer.
. Considerable design flexibility for integration of the system into a Mobile Device.
. Very low stand-by power achievable (implementation dependent).

1.3 Conformance and references
All specifications in this document are mandatory, unless specifically indicated as recommended or
optional or informative. To avoid any doubt, the word “shall” indicates a mandatory behavior of the
specified component, i.e. it is a violation of this System Description Wireless Power Transfer if the
specified component does not exhibit the behavior as defined. In addition, the word “should” indicates a
recommended behavior of the specified component, i.e. it is not a violation of this System Description
Wireless Power Transfer if the specified component has valid reasons to deviate from the defined
behavior. And finally, the word “may” indicates an optional behavior of the specified component, i.e. it is
up to the specified component whether to exhibit the defined behavior (without deviating there from) or
not.
In addition to the specifications provided in this document, product implementations shall also conform to
the specifications provided in the System Descriptions listed below. Moreover, the relevant parts of the
International Standards listed below shall apply as well. If multiple revisions exist of any System
Description or International Standard listed below, the applicable revision is the one that was most
recently published at the release date of this document.
[Part 2] System Description Wireless Power Transfer, Volume I, Part 2, Performance
Requirements.
[Part 3] System Description Wireless Power Transfer, Volume I, Part 3, Compliance
Testing.
[PRMC] Power Receiver Manufacturer Codes, Wireless Power Consortium.

[SI] The International System of Units (SI), Bureau International des Poids et
Mesures.
1.4 Definitions
Active Area The part of the Interface Surface of a Base Station respectively Mobile Device
through which a sufficiently high magnetic flux penetrates when the Base
Station is providing power to the Mobile Device.
Base Station A device that is able to provide near field inductive power as specified in this
System Description Wireless Power Transfer. A Base Station carries a logo to
visually indicate to a user that the Base Station complies with this System
Description Wireless Power Transfer.
Communications and Control Unit
 The functional part of a Power Transmitter respectively Power Receiver that
controls the power transfer. (Informative) Implementation-wise, the
Communications and Control Unit may be distributed over multiple subsystems of
the Base Station respectively Mobile Device.
Control Point The combination of voltage and current provided at the output of the Power
Receiver, and other parameters that are specific to a particular Power Receiver
implementation.
Detection Unit The functional part of a Power Transmitter that detects the presence of a Power
Receiver on the Interface Surface.
Digital Ping The application of a Power Signal in order to detect and identify a Power
Receiver.
Free Positioning A method of positioning a Mobile Device on the Interface Surface of a Base
Station that does not require the user to align the Active Area of the Mobile
Device to the Active Area of the Base Station.
Guided Positioning A method of positioning a Mobile Device on the Interface Surface of a Base
Station that provides the user with feedback to properly align the Active Area of
the Mobile Device to the Active Area of the Base Station.
Interface Surface A flat part of the surface of a Base Station respectively Mobile Device that is
closest to the Primary Coil(s) respectively Secondary Coil.
Mobile Device A device that is able to consume near field inductive power as specified in this
System Description Wireless Power Transfer. A Mobile Device carries a logo to
visually indicate to a user that the Mobile Device complies with this System
Description Wireless Power Transfer.
Operating Frequency The oscillation frequency of the Power Signal.
Operating Point The combination of the frequency, duty cycle and amplitude of the voltage that
is applied to the Primary Cell.
Packet A data structure that the Power Receiver uses to communicate a message to the
Power Transmitter. A Packet consists of a preamble, a header byte, a message,
and a checksum. A Packet is named after the kind of message that it contains.
Power Conversion Unit The functional part of a Power Transmitter that converts electrical energy to a
Power Signal.
Power Pick-up Unit The functional part of a Power Receiver that converts a Power Signal to
electrical energy.
Power Receiver The subsystem of a Mobile Device that acquires near field inductive power and
controls its availability at its output, as defined in this System Description
Wireless Power Transfer. For this purpose, the Power Receiver communicates
its power requirements to the Power Transmitter.

Power Signal The oscillating magnetic flux that is enclosed by a Primary Cell and possibly a
Secondary Coil.
Power Transfer Contract A set of boundary conditions on the parameters that characterize the power
transfer from a Power Transmitter to a Power Receiver. Violation of any of
these boundary conditions causes the power transfer to abort.
Power Transmitter The subsystem of a Base Station that generates near field inductive power and
controls its transfer to a Power Receiver, as defined in this System Description
Wireless Power Transfer.
Primary Cell A single Primary Coil or a combination of Primary Coils that are used to provide
a sufficiently high magnetic flux through the Active Area.
Primary Coil A component of a Power Transmitter that converts electric current to magnetic
flux.
Secondary Coil The component of a Power Receiver that converts magnetic flux to
electromotive force.
Shielding A component in the Power Transmitter respectively Power Receiver that
restricts magnetic fields to the appropriate parts of the Base Station
respectively Mobile Device.
1.5 Acronyms
AC Alternating Current
AWG American Wire Gauge
DC Direct Current
lsb least significant bit
msb most significant bit
N.A. Not Applicable
PID Proportional Integral Differential
RMS Root Mean Square
UART Universal Asynchronous Receiver Transmitter
1.6 Symbols
Cd Capacitance parallel to the Secondary Coil [nF]
Cm Capacitance in the impedance matching network [nF]
 Capacitance in series with the Primary Coil [nF]
CS Capacitance in series with the Secondary Coil [nF]
 Distance between a coil and its Shielding [mm]
 Distance between a coil and the Interface Surface [mm]
 Communications bit rate [kHz]
 Resonant detection frequency [kHz]
 Operating Frequency [kHz]
 Secondary resonance frequency [kHz]
 Primary Coil current modulation depth [mA]
 Power Receiver output current [mA]
 Primary Coil current [mA]
Lm Inductance in the impedance matching network [μH]

 Primary Coil self inductance [μH]
 Secondary Coil self inductance (Mobile Device away from Base Station) [μH]

Secondary Coil self inductance (Mobile Device on top of Base Station) [μH]
 Total amount of power received through the Interface Surface [W]
 Total amount of power transmitted through the Interface Surface [W]
 Power Control Hold-off Time [ms]
 Communications clock period [μs]
 Maximum transition time of the communications [μs]
 Rectified voltage [V]
 Power Receiver output voltage [V]
1.7 Conventions
This Section 1.7 defines the notations and conventions used in this System Description Wireless Power
Transfer.
1.7.1 Cross references
Unless indicated otherwise, cross references to Sections in either this document or documents listed in
Section 1.3, refer to the referenced Section as well as the sub Sections contained therein.
1.7.2 Informative text
With the exception of Sections that are marked as informative, all informative text is set in italics.
1.7.3 Terms in capitals
All terms that start with a capital are defined in Section 1.4. As an exception to this rule, Packet names and
fields are defined in Section 6.3.
1.7.4 Notation of numbers
Real numbers are represented using the digits 0 to 9, a decimal point, and optionally an exponential part.
In addition, a positive and/or negative tolerance may follow a real number. Real numbers that do not
include an explicit tolerance, have a tolerance of half the least significant digit that is specified.
(Informative) For example, a specified value of
comprises the range from 1.21 through 1.24; a
specified value of comprises the range from 1.23 through 1.24; a specified value of
comprises the range from 1.21 through 1.23; a specified value of 1.23 comprises the range from 1.225
through 1.234999…; and a specified value of comprises the range from 1.107 through 1.353.
Integer numbers in decimal notation are represented using the digits 0 to 9.
Integer numbers in hexadecimal notation are represented using the hexadecimal digits 0 to 9 and A to F,
and are preceded by “0x” (unless explicitly indicated otherwise).
Single bit values are represented using the words ZERO and ONE.
Integer numbers in binary notation and bit patterns are represented using sequences of the digits 0 and
1that are enclosed in single quotes (‘’). In a sequence of n bits, the most significant bit (msb) is bit bn–1 and
the least significant bit (lsb) is bit b0; the most significant bit is shown on the left-hand side.
1.7.5 Units of physical quantities
Physical quantities are expressed in units of the International System of Units [SI].
1.7.6 Bit ordering in a byte
The graphical representation of a byte is such that the msb is on the left, and the lsb is on the right. Figure
1-1 defines the bit positions in a byte.

msb
 
 
 
 
 
 
lsb
b7
b6
b5
b4
b3
b2
b1
b0
 
Figure 1-1: Bit positions in a byte
1.7.7 Byte numbering
The bytes in a sequence of n bytes are referred to as B0, B1, …, Bn–1. Byte B0 corresponds to the first byte in
the sequence; byte Bn–1 corresponds to the last byte in the sequence. The graphical representation of a
byte sequence is such that B0 is at the upper left-hand side, and byte Bn–1 is at the lower right-hand side.
1.7.8 Multiple-bit Fields
Unless indicated otherwise, a multiple bit field in a data structure represents an unsigned integer value. In
a multiple-bit field that spans multiple bytes, the msb of the multiple-bit field is located in the byte with
the lowest address, and the lsb of the multiple-bit field is located in the byte with the highest address.
(Informative) Figure 1-2 provides an example of a 6-bit field that spans two bytes.
 
 
 
 
 
 
b5
b4
 
b3
b2
b1
b0
 
 
 
 
B0
 
B1
 
Figure 1-2: Example of multiple-bit field
1.8 Operators
This Section 1.8 defines the operators used in this System Description Wireless Power Transfer, which are
less commonly used. The commonly used operators have their usual meaning.
1.8.1 Exclusive-OR
The symbol ‘.’ represents the exclusive-OR operation.
1.8.2 Concatenation
The symbol ‘||’ represents concatenation of two bit strings. In the resulting concatenated bit string, the
msb of the right-hand side operand directly follows the lsb of the left-hand side operand.

This page is intentionally left blank.

2 System Overview (Informative)
Operation of devices that comply with this System Description Wireless Power Transfer relies on
magnetic induction between planar coils. Two kinds of devices are distinguished, namely devices that
provide wireless power—referred to as Base Stations—and devices that consume wireless power—
referred to as Mobile Devices. Power transfer always takes place from a Base Station to a Mobile Device.
For this purpose, a Base Station contains a subsystem—referred to as a Power Transmitter—that
comprises a Primary Coil,1 and a Mobile Device contains a subsystem—referred to as a Power Receiver—
comprises a Secondary Coil. In fact, the Primary Coil and Secondary Coil form the two halves of a coreless
resonant transformer. Appropriate Shielding at the bottom face of the Primary Coil and the top face of the
Secondary Coil, as well as the close spacing of the two coils, ensures that power transfer occurs with an
acceptable efficiency. In addition, this Shielding minimizes the exposure of users to the magnetic field.
1Note that the Primary Coil may be a “virtual coil,” in the sense that an appropriate array of planar coils
can generate a magnetic field that is similar to the field that a single coil generates.
Typically, a Base Station has a flat surface—referred to as the Interface Surface—on top of which a user
can place one or more Mobile Devices. This ensures that the vertical spacing between Primary Coil and
Secondary Coil is sufficiently small. In addition, there are two concepts for horizontal alignment of the
Primary Coil and Secondary Coil. In the first concept—referred to as Guided Positioning—the user must
actively align the Secondary Coil to the Primary Coil, by placing the Mobile Device on the appropriate
location of the Interface Surface. For this purpose, the Mobile Device provides an alignment aid that is
appropriate to its size, shape and function. The second concept—referred to as Free Positioning—does
not require the active participation in alignment of the Primary Coil and Secondary Coil. One
implementation of Free Positioning makes use of an array of Primary Coils to generate a magnetic field at
the location of the Secondary Coil only. Another implementation of Free Positioning uses mechanical
means to move a single Primary Coil underneath the Secondary Coil.
Figure 2-1 illustrates the basic system configuration. As shown, a Power Transmitter comprises two main
functional units, namely a Power Conversion Unit and a Communications and Control Unit. The diagram
explicitly shows the Primary Coil (array) as the magnetic field generating element of the Power
Conversion Unit. The Control and Communications Unit regulates the transferred power to the level that
the Power Receiver requests. Also shown in the diagram is that a Base Station may contain multiple
Transmitters in order to serve multiple Mobile Devices simultaneously (a Power Transmitter can serve a
single Power Receiver at a time only). Finally, the system unit shown in the diagram comprises all other
functionality of the Base Station, such as input power provisioning, control of multiple Power
Transmitters, and user interfacing.
A Power Receiver comprises a Power Pick-up Unit and a Communications and Control Unit. Similar to the
Power Conversion Unit of the Transmitter, Figure 2-1 explicitly shows the Secondary Coil as the magnetic
field capturing element of the Power Pick-up Unit. A Power Pick-up Unit typically contains a single
Secondary Coil only. Moreover, a Mobile Device typically contains a single Power Receiver. The
Communications and Control Unit regulates the transferred power to the level that is appropriate for the
subsystems connected to the output of the Power Receiver. These subsystems represent the main
functionality of the Mobile Device. An important example subsystem is a battery that requires charging.
The remainder of this document is structured as follows. Section 3 defines the basic Power Transmitter
designs, which come in two basic varieties. The first type of design—type A—is based on a single Primary
Coil (either fixed position or moveable). The second type of design—type B—is based on an array of
Primary Coils. Note that this version 1.0 of the System Description Wireless Power Transfer, Volume I,
Part 1, offers only limited design freedom with respect to actual Power Transmitter implementations. The
reason is that Mobile Devices exhibit a much greater variety of design requirements with respect to the
Power Receiver than a Base Station does to Power Transmitters—for example, a smart phone has design
requirements that differ substantially from those of a wireless headset. Constraining the Power
Transmitter therefore enables interoperability with the largest number of mobile devices.

 Base StationPower TransmitterPower
Conversion UnitCommunications
& Control UnitPower TransmitterPower
Conversion unitCommunications
& Control UnitMobile DevicePower ReceiverPower Pick-up
UnitCommunications
& Control UnitLoadSecondary
CoilOutput
PowerSensing & ControlSystem UnitInput
PowerInput
PowerPrimary
Coil(s)
Figure 2-1: Basic system overview
Section 4 defines the Power Receiver design requirements. In view of the wide variety of Mobile Devices,
this set of requirements has been kept to a minimum. In addition to the design requirements, Section 4 is
complemented with two example designs in Annex A.
Section 5 defines the system control aspects of the power transfer. The interaction between a Power
Transmitter and a Power Receiver comprises four phases, namely selection, ping, identification &
configuration, and power transfer. In the selection phase, the Power Transmitter attempts to discover and
locate objects that are placed on the Interface Surface. In addition, the Power Transmitter attempts to
discriminate between Power Receivers and foreign objects and to select a Power Receiver (or object) for
power transfer. For this purpose, the Power Transmitter may select an object at random and proceed to
the ping phase (and subsequently to the identification & configuration phase) to collect necessary
information. Note that if the Power Transmitter does not initiate power transfer to a selected Power
Receiver, it should enter a low power stand-by mode of operation.2 In the ping phase, the Power
Transmitter attempts to discover if an object contains a Power Receiver. In the identification &
configuration phase, the Power Transmitter prepares for power transfer to the Power Receiver. For this
purpose, the Power Transmitter retrieves relevant information from the Power Receiver. The Power
Transmitter combines this information with information that it stores internally to construct a so-called
Power Transfer Contract, which comprises various limits on the power transfer. In the power transfer
2A definition of such a stand-by mode is outside the scope of this version 1.0 System Description Wireless
Power Transfer, Volume I, Part 1. However, [Part 2] provides requirements on the maximum power use of
a Power Transmitter when it is not actively providing power to a Power Receiver.

phase, the actual power transfer takes place. During this phase, the Power Transmitter and the Power
Receiver cooperate to regulate the transferred power to the desired level. For this purpose, the Power
Receiver communicates its power needs on a regular basis. In addition, the Power Transmitter
continuously monitors the power transfer to ensure that the limits collected in the Power Transfer
Contract are not violated. If a violation occurs anyway, the Power Transmitter aborts the power transfer.
The various Power Transmitter designs employ different methods to adjust the transferred power to the
requested level. Three commonly used methods include frequency control—the Primary Coil current, and
thus the transferred power, is frequency dependent due to the resonant nature of the transformer—duty
cycle control—the amplitude of the Primary Coil current scales with the duty cycle of the inverter that is
used to drive it—and voltage control—the Primary Coil current scales with the driving voltage. Whereas
the details of these control methods are defined in Section 3, Section 5 defines the overall error based
control strategy. This means that the Power Receiver communicates the difference between a desired set
point and the actual set point to the Power Transmitter, which adjusts the Primary Coil current so as to
reduce the error towards zero. There are no constraints on how the Power Receiver derives its set point
from parameters such as power, voltage, current, and temperature. This leaves the option to the Power
Receiver to apply any desired control strategy.
This version 1.0 of the System Description Wireless Power Transfer, Volume I, Part 1, defines
communications from the Power Receiver to the Power Transmitter only. Section 6 defines the
communications interface. On a physical level, communications from the Power Receiver to the Power
Transmitter proceed using load modulation. This means that the Power Receiver switches the amount of
power that it draws from the Power Transmitter between two discrete levels (note that these levels are
not fixed, but depend on the amount of power that is being transferred). The actual load modulation
method is left as a design choice to the Power Receiver. Resistive, capacitive, and inductive schemes are
all possible. On a logical level, the communications protocol uses a sequence of short messages that
contain the relevant data. These messages are contained in Packets, which are transmitted in a simple
UART like format.
Annex A provides two example Power Receiver designs. The design shown in the first example directly
provides the rectified voltage from the Secondary Coil to a single-cell lithium-ion battery for charging at
constant current or voltage. The design shown in the second example uses a post-regulation stage to
create a voltage source at the output of the Power Receiver.
This version 1.0 of the System Description Wireless Power Transfer, Volume I, Part 1, does not define how
a Power Transmitter should detect an object that is placed on the Interface Surface. Annex B discusses
several example methods that a Power Transmitter can use. Some of these methods enable Power
Transmitter implementations that use very low stand-by power—if there are no Power Receivers present
on the Interface Surface, or if there are Power Receivers present that are not engaged in power transfer.
Annex C discusses a few use cases that deal with locating Power Receivers on the Interface Surface of a
type B Power Transmitter. In particular, these use cases describe how to find the optimum location for
the Active Area—through which the Power Transmitter provides power to the Power Receiver—and how
to distinguish between multiple closely spaced Power Receivers.
Finally, Annex D discusses how a Power Transmitter should detect the presence of foreign objects on the
Interface Surface, which are sufficiently close to the Active Area to interfere with the power transfer.
Typical examples of such foreign objects are parasitic metals such as coins, keys, paperclips, etc. If a
parasitic metal is close to the Active Area it could heat up during power transfer due to eddy currents that
result from the oscillating magnetic field. In order to prevent unsafe situations from developing, the
Power Transmitter should abort the power transfer, before the temperature of the parasitic metal rises to
unacceptable levels.

This page is intentionally left blank.

3 Basic Power Transmitter Designs
3.1 Introduction
The Power Transmitter designs, which this version 1.0 of the System Description Wireless Power
Transfer, Volume I, Part 1, defines, are grouped in two basic types.
Type A Power Transmitter designs have a single Primary Coil—and a single Primary Cell, which coincides
with the Primary Coil. In addition, type A Power Transmitter designs include means to realize proper
alignment of the Primary Coil and Secondary Coil. Depending on this means, a type A Power Transmitter
enables either Guided Positioning or Free Positioning.
Type B Power Transmitter designs have an array of Primary Coils. All type B Power Transmitters enable
Free Positioning. For that purpose, type B Power Transmitters can combine one or more Primary Coils
from the array to realize a Primary Cell at different positions across the Interface Surface.
A Power Transmitter serves a single Power Receiver at a time only. However, a Base Station may contain
several Power Transmitters in order to serve multiple Mobile Devices simultaneously. Note that multiple
type B Power Transmitters may share (parts of) the multiplexer and array of Primary Coils (see Section
3.3.1.3).
3.2 Power Transmitter designs that are based on a single Primary Coil
This Section 3.2 defines all type A Power Transmitter designs. In addition to the definitions in this Section
3.2, each Power Transmitter design shall implement the relevant parts of the protocols defined in Section
5, as well as the communications interface defined in Section 6.
3.2.1 Power Transmitter design A1
Power Transmitter design A1 enables Guided Positioning. Figure 3-1 illustrates the functional block
diagram of this design, which consists of two major functional units, namely a Power Conversion Unit and
a Communications and Control Unit.

Control &
CommunicationsUnitInverterPrimary
CoilCurrent
SensePower Conversion UnitInput Power
Figure 3-1: Functional block diagram of Power Transmitter design A1

The Power Conversion Unit on the right-hand side of Figure 3-1 comprises the analog parts of the design.
The inverter converts the DC input to an AC waveform that drives a resonant circuit, which consists of the
Primary Coil plus a series capacitor. Finally, the current sense monitors the Primary Coil current.
The Communications and Control Unit on the left-hand side of Figure 3-1 comprises the digital logic part
of the design. This unit receives and decodes messages from the Power Receiver, executes the relevant
power control algorithms and protocols, and drives the frequency of the AC waveform to control the
power transfer. The Communications and Control Unit also interfaces with other subsystems of the Base
Station, e.g. for user interface purposes.
3.2.1.1 Mechanical details
Power Transmitter design A1 includes a single Primary Coil as defined in Section 3.2.1.1.1, Shielding as
defined in Section 3.2.1.1.2, an Interface Surface as defined in Section 3.2.1.1.3, and an alignment aid as
defined in Section 3.2.1.1.4.
3.2.1.1.1 Primary Coil
The Primary Coil is of the wire-wound type, and consists of no. 20 AWG (0.81 mm diameter) type 2 litz
wire having 105 strands of no. 40 AWG (0.08 mm diameter), or equivalent. As shown in Figure 3-2, the
Primary Coil has a circular shape and consists of multiple layers. All layers are stacked with the same
polarity. Table 3-1 lists the dimensions of the Primary Coil.

dcdodi
Figure 3-2: Primary Coil of Power Transmitter design A1
Table 3-1: Primary Coil parameters of Power Transmitter design A1
Parameter
Symbol
Value
Outer diameter
 
 mm
Inner diameter
 
 mm
Thickness
 
 mm
Number of turns per layer
 
10
Number of layers
2
 
3.2.1.1.2 Shielding
As shown in Figure 3-3, soft-magnetic material protects the Base Station from the magnetic field that is
generated in the Primary Coil. The Shielding extends to at least 2 mm beyond the outer diameter of the
Primary Coil, has a thickness of at least 0.5 mm, and is placed below the Primary Coil at a distance of at
most mm. This version 1.0 of the System Description Wireless Power Transfer, Volume I, Part 1,
limits the composition of the Shielding to a choice from the following list of materials:
. Material 44 — Fair Rite Corporation.
. Material 28 — Steward, Inc.
. CMG22G — Ceramic Magnetics, Inc.


 dsdz2 mm min.
Interface
SurfaceShieldingPrimary CoilBase
Station1.0° max.
5 mm min.317 mm min.
Magnet
Figure 3-3: Primary Coil assembly of Power Transmitter design A1
3.2.1.1.3 Interface Surface
As shown in Figure 3-3, the distance from the Primary Coil to the Interface Surface of the Base Station is
mm, across the top face of the Primary Coil. In addition, the Interface Surface of the Base
Station extends at least 5 mm beyond the outer diameter of the Primary Coil. (Informative) This Primary-
Coil-to-Interface-Surface distance implies that the tilt angle between the Primary Coil and a flat Interface
Surface is at most 1.0.. Alternatively, in case of a non-flat Interface Surface, this Primary-Coil-to-Interface-
Surface distance implies a radius of curvature of the Interface Surface of at least 317 mm, centered on the
Primary Coil. See also Figure 3-3.
3.2.1.1.4 Alignment aid
Power Transmitter design A1 employs a disc shaped bonded Neodymium magnet, which a Power Receiver
design can exploit to provide an effective alignment means (see Section 4.2.1.2). As shown in Figure 3-3,
the magnet is centered within the Primary Coil, and has its north pole oriented towards the Interface
Surface. The (static) magnetic flux density due to the magnet, as measured across the Base Station’s
Interface Surface, has a maximum of
mT. The diameter of the magnet is at most 15.5 mm.
3.2.1.1.5 Inter coil separation
If the Base Station contains multiple type A1 Power Transmitters, the Primary Coils of any pair of those
Power Transmitters shall have a center-to-center distance of at least 50 mm.
3.2.1.2 Electrical details
As shown in Figure 3-4, Power Transmitter design A1 uses a half-bridge inverter to drive the Primary Coil
and a series capacitance. Within the Operating Frequency range specified below, the assembly of Primary
Coil, Shielding, and magnet has a self inductance μH. The value of the series capacitance is
nF. The input voltage to the half-bridge inverter is V. (Informative) Near resonance, the
voltage developed across the series capacitance can reach levels exceeding 200 V pk-pk.
Power Transmitter design A1 uses the Operating Frequency and duty cycle of the Power Signal in order to
control the amount of power that is transferred. For this purpose, the Operating Frequency range of the
half-bridge inverter is kHz with a duty cycle of 50%; and its duty cycle range is 10…50%
at an Operating Frequency of 205 kHz. A higher Operating Frequency or lower duty cycle result in the
transfer of a lower amount of power. In order to achieve a sufficiently accurate adjustment of the amount
of power that is transferred, a type A1 Power Transmitter shall control the Operating Frequency with a
resolution of
. kHz, for fop in the 110…175 kHz range;
. kHz, for fop in the 175…205 kHz range;


or better. In addition, a type A1 Power Transmitter shall control the duty cycle of the Power Signal with a
resolution of 0.1% or better.
When a type A1 Power Transmitter first applies a Power Signal (Digital Ping; see Section 5.2.1), it shall use
an initial Operating Frequency of 175 kHz (and a duty cycle of 50%).
Control of the power transfer shall proceed using the PID algorithm, which is defined in Section 5.2.3.1.
The controlled variable introduced in the definition of that algorithm represents the Operating
Frequency. In order to guarantee sufficiently accurate power control, a type A1 Power Transmitter shall
determine the amplitude of the Primary Cell current—which is equal to the Primary Coil current—with a
resolution of 7 mA or better. Finally, Table 3-2, Table 3-3, and Table 3-4 provide the values of several
parameters, which are used in the PID algorithm.

LPCPHalf-bridge
InverterInput
Voltage+
.
Control
Figure 3-4: Electrical diagram (outline) of Power Transmitter design A1
Table 3-2: PID parameters for Operating Frequency control
Parameter
Symbol
Value
Unit
Proportional gain
 
10
mA-1
Integral gain
 
0.05
mA-1ms-1
Derivative gain
 
0
mA-1ms
Integral term limit
 
3,000
N.A.
PID output limit
 
20,000
N.A.
 
Table 3-3: Operating Frequency dependent scaling factor
Frequency Range [kHz]
Scaling Factor [Hz]
110…140
1.5
140…160
2
160…180
3
180…205
5
 

Table 3-4: PID parameters for duty cycle control
Parameter
Symbol
Value
Unit
Proportional gain
 
10
mA-1
Integral gain
 
0.05
mA-1ms-1
Derivative gain
 
0
mA-1ms
Integral term limit
 
3,000
N.A.
PID output limit
 
20,000
N.A.
Scaling factor
 
–0.01
%
 

3.2.2 Power Transmitter design A2
Power Transmitter design A2 enables Free Positioning. Figure 3-5 illustrates the functional block diagram
of this design, which consists of three major functional units, namely a Power Conversion Unit, a Detection
Unit, and a Communications and Control Unit.

Communications& Control UnitInverterPrimary
CoilVoltageSensePower Conversion UnitInput PowerPositioning
StageDetection Unit
Figure 3-5: Functional block diagram of Power Transmitter design A2
The Power Conversion Unit on the right-hand side of Figure 3-5 and the Detection Unit of the bottom of
Figure 3-5 comprise the analog parts of the design. The Power Conversion Unit is similar to the Power
Conversion Unit of Power Transmitter design A1. The inverter converts the DC input to an AC waveform
that drives a resonant circuit, which consists of the Primary Coil plus a series capacitor. The Primary Coil
is mounted on a positioning stage to enable accurate alignment of the Primary Coil to the Active Area of
the Mobile Device. Finally, the voltage sense monitors the Primary Coil voltage.
The Communications and Control Unit on the left-hand side of Figure 3-5 comprises the digital logic part
of the design. This unit is similar to the Communications and Control Unit of Power Transmitter design
A1. The Commnuications and Control Unit receives and decodes messages from the Power Receiver,
executes the relevant power control algorithms and protocols, and drives the input voltage of the AC
waveform to control the power transfer. In addition, the Communications and Control Unit drives the
positioning stage and operates the Detection Unit. The Communications and Control Unit also interfaces
with other subsystems of the Base Station, e.g. for user interface purposes.
The Detection Unit determines the approximate location of objects and/or Power Receivers on the
Interface Surface. This version 1.0 of the System Description Wireless Power Transfer, Volume I, Part 1,
does not specify a particular detection method. However, it is recommended that the Detection Unit
exploits the resonance in the Power Receiver at the detection frequency (see Section 4.2.2.1). The

reason is that this approach minimizes movements of the Primary Coil, because the Power Transmitter
does not need to attempt to identify objects that do not respond at this resonant frequency. Annex C.3
provides an example resonant detection method.
3.2.2.1 Mechanical details
Power Transmitter design A2 includes a single Primary Coil as defined in Section 3.2.2.1.1, Shielding as
defined in Section 3.2.2.1.2, an Interface Surface as defined in Section 3.2.2.1.3, and a positioning stage as
defined in Section 3.2.2.1.4.
3.2.2.1.1 Primary Coil
The Primary Coil is of the wire-wound type, and consists of litz wire having 30 strands of 0.1 mm
diameter, or equivalent. As shown in Figure 3-6, the Primary Coil has a circular shape and consists of
multiple layers. All layers are stacked with the same polarity. Table 3-5 lists the dimensions of the
Primary Coil.

dcdodi
Figure 3-6: Primary Coil of Power Transmitter design A2
Table 3-5: Primary Coil parameters of Power Transmitter design A2
Parameter
Symbol
Value
Outer diameter
 
 mm
Inner diameter
 
 mm
Thickness
 
 mm
Number of turns per layer
 
10
Number of layers
2
 

3.2.2.1.2 Shielding
As shown in Figure 3-7, soft-magnetic material protects the Base Station from the magnetic field that is
generated in the Primary Coil. The Shielding extends to at least 2 mm beyond the outer diameter of the
Primary Coil, has a thickness of at least 0.20 mm and is placed below the Primary Coil at a distance of at
most mm. This version 1.0 of the System Description Wireless Power Transfer, Volume I, Part 1,
limits the composition of the Shielding to a choice from the following list of materials:
. DPR-MF3 — Daido Steel
. HS13-H — Daido Steel

 
dsdz2 mm min.
Interface
SurfaceShieldingPrimary
CoilBase
Station1.0° max.
5 mm min.
Figure 3-7: Primary Coil assembly of Power Transmitter design A2
3.2.2.1.3 Interface Surface
As shown in Figure 3-7, the distance from the Primary Coil to the Interface Surface of the Base Station is
mm, across the top face of the Primary Coil. In addition, the Interface Surface of the Base
Station extends at least 5 mm beyond the outer diameter of the Primary Coil.
3.2.2.1.4 Positioning stage
The positioning stage shall have a resolution of 0.1 mm or better in each of the two orthogonal directions
parallel to the Interface Surface.
3.2.2.2 Electrical details
As shown in Figure 3-8, Power Transmitter design A2 uses a full-bridge inverter to drive the Primary Coil
and a series capacitance. At the fixed Operating Frequency of 140 kHz, the assembly of Primary Coil and
Shielding has a self inductance μH. The value of the series capacitance is nF.
(Informative) Near resonance, the voltage developed across the series capacitance can reach levels up to
50 V pk-pk.
Power Transmitter design A2 uses the input voltage to the full-bridge inverter to control the amount of
power that is transferred. For this purpose, the input voltage range is 3…12 V, where a lower input
voltage results in the transfer of a lower amount of power. In order to achieve a sufficiently accurate
adjustment of the power that is transferred, a type A2 Power Transmitter shall be able to control the input
voltage with a resolution of 50 mV or better.
When a type A2 Power Transmitter first applies a Power Signal (Digital Ping; see Section 5.2.1), it shall use
an initial input voltage of 8 V.

 LPCPFull-bridge
InverterInput
Voltage+
.
Control
Figure 3-8: Electrical diagram (outline) of Power Transmitter design A2
Control of the power transfer shall proceed using the PID algorithm, which is defined in Section 5.2.3.1.
The controlled variable introduced in the definition of that algorithm represents the input voltage to
the full-bridge inverter. In order to guarantee sufficiently accurate power control, a type A2 Power
Transmitter shall determine the amplitude of the Primary Cell voltage—which is equal to the Primary Coil
voltage—with a resolution of 5 mV or better. Finally, Table 3-6 provides the values of several parameters,
which are used in the PID algorithm.
 
Table 3-6: PID parameters for voltage control
Parameter
Symbol
Value
Unit
Proportional gain
 
1
mA-1
Integral gain
 
0
mA-1ms-1
Derivative gain
 
0
mA-1ms
Integral term limit
 
N.A.
N.A.
PID output limit
 
1,500
N.A.
Scaling factor
 
–0.5
mV
 

3.3 Power Transmitter designs that are based on an array of Primary Coils
This Section 3.3 defines all type B Power Transmitter designs. In addition to the definitions in this Section
3.3, each Power Transmitter design shall implement the relevant parts of the protocols defined in Section
5, as well as the communications interface defined in Section 6.
3.3.1 Power Transmitter design B1
Power Transmitter design B1 enables Free Positioning. Figure 3-9 illustrates the functional block diagram
of this design, which consists of two major functional units, namely a Power Conversion Unit and a
Communications and Control Unit.

Power Conversion UnitCommunications& Control UnitInverterSensingImpedance
MatchingInput PowerMultiplexerPrimary
Coil Array
Figure 3-9: Functional block diagram of Power Transmitter design B1
The Power Conversion Unit on the right-hand side of Figure 3-9 comprises the analog parts of the design.
The design uses an array of partly overlapping Primary Coils to provide for Free Positioning. Depending
on the position of the Power Receiver, the multiplexer connects and/or disconnects the appropriate
Primary Coils. The impedance matching network forms a resonant circuit with the parts of the Primary
Coil array that are connected. The sensing circuits monitor (amongst others) the Primary Cell current and
voltage, and the inverter converts the DC input to an AC waveform that drives the Primary Coil array.
The Communications and Control Unit on the left-hand side of Figure 3-9 comprises the digital logic part
of the design. This unit receives and decodes messages from the Power Receiver, configures the
multiplexer to connect the appropriate parts of the Primary Coil array, executes the relevant power
control algorithms and protocols, and drives the frequency and input voltage to the inverter to control the
amount of power provided to the Power Receiver. The Communications and Control Unit also interfaces
with the other subsystems of the Base Station, e.g. for user interface purposes.

3.3.1.1 Mechanical details
Power Transmitter design B1 includes a Primary Coil array as defined in Section 3.3.1.1.1, Shielding as
defined in Section 3.3.1.1.2, and an Interface Surface as defined in Section 3.3.1.1.3.
3.3.1.1.1 Primary Coil array
The Primary Coil array consists of 3 layers. Figure 3-10(a) shows a top view of a single Primary Coil,
which is of the wire-wound type, and consists of litz wire having 24 strands of no. 40 AWG (0.08 mm
diameter), or equivalent.

(a)(b)
(c)
321dcdctop3t2t321dhdcdadcdodi
Figure 3-10: Primary Coil array of Power Transmitter design B1
As shown in Figure 3-10(a), the Primary Coil has a circular shape and consists of a single layer. Figure
3-10(b) shows a side view of the layer structure of the Primary Coil array. Figure 3-10(c) provides a top
view of the Primary Coil array, showing that the individual Primary Coils are packed in a hexagonal grid.
The solid hexagons show the closely packed structure of the grid of Primary Coils on layer 1 of the
Primary Coil array. The dashed hexagon illustrates that the grid of Primary Coils on layer 2 is offset over a
distance to the right, such that the centers of the Primary Coils in layer 2 coincide with the corners of

Primary Coils in layer 1. Likewise, the dash-dotted hexagon illustrates that the grid of Primary Coils on
layer 3 is offset over a distance to the left, such that the centers of the Primary Coils in layer 3 coincide
with the corners of Primary Coils in layer 1. As a result, the centers, respectively corners, of the Primary
Coils on layer 2 and the corners, respectively centers, of the Primary Coils on layer 3 coincide as well. All
Primary Coils are stacked with the same polarity. See Section 3.3.1.2 for the meaning of the shaded
hexagons.
Table 3-7 lists the relevant parameters of the Primary Coil array.
Table 3-7: Primary Coil array parameters of Power Transmitter design B1
Parameter
Symbol
Value
Outer diameter
 
 mm
Inner diameter
 
 mm
Layer thickness*
 

mm
Number of turns
 
16
Array thickness
 

mm
Center-to-center distance
 
 mm
Offset 2nd layer array
 
 mm
Offset 3rd layer array
 
 mm
*Value includes thickness of connection wires
 
3.3.1.1.2 Shielding
As shown in Figure 3-11, Transmitter design B1 employs Shielding to protect the Base Station from the
magnetic field that is generated in the Primary Coil array. The Shielding extends to at least 2 mm beyond
the outer edges of the Primary Coil array, and is placed at a distance of at most mm below the
Primary Coil array.
The Shielding consists of soft magnetic material that has a thickness of at least 0.5 mm. This version 1.0
of the System Description Wireless Power Transfer, Volume I, Part 1, limits the composition of the
Shielding to a choice from the following list of materials:
. Material 78 — Fair Rite Corporation.
. 3C94 — Ferroxcube.
. N87 — Epcos AG.
. PC44 — TDK Corp.

 
dsdz2 mm min.
Interface
SurfacePrimary Coil ArrayBase
StationShielding
Figure 3-11: Primary Coil array assembly of Power Transmitter design B1

3.3.1.1.3 Interface Surface
As shown in Figure 3-11, the distance from the Primary Coil array to the Interface Surface of the Base
Station is
mm, across the top face of the Primary Coil array. In addition, the Interface Surface
extends at least 5 mm beyond the outer edges of the Primary Coil array.
3.3.1.2 Electrical details
As shown in Figure 3-12, Power Transmitter design B1 uses a half-bridge inverter to drive the Primary
Coil array. In addition, Power Transmitter design B1 uses a multiplexer to select the position of the Active
Area. The multiplexer shall configure the Primary Coil array in such a way that one, two, or three Primary
Coils are connected—in parallel—to the driving circuit. The connected Primary Coils together constitute a
Primary Cell. As an additional constraint, the multiplexer shall select the Primary Coils such that each
selected Primary Coil has an overlap with every other selected Primary Coil; see Figure 3-10(c) for an
example.

Cm1MultiplexerControlImpedance
Matching CircuitHalf-bridge
InverterInput
Voltage+
.
ControlC1C2LmCm23S
Figure 3-12: Electrical diagram (outline) of Power Transmitter design B1
Within the Operating Frequency range kHz, the assembly of Primary Coil array and
Shielding has an inductance of μH for each individual Primary Coil in layer 1 (closest to the Interface
Surface), μH for each individual Primary Coil in layer 2, and μH for each individual Primary
Coil in layer 3. The capacitances and inductance in the impedance matching circuit are, respectively,
nF, nF,and μH. The capacitances and in the half-bridge
inverter both are 68 μF. The switch is open if the Primary Cell consists of a single Primary Coil;
otherwise, the swich is closed. (Informative) The voltage across the capacitance can reach levels
exceeding 36 V pk-pk.
Power Transmitter design B1 uses the input voltage to the half-bridge inverter to control the amount of
power that is transferred. For this purpose, the input voltage range is 0…20 V, where a lower input
voltage results in the transfer of a lower amount of power. In order to achieve a sufficiently accurate
adjustment of the power that is transferred, a type B1 Power Transmitter shall be able to control the input
voltage with a resolution of 35 mV or better.
When a type B1 Power Transmitter first applies a Power Signal (Digital Ping; see Section 5.2.1), it shall use
an initial input voltage of 12 V.

Control of the power transfer shall proceed using the PID algorithm, which is defined in Section 5.2.3.1.
The controlled variable introduced in the definition of that algorithm represents the input voltage to
the half-bridge inverter. In order to guarantee sufficiently accurate power control, a type B1 Transmitter
shall determine the amplitude of the current into the Primary Cell with a resolution of 5 mA or better. In
addition to the PID algorithm, a type B1 Power Transmitter shall limit the current into the Primary Cell to
at most 4 A RMS in the case that the Primary Cell consists of two or three Primary Coils, or at most 2 A
RMS in the case that the Primary Cell consists of one Primary Coil. For that purpose, the Power
Transmitter may limit the input voltage to the half-bridge inverter to value that is lower than 20 V.
Finally, Table 3-8 provides the values of several parameters, which are used in the PID algorithm.
Table 3-8: PID parameters for voltage control
Parameter
Symbol
Value
Unit
Proportional gain
 
1
mA-1
Integral gain
 
0
mA-1ms-1
Derivative gain
 
0
mA-1ms
Integral term limit
 
N.A.
N.A.
PID output limit
 
2,000
N.A.
Scaling factor
 
–1
mV
 
3.3.1.3 Scalability
Sections 3.3.1.1 and 3.3.1.2 define the mechanical and electrical details of Power Transmitter design B1.
As defined in Section 3.1, a type B1 Power Transmitter serves a single Power Receiver only. In order to
serve multiple Power Receivers simultaneously, a Base Station may contain multiple type B1 Power
Transmitters. As shown in Figure 3-13, these Power Transmitters may share the Primary Coil array and
multiplexer. However, each individual Power Transmitter shall have a separately controllable inverter,
impedance matching circuit, and means to determine the Primary Cell current, as defined in Section
3.3.1.2. In addition, the multiplexer shall ensure that it does not connect multiple inverters to any
individual Primary Coil.

 Half-bridge
InverterImpedance
MatchingControlInput Voltage 1Half-bridge
InverterImpedance
MatchingHalf-bridge
InverterImpedance
MatchingMultiplexer (shared)
Primary Coil Array (shared)
Control1st Transmitter (design B1)
2nd Transmitter (design B1)
nth Transmitter (design B1)
Input Voltage 2Input Voltage n
Figure 3-13: Multiple type B1 Power Transmitters sharing a multiplexer and Primary Coil array

3.3.2 Power Transmitter design B2
Power Transmitter design B2 enables Free Positioning. The main difference between Power Transmitter
design B2 and Power Transmitter design B1 is the Primary Coil array. Power Transmitter design B2 is
based on a Printed Circuit Board (PCB) type Primary Coil array. The functional block diagram of a type B2
Power Transmitter is identical to the functional block diagram of a type B1 Power Transmitter; see Figure
3-9 and the descriptive text in Section 3.3.1.
3.3.2.1 Mechanical details
Power Transmitter design B2 includes a Primary Coil array as defined in Section 3.3.2.1.1, Shielding as
defined in Section 3.3.2.1.2, and an Interface Surface as defined in Section 3.3.2.1.3.
3.3.2.1.1 Primary Coil array
The Primary Coil array consists of a 8 layer PCB. The inner six layers of the PCB each contain a grid of
Primary Coils, and the bottom layer contains the leads to each of the individual Primary Coils. The top
layer can be used for any purpose, but shall not influence the inductance values of the Primary Coils.
Figure 3-14(a) shows a top view of a single Primary Coil, which consists of a trace that runs through 18
hexagonal turns. As shown in the top inset of Figure 3-14(a), the corners of this hexagonal shape are
rounded. The bottom inset of Figure 3-14(a) shows the width of the trace as well as the distance between
two adjacent turns. Figure 3-14(b) shows a side view of the layer structure of the PCB. Copper layers 2, 3,
4, 5, 6, and 7 each contain a grid of Primary Coils. Copper layer 8 contains the leads to each of the Primary
Coils. Figure 3-14(c) provides a top view of the Primary Coil array, showing that the individual Primary
Coils are packed in a hexagonal grid. The solid hexagons show the closely packed structure of the grids of
Primary Coils on layer 2 and layer 7 of the Primary Coil array. Each solid hexagon represents a set of two
identical Primary Coils—in this case one Primary Coil on layer 2 and one Primary Coil on layer 7,
respectively—which are connected in parallel. The dashed hexagon illustrates that the grids of Primary
Coils on layer 3 and layer 6 are offset over a distance to the right, such that the centers of the Primary
Coils in layer 3 and layer 6 coincide with the corners of Primary Coils in layer 2 and layer 7. Likewise, the
dash-dotted hexagon illustrates that the grids of Primary Coils on layer 4 and layer 5 are offset over a
distance to the left, such that the centers of the Primary Coils in layer 4 and layer 5 coincide with the
corners of Primary Coils in layer 2 and layer7. As a result, the centers, respectively corners, of the Primary
Coils on layer 3 and layer 6 and the corners, respectively centers, of the Primary Coils on layer 4 and layer
5 coincide as well. See Section 3.3.2.2 for the meaning of the shaded hexagons.
Table 3-9: Primary Coil array parameters of Power Transmitter design B2
Parameter
Symbol
Value
Outer diameter
 
 mm
Track width
 
 mm
Track width plus spacing
 
 mm
Corner rounding*
 
 mm
Number of turns
 
18
Track thickness
 
 mm
Dielectric thickness 1
 

mm
Dielectric thickness 2
 
 mm
Array thickness
 
 mm
Center-to-center distance
 
31.855±0.2 mm
Offset 2nd layer array
 
18.4±0.1 mm
Offset 3rd layer array
 
18.4±0.1 mm
*Value applies to the outermost winding
 

Table 3-9 lists the relevant parameters of the Primary Coil array. The finished PCB thickness is
mm.

dwdsrc(a)
(b)
8765dCudCudCud1d1d2topdo3t2t321dh(c)
43dCudCud1d2d11da2dCud2
Figure 3-14: Primary Coil array of Power Transmitter design B2

3.3.2.1.2 Shielding
Power Transmitter design B2 employs Shielding that is identical to the Shielding of Power Transmitter
design B1. See Section 3.3.1.1.2.
3.3.2.1.3 Interface Surface
The distance from the Primary Coil array to the Interface Surface of the Base Station is
mm,
across the top face of the Primary Coil array. See also Figure 3-11 in Section 3.3.1.1.3. In addition, the
Interface Surface extends at least 5 mm beyond the outer edges of the Primary Coil array.
3.3.2.2 Electrical details
The outline of the electrical diagram of Power Transmitter design B2 follows the outline of the electrical
diagram of Power Transmitter design B1. See also Figure 3-12 in Section 3.3.1.2.
Power Transmitter design B2 uses a half-bridge inverter to drive the Primary Coil array. In addition,
Power Transmitter design B2 uses a multiplexer to select the position of the Active Area. The multiplexer
shall configure the Primary Coil array in such a way that one, two, or three sets of two Primary Coils are
connected—in parallel—to the driving circuit. The connected Primary Coils together constitute a Primary
Cell. As an additional constraint, the multiplexer shall select the Primary Coils such that each selected
Primary Coil has an overlap with every other selected Primary Coil; see Figure 3-14(c) for an example.
Within the Operating Frequency range kHz, the assembly of Primary Coil array and
Shielding has an inductance of μH for each set of Primary Coils in layer 2 and layer 7 (connected in
parallel), μH for each set of Primary Coils in layer 3 and layer 6 (connected in parallel), and
μH for each set of Primary Coils in layer 4 and 5 (connected in parallel). The capacitance and
inductance in the impedance matching circuit (Figure 3-12) are, respectively, nF
nF and μH. The capacitances and in the half-bridge inverter both are 68 μF.
The switch is open if the Primary Cell consists of a single Primary Coil; otherwise, the swich is closed.
(Informative) The voltage across the capacitance can reach levels exceeding 36 V pk-pk.
Power Transmitter design B2 uses the input voltage to the half-bridge inverter to control the amount of
power that is transferred. For this purpose, the input voltage range is 0…20 V, where a lower input
voltage results in the transfer of a lower amount of power. In order to achieve a sufficiently accurate
adjustment of the power that is transferred, a type B2 Power Transmitter shall be able to control the input
voltage with a resolution of 35 mV or better.
When a type B2 Power Transmitter first applies a Power Signal (Digital Ping; see Section 5.2.1), it shall use
an initial input voltage of 12 V.
Control of the power transfer shall proceed using the PID algorithm, which is defined in Section 5.2.3.1.
The controlled variable introduced in the definition of that algorithm represents the input voltage to
the half-bridge inverter. In order to guarantee sufficiently accurate power control, a type B2 Transmitter
shall determine the amplitude of the current into the Primary Cell (i.e. the sum of the currents through
each of its three constituent Primary Coils) with a resolution of 5 mA or better. In addition to the PID
algorithm, a type B2 Power Transmitter shall limit the current into the Primary Cell to at most 3.5 A RMS
in the case that the Primary Cell consists of two or three Primary Coils, or at most 1.75 A RMS in the case
that the Primary Cell consists of one Primary Coil. For that purpose, the Power Transmitter may limit the
input voltage to the half-bridge inverter to value that is lower than 20 V. Finally, Table 3-8 in Section
3.3.1.2 provides the values of several parameters, which are used in the PID algorithm.
3.3.2.3 Scalability
Power Transmitter Design B2 offers the same scalability options as Power Transmitter design B1. See
Section 3.3.1.3.

4 Power Receiver Design Requirements
4.1 Introduction
Figure 4-1 illustrates an example functional block diagram of a Power Receiver.

Power Pick-up UnitSecondary CoilRectification
CircuitCommunications
ModulatorOutput
DisconnectCommunications
& Control UnitLoadSensing & ControlVoltage Sense
Figure 4-1: Example functional block diagram of a Power Receiver
In this example, the Power Receiver consists of a Power Pick-up Unit and a Communications and Control
Unit. The Power Pick-up Unit on the left-hand side of Figure 4-1 comprises the analog components of the
Power Receiver:
. A dual resonant circuit consisting of a Secondary Coil plus series and parallel capacitances to
enhance the power transfer efficiency and enable a resonant detection method (see Section
4.2.2.1).
. A rectification circuit that provides full-wave rectification of the AC waveform, using e.g. four
diodes in a full-bridge configuration, or a suitable configuration of active components (see Section
4.2.2.2). The rectification circuit may perform output smoothing as well. In this example, the
rectification circuit provides power to both the Communications and Control Unit of the Power
Receiver and the output of the Power Receiver


. A communications modulator (see Section 4.2.2.4). On the DC side of the Power Receiver, the
communications modulator typically consists of a resistor in series with a switch. On the AC side
of the Power Receiver, the communications modulator typically consists of a capacitor in series
with a switch (not shown in Figure 4-1).
. An output disconnect switch, which prevents current from flowing to the output when the Power
Receiver does not provide power at its output. In addition, the output disconnect switch prevents
current back flow into the Power Receiver when the Power Receiver does not provide power at
its output. Moreover, the output disconnect switch minimizes the power that the Power Receiver
draws from the Power Transmitter when a Power Signal is first applied to the Secondary Coil.
. A rectified voltage sense.

The Communications and Control Unit on the right-hand side of Figure 4-1 comprises the digital logic part
of the Power Receiver. This unit executes the relevant power control algorithms and protocols; drives the
communications modulator; controls the output disconnect switch; and monitors several sensing circuits,
in both the Power Pick-up Unit and the load—a good example of a sensing circuit in the load is a circuit
that measures the temperature of, e.g., a rechargeable battery.
Note that this version 1.0 of the System Description Wireless Power Transfer, Volume I, Part 1, minimizes
the set of Power Receiver design requirements (see Section 4.2). Accordingly, compliant Power Receiver
designs that differ from the example functional block diagram shown in Figure 4-1 are possible. For
example, an alternative design includes post-regulation of the output of the rectification circuit (e.g., using
a buck converter, battery charging circuit, power management unit, etc.). In yet another design, the
Communications and Control Unit interfaces with other subsystems of the Mobile Device, e.g. for user
interface purposes.
4.2 Power Receiver design requirements
The design of a Power Receiver shall comply with the mechanical requirements listed in Section 4.2.1 and
the electrical requirements listed in Section 4.2.2. In addition, a Power Receiver shall implement the
relevant parts of the protocols defined in Section 5, as well as the communications interface defined in
Section 6.
4.2.1 Mechanical requirements
A Power Receiver design shall include a Secondary Coil, and an Interface Surface as defined in Section
4.2.1.1. In addition, a Power Receiver design shall include an alignment aid as defined in Section 4.2.1.2.
4.2.1.1 Interface Surface
The distance from the Secondary Coil to the Interface Surface of the Mobile Device shall not exceed
mm, across the bottom face of the Secondary Coil. See Figure 4-2.

dzInterface
SurfaceSecondary
CoilShielding
(optional)
Mobile
Device
Figure 4-2: Secondary Coil assembly

4.2.1.2 Alignment aid
The design of a Mobile Device shall include means that helps a user to properly align the Secondary Coil of
its Power Receiver to the Primary Coil of a Power Transmitter that enables Guided Positioning. This
means shall provide the user with directional guidance—i.e. where to the user should move the Mobile
Device—as well as alignment indication—i.e. feedback that the user has reached a properly aligned
position.3
3The design requirements of the Mobile Device to determine the range of lateral displacements that
constitute proper alignment.
4The switch shall remain closed even if no power is available from the Secondary Coil.
(Informative) An example of such means is a piece of hard or soft magnetic material, which is attracted to
the magnet that is provided in Power Transmitter design A1. The attractive force should provide the user
with tactile feedback, when placing the Mobile Device on the Interface Surface. Note that the Mobile Device
cannot rely on the presence of any alignment support from the Base Station, other than the alignment aids
specified in Section 3.
4.2.1.3 Shielding
An important consideration for a Power Receiver designer is the impact of the Power Transmitter’s
magnetic field on the Mobile Device. Stray magnetic fields could interact with the Mobile Device and
potentially cause its intended functionality to deteriorate, or cause its temperature to increase due to the
power dissipation of generated eddy currents.
It is recommended to limit the impact of magnetic fields by means of Shielding on the top face of the
Secondary Coil. See also Figure 4-2. This Shielding should consist of material that has parameters similar
to the materials listed in Sections 3.2.1.1.2 and 3.3.1.1.2. The Shielding should cover the Secondary Coil
completely. Additional Shielding beyond the outer diameter of the Secondary Coil might be necessary
depending upon the impact of stray magnetic fields.
The example Power Receiver designs discussed in Annex A.1 and Annex A.2 both include Shielding.
4.2.2 Electrical requirements
A Receiver design shall include a dual resonant circuit as defined in Section 4.2.2.1, a rectification circuit
as defined in Section 4.2.2.2, sensing circuits as defined in Section 4.2.2.3, a communications modulator as
defined in Section 4.2.2.4, and an output disconnect switch as defined in Section 4.2.2.5.
4.2.2.1 Dual resonant circuit
The dual resonant circuit of the Power Receiver comprises the Secondary Coil and two resonant
capacitances. The purpose of the first resonant capacitance is to enhance the power transfer efficiency.
The purpose of the second resonant capacitance is to enable a resonant detection method. Figure 4-3
illustrates the dual resonant circuit. The switch in the dual resonant circuit is optional. If the switch is not
present, the capacitance shall have a fixed connection to the Secondary Coil . If the switch is present,
it shall remain closed4 until the Power Receiver transmits its first Packet (see Section 5.3.1).

CSCdLS
Figure 4-3: Dual resonant circuit of a Power Receiver

The dual resonant circuit shall have the following resonant frequencies:

In these equations,
is the self inductance of the Secondary Coil when placed on the Interface Surface of a
Power Transmitter and—if necessary—aligned to the Primary Cell; and is the self inductance of the
Secondary Coil without magnetically active material that is not part of the Power Receiver design close to
the Secondary Coil (e.g. away from the Interface Surface of a Power Transmitter). Moreover, the
tolerances and on the resonant frequency are for Power Receivers that specify a
Maximum Power value in the Configuration Packet of 3 W and above, and and for all
other Power Receivers. The quality factor Q of the loop consisting of the Secondary Coil, switch (if
present), resonant capacitance and resonant capacitance , shall exceed the value 77. Here the quality
factor Q is defined as:

with the DC resistance of the loop with the capacitances and short-circuited.
Figure 4-4 shows the environment that is used to determine the self-inductance
of the Secondary Coil.
The primary Shielding shown in Figure 4-4 consists of material PC44 from TDK Corp. The primary
Shielding has a square shape with a side of 50 mm and a thickness of 1 mm. The center of the Secondary
Coil and the center of the primary Shielding shall be aligned. The distance from the Receiver Interface
Surface to the primary Shielding is mm. Shielding on top of the Secondary Coil is present only if
the Receiver design includes such Shielding. Other Mobile Device components that influence the
inductance of the Secondary Coil shall be present as well when determining the resonant frequencies—the
magnetic attractor shown in Figure 4-4 is example of such a component. The excitation signal that is used
to determine and
shall have an amplitude of 1 V RMS and a frequency of 100 kHz.

dzInterface
SurfaceSecondary CoilShielding (optional)
Mobile
DevicePrimary ShieldingSpacerMagnetic
Attractor
(example)
Figure 4-4: Characterization of resonant frequencies
4.2.2.2 Rectification circuit
The rectification circuit shall use full-wave rectification to convert the AC waveform to a DC power level.
4.2.2.3 Sensing circuits
The Power Receiver shall monitor the DC voltage directly at the output of the rectification circuit.

4.2.2.4 Communications modulator
The Power Receiver shall have the means to modulate the Primary Cell current and Primary Cell voltage
as defined in Section 6.2.1.5 This version 1.0 of the System Description Wireless Power Transfer, Volume
I, Part 1, leaves the specific loading method as a design choice to the Power Receiver. Typical example
methods include modulation of a resistive load on the DC side of the Power Receiver, and modulation of a
capacitive load on the AC side of the Power Receiver.
5(Informative) Note that the dual resonant circuit as depicted in Figure 4-3 does not prohibit
implementation of the communications modulator directly at the Secondary Coil.
4.2.2.5 Output disconnect
The Power Receiver shall have the means to disconnect its output from the subsystems connected thereto.
If the Power Receiver has disconnected its output, it shall ensure that it still draws a sufficient amount of
power from the Power Transmitter, such that Power Receiver to Power Transmitter communications
remain possible (see also Section 6.2.1).
The Power Receiver shall keep its output disconnected until it reaches the power transfer phase for the
first time after a Digital Ping (see also Section 5). Subsequently, the Power Receiver may operate the
output disconnect switch any time while the Power Transmitter applies a Power Signal. This also means
that the Power Receiver may keep its output connected if it reverts from the power transfer phase to the
identification & configuration phase.
(Informative) Note that the Power Receiver may experience a voltage peak when operating the output
disconnect switch (and changing between maximum and near-zero power dissipation).

This page is intentionally left blank.

5 System Control
5.1 Introduction
From a system control perspective, power transfer from a Power Transmitter to a Power Receiver
comprises four phases, namely selection, ping, identification & configuration, and power transfer. Figure
5-1 illustrates the relation between the phases. The solid arrows indicate transitions, which the Power
Transmitter initiates; and the dash-dotted arrows indicate transitions that the Power Receiver initiates.
By definition, if the Power Transmitter is not applying a Power Signal, the sytem is in the selection phase.
This means that a transition from any of the other phases to the selection phase involves the Power
Transmitter removing the Power Signal.

pingidentification &
configurationpower transferextend Digital PingPower Transfer Contract establishedPower transfer completeno Power Transfer Contractunexpected Packettransmission errortime-outReconfigurePower Transfer Contract violationunexpected Packettime-outapply Power Signalselectionno responseabort Digital Pingpower transfer complete
Figure 5-1: Power transfer phases
The main activity in each of these phases is the following:
. selection In this phase, the Power Transmitter typically monitors the Interface Surface for the
placement and removal of objects. The Power Transmitter may use a variety of methods for this
purpose. See Annex B for some examples. If the Power Transmitter discovers one or more
objects, it should attempt to locate those objects—in particular if it supports Free Positioning. In
addition, the Power Transmitter may attempt to differentiate between Power Receivers and
foreign objects—keys, coins, etc. Moreover, the Power Transmitter should attempt to select a
Power Receiver for power transfer. If initially the Power Transmitter does not have sufficient
information for these purposes, the Power Transmitter may repeatedly proceed to the ping and
subsequently to the identification & configuration phases—each time selecting a different Primary
Cell—and revert to the selection phase after collecting relevant information. See Annex C for
examples. Finally, if the Power Transmitter selects a Primary Cell, which it intends to use for


power transfer to a Power Receiver, the Power Transmitter proceeds to the ping phase—and
eventually to the power transfer phase. On the other hand, if the Power Transmitter does not
select a Power Receiver for power transfer—and is not actively providing power to a Power
Receiver for an extended amount of time—the Power Transmitter should enter a stand-by mode
of operation.6 See [Part 2] for performance requirements on such a mode of operation.
. ping In this phase, the Power Transmitter executes a Digital Ping, and listens for a response. If
the Power Transmitter discovers a Power Receiver, the Power Transmitter may extend the Digital
Ping, i.e. maintain the Power Signal at the level of the Digital Ping. This causes the system to
proceed to the identification & configuration phase. If the Power Transmitter does not extend the
Digital Ping, the system shall revert to the selection phase.
. identification & configuration In this phase, the Power Transmitter identifies the selected Power
Receiver, and obtains configuration information such as the maximum amount of power that the
Power Receiver intends to provide at its output. The Power Transmitter uses this information to
create a Power Transfer Contract. This Power Transfer Contract contains limits for several
parameters that characterize the power transfer in the power transfer phase. At any time before
proceeding to the power transfer phase, the Power Transmitter may decide to terminate the
extended Digital Ping—e.g. to discover additional Power Receivers. This reverts the system to the
selection phase.
. power transfer In this phase, the Power Transmitter continues to provide power to the Power
Receiver, adjusting its Primary Cell current in response to control data that it receives from the
Power Receiver. Throughout this phase, the Power Transmitter monitors the parameters that are
contained in the Power Transfer Contract. A violation of any of the stated limits on any of those
parameters causes the Power Transmitter to abort the power transfer—returning the system to
the selection phase. Finally, the system may also leave the power transfer phase on request of the
Power Receiver. For example, the Power Receiver can request to terminate the power transfer—
battery fully charged—reverting the system to the selection phase, or request to renegotiate the
Power Transfer Contract—change to trickle charging the battery using a lower maximum amount
of power—reverting the system to the identification & configuration phase.

6Note that it is up to the Power Transmitter implementation to determine whether this stand-by mode of
operation is part of the selection phase or is separate from the selection phase.
Section 5.2 defines the system control protocols in the ping, identification & configuration, and power
transfer phases from a Power Transmitter perspective. Section 5.3 defines the system control protocols in
these four phases from a Power Receiver perspective. Note that this version 1.0 of the System
Description Wireless Power Transfer, Volume I, Part 1, does not define the system control protocol in the
selection phase. Further note that—from a power transfer point of view—the Power Receiver remains
passive throughout most of the selection phase.
At any time a user can remove a Mobile Device that is receiving power. The Power Transmitter can
recognize such an event from a time-out in the communications from the Power Receiver, or from a
violation of the Power Transfer Contract. As a result, the Power Transmitter aborts the power transfer
and the system reverts to the selection phase.
Throughout the power transfer phase, the Power Transmitter and Power Receiver control the amount of
power that is transferred. The Figure 5-2 illustrates a schematic diagram of the power transfer control
loop, which basically operates as follows: The Power Receiver selects a desired Control Point—a desired
output current and/or voltage, a temperature measured somewhere in the Mobile Device, etc. In addition,
the Power Receiver determines its actual Control Point. Note that the Power Receiver may use any
approach to determine a Control Point. Moreover, the Power Receiver may change this approach at any
time during the power transfer phase. Using the desired Control Point and actual Control Point, the Power
Receiver calculates a Control Error Value—for example simply taking the (relative) difference of the two
output voltages or currents—such that the result is negative if the Power Receiver requires less power in
order to reach its desired Control Point, and positive if the Power Receiver requires more power in order
to reach its desired Control Point. Subsequently, the Power Receiver transmits this Control Error Value to
the Power Transmitter.

The Power Transmitter uses the Control Error Value and the actual Primary Cell current to determine a
new Primary Cell current. After the system stabilizes from the communications of the Control Error
Packet, the Power Transmitter has a short time window to control its actual Primary Cell current towards
the new Primary Cell current. Within this window, the Power Transmitter reaches a new Operating
Point—the amplitude, frequency, and duty cycle of the AC voltage that is applied to the PrimaryCell.
Subsequently, the Power Transmitter keeps its Operating Point fixed in order to enable the Power
Receiver to communicate additional control and status information. See Section 5.2.3.1 for details.

Control towards
new Primary Cell
currentDetermine new
Primary Cell
currentPower Conversion
UnitControl Error
PacketCalculateControl Error
ValueDetermine actual
Control PointPower Pick-up
UnitDetermine actual
Primary Cell
currentSelect desired
Control PointPower ReceiverPower TransmitterSet new Operating
Point
Figure 5-2: Power transfer control loop

5.2 Power Transmitter perspective
Section 5.2.1 defines the protocol that the Power Transmitter shall execute in order to select a Power
Receiver for power transfer. This protocol comprises a Digital Ping. Section 5.2.2 defines the protocol
that the Power Transmitter shall execute in order to identify the Power Receiver and establish a Power
Transfer Contract. This protocol extends the Digital Ping, in order to enable the Power Receiver to
communicate the necessary information. Section 5.2.3 defines the protocol that the Power Transmitter
shall execute after is has established the Power Transfer Contract. During execution of this protocol, the
Power Transmitter controls its Primary Cell current in response to control data that it receives from the
Power Receiver.
5.2.1 Ping phase
In the ping phase, the Power Transmitter shall execute a Digital Ping. This Digital Ping shall proceed as
follows, with conditions appearing earlier in this list take precedence over conditions appearing later:
. The Power Transmitter shall apply a Power Signal at the Operating Point as defined for the
particular Power Transmitter design (see Section 3), and attempt to receive an incoming Packet.
. If the Power Transmitter does not detect the start bit of the header byte of the first incoming
Packet within ms of first applying the Power Signal, the Power Transmitter shall remove the
Power Signal (i.e. reduce the Primary Cell current to zero) within ms. See Figure 5-3(a).
. If the Power Transmitter correctly receives a Signal Strength Packet, the Power Transmitter may
proceed to the identification & configuration phase of the power transfer, maintaining the Power
Signal at the Operating Point as defined for the particular Power Transmitter design. See Figure
5-3(b). If the Power Transmitter does not proceed to the identification & configuration phase, the
Power Transmitter shall remove the Power Signal within ms after receiving the stop bit of
the Signal Strength Packet’s checksum byte. See Figure 5-3(c).
. If the Power Transmitter does not correctly receive (see Section 6.2.4) the first Packet within
ms after detecting the start bit of the first incoming Packet, the Power Transmitter shall
remove the Power Signal within ms. See Figure 5-3(d).
. If the Power Transmitter correctly receives any other Packet than a Signal Strength Packet, and in
particular if the Power Transmitter receives an End Power Transfer Packet, the Power
Transmitter shall remove the Power Signal within ms after receiving the stop bit of the
Packet’s checksum byte. See Figure 5-3(e).

If the Power Transmitter does not proceed to the identification & configuration phase, the Power
Transmitter shall revert to the selection phase.
Note that the thick line in Figure 5-3 represents the amplitude of the Power Signal, which is zero at the
left-hand side of the diagrams. The dashed line represents possible communications from the Power
Receiver, which the Power Transmitter shall ignore—as follows from the above conditions.
Table 5-1: Power Transmitter timing in the ping phase
Parameter
Symbol
Value
Unit
Maximum Digital Ping duration
 
65
ms
Power Signal termination time
 
28
ms
First Packet time out
 
17
ms
Power Signal expiration time
 
28
ms
 
 

 tpingtterminate(a)
tpingtfirsttterminate(d)
tpingtfirsttexpire(c)
Signal Strengthtpingtfirsttterminate(e)
tpingtfirst(b)
Signal Strength
Figure 5-3: Power Transmitter timing in the ping phase
5.2.2 Identification & configuration phase
In the identification & configuration phase, the Power Transmitter shall identify the Power Receiver and
collect configuration information. For this purpose, the Power Transmitter shall correctly receive the
following sequence of Packets, in the order shown, and without changing its Operating Point:
. If the Power Transmitter enters the identification & configuration phase from the ping phase, an
Identification Packet.
. If the Ext bit of the preceding Identification Packet is set to ONE, an Extended Identification
Packet.
. Up to 7 optional configuration Packets from the following set (the order in which the Power
Transmitter receives these Packets, if any, is not relevant):
o A Power Control Hold-off Packet. If the Power Transmitter receives multiple Power
Control Hold-off Packets, the Power Transmitter shall retain the Power Control Hold-off
Time contained in the last Power Control Hold-off Packet received (see below).
o Any Proprietary Packet (as listed in Table 6-3). If the Power Transmitter does not know
how to handle the message contained in the Proprietary Packet, the Power Transmitter
shall ignore that message.
 


o Any reserved Packet (as indicated in Table 6-3). The Power Transmitter shall ignore the
message contained in the reserved Packet.

. A Configuration Packet. If the number of optional configuration Packets, which the Power
Transmitter has received, is not equal to the value contained in the Count field of the
Configuration Packet, the Power Transmitter shall remove the Power Signal within ms
after receiving the stop bit of the Configuration Packet’s checksum byte, and return to the
selection phase.

The Power Transmitter shall receive the above sequence of Packets subject to the following timing
constraints:
. If the Power Transmitter does not detect the start bit of the header byte of a next Packet in the
sequence within ms after receiving the stop bit of the checksum byte of the directly preceding
Packet in the sequence, the Power Transmitter shall remove the Power Signal within ms. See
Figure 5-4(a). In this context, the directly preceding Packet of the Identification Packet is the Signal
Strength Packet, which the Power Transmitter has received in the ping phase. In addition, if the
Power Transmitter has entered the identification & configuration phase from the power transfer
phase, the directly preceding Packet of the first Packet in the sequence—either the Configuration
Packet if the sequence does not contain optional configuration Packets, or the first optional
configuration Packet—is the End Power Transfer Packet, which the Power Transmitter has received
in the power transfer phase.
. If the Power Transmitter does not correctly receive a Packet in the sequence within ms after
receiving the start bit of the header byte of that Packet, the Power Transmitter shall remove the
Power Signal within ms. See Figure 5-4(b).
. If the Power Transmitter correctly receives a next Packet that does not comply with the above
sequence, the Power Transmitter shall remove the Power Signal within ms after receiving the
stop bit of that Packet’s checksum byte. See Figure 5-4(c).

In addition to these timing constraints, if the Power Transmitter does not receive a Packet correctly (see
Section 6.2.4), the Power Transmitter shall remove the Power Signal within ms after detecting the
error.
After the Power Transmitter has received the Configuration Packet, the Power Transmitter shall execute
the following steps, in the order shown:
. If the relation
is not satisfied, the Power Transmitter shall revert to the
selection phase. Moreover, if the Power Transmitter reverts to the selection phase, the Power
Transmitter shall remove the Power Signal within ms after receiving the stop bit of the
Configuration Packet’s checksum byte. If the Power Transmitter has not received a Power Control
Hold-off Packet, the Power Transmitter shall proceed to use
.
. If the Power Transmitter has correctly received all Packets in the sequence (see Figure 5-4(d)),
the Power Transmitter may create a Power Transfer Contract. See below.
. If the Power Transmitter has created a Power Transfer Contract, the Power Transmitter may
proceed to the power transfer phase. If the Power Transmitter does not proceed to the power
transfer phase, the Power Transmitter shall remove the Power Signal within ms after
receiving the stop bit of the Configuration Packet’s checksum byte. See Figure 5-4(e)
. If the Power Transmitter has removed the Power Signal—and does not proceed to the power
transfer phase—the Power Transmitter shall revert to the selection phase.


Table 5-2: Power Transmitter timing in the identification & configuration phase
Parameter
Symbol
Value
Unit
Next Packet time out
 
20
ms
Maximum Packet length
 
170
ms
 
Table 5-3: Power control hold-off time
Parameter
Symbol
Value
Unit
Power Control Hold-off Time
 

5
ms
Power Control Hold-off Time
 

205
ms
 
 

tnextConfigurationPreceding Packet(e)
texpiretterminate(a)
tnextPreceding PackettnextNext PacketPreceding Packet(b)
tterminatetmaxtnextPreceding Packet(c)
tterminatetmaxtnextNext PacketPreceding Packet(d)
tmaxConfigurationtmax
Figure 5-4: Power Transmitter timing in the identification & configuration phase

Based on the configuration information received from the Power Receiver, the Power Transmitter can
create a Power Transfer Contract. This version 1.0 of the System Description Wireless Power Transfer,
Volume I, Part 1, does not define the parameters that comprise a Power Transfer Contract. However, it is
recommended that the Power Transfer Contract contains at least the following parameters:
. The maximum power that the Power Receiver intends to provide at its output (as obtained from
the Maximum Power field of the Configuration Packet).

5.2.3 Power transfer phase
In the power transfer phase, the Power Transmitter controls the power transfer to the Power Receiver, in
response to control data that it receives from the latter. For this purpose, the Power Transmitter shall
receive zero or more of the following Packets:
. Control Error Packet.
. Rectified Power Packet.
. Charge Status Packet.
. End Power Transfer Packet.
. Any Proprietary Packet (as listed in Table 6-3). If the Power Transmitter does not know how to
handle the message contained in the Proprietary Packet, the Power Transmitter shall ignore that
message.
. Any reserved Packet (as indicated in Table 6-3). The Power Transmitter shall ignore the message
contained in the reserved Packet.

The Power Transmitter shall receive the above Packets subject to the following timing constraints:
. If the Power Transmitter does not correctly receive the first Control Error Packet within
ms after receiving the stop bit of the checksum byte of the Configuration Packet, which the
Power Transmitter has received in the identification & configuration phase, the Power
Transmitter shall remove the Power Signal within ms. If the Power Transmitter does not
correctly receive a Control Error Packet within ms after receiving the stop bit of the
checksum byte of the preceding Control Error Packet, the Power Transmitter shall remove the
Power Signal within ms. See Figure 5-5(a).
. If the Power Transmitter does not correctly receive the first Rectified Power Packet within s
after receiving the stop bit of the checksum byte of the Configuration Packet, which the Power
Transmitter has received in the identification & configuration phase, the Power Transmitter shall
remove the Power Signal within ms. If the Power Transmitter does not correctly receive
a Rectified Power Packet within s after receiving the stop bit of the preceding Rectified
Power Packet, the Power Transmitter shall remove the Power Signal within ms. See
Figure 5-5 (f).

In addition to the above timing constraints, the Power Transmitter shall execute the following actions:
. Upon receiving a Control Error Value, the Power Transmitter shall make adjustments to its
Operating Point for at most ms, as defined in Section 5.2.3.1. Prior to making any
adjustment, the Power Transmitter shall wait for ms to enable the Primary Cell current to
stabilize again after communications. See Figure 5-5 (b).
. If the Power Transmitter correctly receives a Packet that does not comply with the above
sequence, the Power Transmitter shall remove the Power Signal within ms after receiving
the stop bit of that Packet’s checksum byte. See Figure 5-5 (c).
. If the Power Transmitter receives an End Power Transfer Packet, the Power Transmitter shall:
o Revert to the identification & configuration phase without changing its Operating Point, if
the End Power Transfer Code is 0x07 (reconfigure). See Figure 5-5 (d).
o Remove the Power Signal within ms after receiving the stop bit of the End Power
Transfer Packet’s checksum byte, if the End Power Transfer Code has any other value
than 0x07. See Figure 5-5 (e).
 


. The Power Transmitter shall monitor the parameters contained in the Power Transfer Contract
throughout the power transfer phase. If the Power Transmitter detects that the actual value of
any of those parameters exceeds the limits contained in the Power Transfer Contract, the Power
Transmitter shall remove the Power Signal within ms.
. If the Power Transmitter has removed the Power Signal, the Power Transmitter shall revert to the
selection phase.

 
(a)
ttimeouttterminate(b)
ttimeoutNext Control Errortactivetdelay(c)
tterminate(d)
End Power Transfer(e)
End Power TransfertterminatePreceding Control ErrorPreceding Control ErrortdelayPreceding Control ErrortdelayPreceding Control ErrortdelayPreceding Control Errortactivetactivetactive(f)
tpowertterminatePreceding Rectified Powertsettletsettletsettletsettle
Figure 5-5: Power Transmitter timing in the power transfer phase

Table 5-4: Power Transmitter timing in the power transfer phase
Parameter
Symbol
Value
Unit
Control Error Packet time out
 
1250
ms
Power control active time
 
20
ms
Power control settling time
 
3
ms
Rectified Power Packet time
 
30
s
 
5.2.3.1 Power transfer control
This version 1.0 of the System Description Wireless Power Transfer, Volume I, Part 1, defines a specific
method, which the Power Transmitter shall use to control its Primary Cell current towards the new
Primary Cell current (see also Section 5.1). This method is based on a discrete proportional-integral-
differential (PID) algorithm as illustrated in Figure 5-6.

Power
Conversion UnitControl Error
MessageΣ+
+
+
TransmitterΣ+
-
.. ..,..
d
..
a
.. 1
.. ..,.. 1
p .. ..,..
..,..
..,.. 1
..,.. ..,.. 1 +
i .. ..,.. inner
d
.. ..,.. .. ..,.. 1
inner
.. ..,..
.. ..,..
..,.. 1
v .. ..,..
..,..
..,.. 1
a
..,..
a
..,.. 1
a
.. 1 1+
.. ..
128

Figure 5-6: PID control algorithm
To execute this algorithm, the Power Transmitter shall execute the steps listed below, in the order of
appearance. In the definitions of these steps, the index .. labels the sequence of Control Error
Packets, which the Power Transmitter receives.
. Upon receipt of the .. Control Error Packet, the Power Transmitter shall calculate the new
Primary Cell current
as

 
..

where
represents the actual Primary Cell current—reached in response to the previous
Control Error Packet—and .. represents the Control Error Value contained in the .. Control
Error Packet. Note that
represents the Primary Cell current at the start of the power transfer
phase.

. If the Control Error Value .. is non-zero, the Power Transmitter shall make adjustments to its
Primary Cell current for ms. For this purpose, the Power Transmitter shall execute a loop
comprising of the steps listed below. The index .. .. labels the iterations of this loop.
o The Power Transmitter shall calculate the difference between the new Primary Cell and
the actual Primary Cell current as the error
 

..

Where
represents the Primary Cell current determined in iteration .. of the
loop. Note that
represents the actual Primary Cell current at the start of the loop.
o The Transmitter shall calculate the proportional, integral, and derivative terms (in any
order):
 

 ..
..
..
.. ..

where is the proportional gain, is the integral gain, is the derivative gain, and
is the time required to execute a single iteration of the loop. In addition, the integral
term , and the error .. . The Power Transmitter shall limit the integral
term such that it remains within the range —if necessary, the Power
Transmitter shall replace the calculated integral term with the appropriate boundary
value.
o The Power Transmitter shall calculate the sum of the proportional, integral, and
derivative terms:
 

 .. ..
In this calculation, the Power Transmitter shall limit the sum .. such that it remains
within the range – .
o The Power Transmitter shall calculate the new value of the controlled variable
 

 ..
where is a scaling factor that depends on the controlled variable. In addition, the
controlled variable , with representing the actual value of the
controlled variable at the start of the power transfer phase. The controlled variable is
either the Operating Frequency, the duty cycle of the inverter, or the voltage input to the
inverter. If the calculated exceeds the specified range (see the definition of the
individual Power Transmitter designs in Section 3), the Power Transmitter shall replace
the calculated with the appropriate limiting value.
o The Power Transmitter shall apply the new value of the controlled variable to its
Power Conversion Unit.
o The Power Transmitter shall determine the actual Primary Cell current
.
 

The maximum number of iterations of the loop .. , and the time required to execute a
single iteration of the loop shall satisfy the following relation:
..
. The Power Transmitter shall determine the Primary Cell current
exactly
ms after receiving the stop bit of the checksum byte of the .. Control Error Packet.

See the definition of the individual Power Transmitter designs in Section 3 for the values of , , , ,
and .

5.3 Power Receiver perspective
Section 5.3.1 defines the initial response of the Power Receiver to the application of a Power Signal. As
part of this initial response, the Power Receiver wakes up its Communications and Control Unit—if that is
not already up and running. Section 5.3.2 defines the response of a Power Receiver to a Digital Ping. This
response ensures the Power Transmitter that it is dealing with a Power Receiver (rather than some
unknown object). Section 5.3.3 defines the response of a Power Receiver to an extended Digital Ping. This
response enables the Power Transmitter to identify the Power Receiver and establish a Power Transfer
Contract. Finally, Section 5.3.4 defines the protocol that the Power Receiver shall execute in order to
control the power transfer from the Power Transmitter.
In addition to the timing constraints given in Sections 5.3.1, 5.3.2, 5.3.3, and 5.3.4, the Power Receiver shall
leave the ping, identification & communication, or power transfer phase at most ms (see Table 5-5)
after the Power Transmitter removes the Power Signal. Note that this version 1.0 of the System
Description Wireless Power Transfer, Volume I, Part 1, does not define how the Power Receiver should
detect that the Power Transmitter removes the Power Signal.
Table 5-5: Power Receiver timing in any phase
Parameter
Symbol
Value
Unit
Power Receiver reset time
 
28
ms
 
Moreover, notwithstanding the timing constraints given in Sections 5.3.1, 5.3.2, 5.3.3, and 5.3.4, the Power
Receiver may stop transmitting Packets to the Power Transmitter at any time. (Informative) This
behavior causes the Power Transmitter to remove the Power Signal, possibly under the assumption that a
user has removed the Power Receiver from the Interface Surface. The recommended behavior to cause the
Power Transmitter to remove the Power Signal (when a user has not removed the Power Receiver from the
Interface Surface) is to transmit an End Power Transfer Packet as defined in Sections 5.3.2 and 5.3.4.
5.3.1 Selection phase
As soon as the Power Transmitter applies a Power Signal, the Power Receiver shall enter the selection
phase.7 Note that this version 1.0 of the System Description Wireless Power Transfer, Volume I, Part 1,
does not define how the Power Receiver should detect that the Power Transmitter applies a Power Signal.
If the Power Receiver considers the rectified voltage to be sufficiently high, the Power Receiver shall
proceed to the ping phase subject to the following timing constraints:
7If the Power Receiver is not in the selection phase already. Note that if the Power Receiver needs time to
start up its Communications and Control Unit, the Power Receiver shall consider itself to be in the
selection phase during that start-up time. In general, the Power Receiver may consider itself to be in the
selection phase whenever it is neither in the ping phase, nor in the identification & configuration phase, nor
in the power transfer phase.
. The Power Receiver shall not proceed to the ping phase until the Power Transmitter has
continuously applied the Power Signal for at least
ms.
. The Power Receiver shall proceed to the ping phase at the latest
ms after the Power
Transmitter has first applied the Power Signal.

If the Power Receiver does not proceed to the ping phase, the Power Receiver shall not transmit any
Packet.
See Figure 5-7 and Table 5-6, where
.

 twakeFirst Packet
Figure 5-7: Power Receiver timing in the selection phase
Table 5-6: Power Receiver timing in the selection phase
Parameter
Symbol
Value
Unit
Wake up time (early)
 

15
ms
Wake up time (late)
 

58
ms
 
5.3.2 Ping phase
If the Power Receiver responds to te Digital Ping, the Power Receiver shall transmit either a Signal
Strength Packet, or an End Power Transfer Packet as its first Packet. The Power Receiver shall transmit
this first Packet immediately upon entering the ping phase.

First Packet
Figure 5-8: Power Receiver timing in the ping phase
After the Power Receiver has transmitted a Signal Strength Packet, the Power Receiver shall proceed to
the identification & configuration phase. After the Power Receiver has transmitted an End Power Transfer
Packet, shall remain in the ping phase. In that case, the Power Receiver should transmit additional End
Power Transfer Packets.8
8The Power Transmitter can miss the first End Power Transfer Packet, e.g. due to a communications error,
and continue to apply the Power Signal.
5.3.3 Identification & configuration phase
In the identification & configuration phase, the Power Receiver shall transmit the following sequence of
Packets:
. If the Power Receiver enters the identification & configuration phase from the ping phase, an
Identification Packet.
. If the Ext bit of the preceding Identification Packet is set to ONE, an Extended Identification
Packet.
. Up to 7 optional configuration Packets from the following set (the order in which the Power
Receiver transmits these Packets, if any, is not relevant):
o A Power Control Hold-off Packet. The Power Control Hold-off Time contained in
this Packet shall satisfy the relation
. See Table 5-3.
o Any Proprietary Packet (as listed in Table 6-3).

. A Configuration Packet.

The Power Receiver shall transmit the above sequence of Packets subject to the following timing
constraints:
. The Power Receiver shall not start to transmit the preamble of the next Packet in the sequence
within ms after transmitting the stop bit of the checksum byte of the directly preceding
Packet in the sequence.


. The Power Receiver shall start to transmit the start bit of the next Packet in the sequence at the
latest ms after transmitting the stop bit of the checksum byte of the directly preceding Packet
in the sequence.

With respect to the above timing constraints, if the Power Receiver has entered the identification &
configuration phase from the ping phase, the directly preceding Packet of the Identification Packet is the
Signal Strength Packet, which the Power Receiver has transmitted in the ping phase. In addition, if the
Power Receiver has entered the identification & configuration phase from the power transfer phase, the
directly preceding Packet of the first Packet in the sequence—either the Configuration Packet if the
sequence does not contain optional configuration Packets, or the first optional configuration Packet—is
the End Power Transfer Packet, which the Power Receiver has transmitted in the power transfer phase.
See Figure 5-9 and Table 5-7.
After the Power Receiver has transmitted a Configuration Packet, the Power Receiver shall proceed to the
power transfer phase.

tnextNext PacketPreceding Packettsilent
Figure 5-9: Power Receiver timing in the identification & configuration phase
Table 5-7: Power Receiver timing in the identification & configuration phase
Parameter
Symbol
Value
Unit
Silent time out
 
7
ms
Next Packet time out
 
20
ms
 
5.3.4 Power transfer phase
In the power transfer phase, the Power Receiver controls the power transfer from the Power Transmitter,
by means of control data that it transmits to the latter. For this purpose, the Power Receiver shall
transmit zero or more of the following Packets:
. Control Error Packet. The Power Receiver shall set the Control Error Value to zero if the actual
Control Point is equal to the desired Control Point. The Power Receiver shall set the Control Error
Value to a negative value to request a decrease of the Primary Cell current. The Power Receiver
shall set the Control Error Value to a positive value to request an increase of the Primary Cell
current. See also Sections 5.1 and 5.2.3.1.
. Rectified Power Packet.
. Charge Status Packet.
. End Power Transfer Packet.
. Any Proprietary Packet (as listed in Table 6-3).

The Power Receiver shall transmit the above Packets subject to the following timing constraints:
. The Power Receiver shall not start to transmit the preamble of any Packet within ms after
transmitting the stop bit of the checksum byte the directly preceding Packet. As an additional
constraint, the Power Receiver shall not start to transmit the preamble of any Packet within
ms after transmitting the stop bit of the checksum byte of a Control Error Packet,
where the Power Control Hold-off value, which the Power Receiver has transmitted using
the last Power Control Hold-off Packet in the identification & configuration phase. If the Power


Receiver has not transmitted a Power Control Hold-off Packet to the Power Transmitter, the
Power Receiver shall use
(see Table 5-3).
. The Power Receiver shall start to transmit the start bit of the first Control Error Packet at the
latest ms after transmitting the stop bit of the checksum byte of the Configuration Packet
in the sequence, which the Power Receiver has transmitted in the identification & configuration
phase. The Power Receiver shall transmit the start bit of the header byte of a next Control Error
Packet within ms after transmitting the stop bit of the checksum byte of the preceding
Control Error Packet.
. It is recommended that the Power Receiver determines its actual Control Point ms
after transmitting the stop bit of the checksum byte of a Control Error Packet.
. The Power Receiver shall start to transmit the start bit of the header byte of the first Rectified
Power Packet within s after transmitting the stop bit of the checksum byte of the
Configuration Packet, which the Power Receiver has transmitted in the identification &
configuration phase. The Power Receiver shall transmit the start bit of the header byte of a next
Rectified Power Packet within s after transmitting the stop bit of the checksum byte of the
preceding Rectified Power Packet.

See Figure 5-10 and Table 5-8.
In addition to the above timing constraints, if the Power Receiver has transmitted an End Power Transfer
Packet, which contains an End Power Transfer Code of 0x07, the Power Receiver shall revert to the
identification & configuration phase. Moreover, if the Power Receiver has transmitted an End Power
Transfer Packet, which contains any other End Power Transfer Code, the Power Receiver shall remain in
the power transfer phase (and therefore shall continue to transmit Control Error Packets), until the Power
Transmitter removes the Power Signal. Furthermore, the Power Receiver should transmit additional End
Power Transfer Packets if the Power Transmitter does not remove the Power Signal.9
9(Informative) The Power Transmitter can miss the first and possibly subsequent End Power Transfer
Packets, e.g. due to communications errors, and continue to apply the Power Signal. However, eventually the
Power Transmitter should remove the Power Signal due to a time-out as defined in Section 5.2.3.

tintervaltcontrolControl ErrorNext Control Errortdelay(a)
trectifiedRectified PowerNext Rectified Power(b)
Figure 5-10: Power Receiver timing in the power transfer phase
Table 5-8: Power Receiver timing in the power transfer phase
Parameter
Symbol
Value
Unit
Interval
 
250
ms
Controller time*
 
23
ms
Rectified Power Packet time
 
5
s
*Note that (see Table 5-4)
 

This page is intentionally left blank.

6 Communications Interface
6.1 Introduction
The Power Receiver communicates to the Power Transmitter using backscatter modulation. For this
purpose, the Power Receiver modulates the amount of power, which it draws from the Power Signal. The
Power Transmitter detects this as a modulation of the current through and/or voltage across the Primary
Cell. In other words, the Power Receiver and Power Transmitter use an amplitude modulated Power
Signal to provide a Power Receiver to Power Transmitter communications channel.
6.2 Physical and data link layers
This Section 6.2 defines both the physical layer and the data link layer of the communications interface.
6.2.1 Modulation scheme
The Power Receiver shall modulate the amount of power, which it draws from the Power Signal, such that
the Primary Cell current and/or Primary Cell voltage assume two states, namely a HI state and a LO
state.10 A state is characterized in that the amplitude is constant within a certain variation Δ for at least
ms. If the Power Receiver is properly aligned to the Primary Cell of a type A1 Power Transmitter, and
for all appropriate loads, at least one of the following three conditions shall apply:11
10(Informative) Note that the HI and LO states do not correspond to fixed Primary Cell current and/or
Primary Cell voltage levels.
11The design requirements of the Mobile Device determine both the range of lateral displacements that
constitute proper alignment, and the range of loading conditions on its Power Receiver.
12The start of the cycle corresponds the closing of the top switch in the half-bridge inverter.
. The difference of the amplitude of the Primary Cell current in the HI and LO state is at least
15 mA.
. The difference of the Primary Cell current, as measured at instants in time that correspond to one
quarter of the cycle of the control signal driving the half-bridge inverter (see Figure 3-4),12 in the
HI and LO state is at least 15 mA.
. The difference of the amplitude of the Primary Cell voltage in the HI and LO state is at least
200 mV.

During a transition the Primary Cell current and Primary Cell voltage are undefined. See Figure 6-1 and
Table 6-1.

LO StateHI StatePrimary Cell CurrentPrimary Cell VoltagetTHI StateLO State100%
ModulationDepthtTtStStStSΔΔ
Figure 6-1: Amplitude modulation of the Power Signal

Table 6-1: Amplitude modulation of the Power Signal
Parameter
Symbol
Value
Unit
Maximum transition time
 
100
 s
Minimum stable time
 
150
 s
Current amplitude variation
 
8
mA
Voltage amplitude variation
 
110
mV
 
6.2.2 Bit encoding scheme
The Power Receiver shall use a differential bi-phase encoding scheme to modulate data bits onto the
Power Signal. For this purpose, the Power Receiver shall align each data bit to a full period tCLK of an
internal clock signal, such that the start of a data bit coincides with the rising edge of the clock signal. This
internal clock signal shall have a frequency kHz.
The Receiver shall encode a ONE bit using two transitions in the Power Signal, such that the first
transition coincides with the rising edge of the clock signal, and the second transition coincides with the
falling edge of the clock signal. The Receiver shall encode a ZERO bit using a single transition in the Power
Signal, which coincides with the rising edge of the clock signal. Figure 6-2 shows an example.
 ZEROZEROZEROZEROONEONEONEONEtCLK
Figure 6-2: Example of the differential bi-phase encoding
6.2.3 Byte encoding scheme
The Power Receiver shall use an 11-bit asynchronous serial format to transmit a data byte. This format
consists of a start bit, the 8 data bits of the byte, a parity bit, and a single stop bit. The start bit is a ZERO.
The order of the data bits is lsb first. The parity bit is odd. This means that the Power Receiver shall set
the parity bit to ONE if the data byte contains an even number of ONE bits. Otherwise, the Power Receiver
shall set the parity bit to ZERO. The stop bit is a ONE. Figure 6-3 shows the data byte format—including
the differential bi-phase encoding of each individual bit—using the value 0x35 as an example.
 b0StartParityStopb1b2b3b4b5b6b7
Figure 6-3: Example of the asynchronous serial format
6.2.4 Packet structure
The Power Receiver shall communicate to the Power Transmitter using Packets. As shown in Figure 6-4, a
Packet consists of 4 parts, namely a preamble, a header, a message, and a checksum. The preamble
consists of a minimum of 11 and a maximum of 25 bits, all set to ONE, and encoded as defined in Section
6.2.2. The preamble enables the Power Transmitter to synchronize with the incoming data and accurately
detect the start bit of the header.

The header, message, and checksum consist of a sequence of three or more bytes encoded as defined in
Section 6.2.3.13
13The Power Receiver should turn off its communications modulator after transmitting a Packet. This may
cause an additional HI state to LO state or LO state to HI state transition in the Primary Cell current.

PreambleHeaderMessageChecksum
Figure 6-4: Packet format
The Power Transmitter shall consider a Packet as received correctly if:
. The Power Transmitter has detected at least 4 preamble bits that are followed by a start bit.
. The Power Transmitter has not detected a parity error in any of the bytes that comprise the
Packet. This includes the header byte, the message bytes and the checksum byte.
. The Power Transmitter has detected the stop bit of the checksum byte.
. The Power Transmitter has determined that the checksum byte is consistent (see Section 6.2.4.3).

If the Power Transmitter does not receive a Packet correctly, the Power Transmitter shall discard the
Packet, and not use any of the information contained therein. (Informative) In the ping phase as well as in
the identification and configuration phase, this typically leads to a time-out, which causes the Power
Transmitter to remove the Power Signal.
6.2.4.1 Header
The header consists of a single byte that indicates the Packet type. In addition, the header implicitly
provides the size of the message contained in the Packet. The number of bytes in a message is calculated
from the value contained in the header of the Packet, as shown in the center column of Table 6-2.
Table 6-2: Message size
Header
Message Size*
Comment
0x00…0x1F
1 + (Header – 0) / 32
1 .32 messages (size 1)
0x20…0x7F
2 + (Header – 32) / 16
6 . 16 messages (size 2…7)
0x80…0xDF
8 + (Header – 128) / 8
12 . 8 messages (size 8…19)
0xE0…0xFF
20 + (Header – 224) / 4
8 . 4 messages (size 20…27)
*Values in this column are truncated to an integer
 
Table 6-3 lists the Packet types defined in this version 1.0 of the System Description Wireless Power
Transfer, Volume I, Part 1. The formats of the messages contained in each of these Packet types are
defined in Section 6.3. The format of the messages contained in Packet types, which are listed as
Proprietary, is implementation dependent. Header values that are not listed in Table 6-3 are reserved.
The Power Receiver shall not transmit Packets that have one of the reserved values as the header.

Table 6-3: Packet types
Header*
Packet Types
Message Size
ping phase
0x01
Signal Strength
1
identification & configuration phase
0x06
Power Control Hold-off
1
0x51
Configuration
5
0x71
Identification
7
0x81
Extended Identification
8
power transfer phase
0x02
End Power Transfer
1
0x03
Control Error
1
0x04
Rectified Power
1
0x05
Charge Status
1
identification & configuration / power transfer phase
0x18
Proprietary
1
0x19
Proprietary
1
0x28
Proprietary
2
0x29
Proprietary
2
0x38
Proprietary
3
0x48
Proprietary
4
0x58
Proprietary
5
0x68
Proprietary
6
0x78
Proprietary
7
0x84
Proprietary
8
0xA4
Proprietary
12
0xC4
Proprietary
16
0xE2
Proprietary
20
0xF2
Proprietary
24
*Header values not listed in this table correspond to reserved Packet types
 
6.2.4.2 Message
The Power Receiver shall ensure that the message contained in the Packet is consistent with the Packet
type indicated in the header. See Section 6.3 for a detailed definition of the possible messages. The first
byte of the message, byte B0, directly follows the header.
6.2.4.3 Checksum
The checksum consists of a single byte, which enables the Power Transmitter to check for transmission
errors. The Power Transmitter shall calculate the checksum as follows:

 ,
where C represents the calculated checksum, H represents the header byte, and B0, B1,…, Blast represent the
message bytes.
If the calculated checksum and the checksum byte contained in the Packet are not equal, the Power
Transmitter shall determine that the checksum is inconsistent.
6.3 Logical layer
This Section 6.3 defines the format of the messages of the communications interface.
6.3.1 Signal Strength Packet (0x01)
Table 6-4 defines the format of the message contained in a Signal Strength Packet
Table 6-4: Signal Strength
 
b7
b6
b5
b4
b3
b2
b1
b0
B0
Signal Strength Value
 
Signal Strength Value The unsigned integer value in this field indicates the degree of coupling between
the Primary Cell and Secondary Coil, with the purpose to enable Power Transmitters that use Free
Positioning to determine the Primary Cell that provides optimum power transfer (see also Annex C). To
determine the degree of coupling, the Power Receiver shall monitor the value of a suitable variable during
a Digital Ping. Examples of such variables are:
. The rectified voltage.
. The open circuit voltage (as measured at the output disconnect switch).
. The received Power (if the rectified voltage is actively or passively clamped during a Digital Ping).

The variable that is chosen shall result in a Signal Strength Value that increases monotonically with
increasing degree of coupling. The Signal Strength Value is reported as

where is the monitored variable, and is the maximum value, which the Power Receiver expects for
that variable during a Digital Ping. Note that the Power Receiver shall set the Signal Strength Value to 255
in the case that .

6.3.2 End Power Transfer Packet (0x02)
Table 6-3 defines the format of the message contained in an End Power Transfer Packet.
Table 6-5: End Power Transfer
 
b7
b6
b5
b4
b3
b2
b1
b0
B0
End Power Transfer Code
 
End Power Transfer Code This field identifies the reason for the End Power Transfer request, as listed
in Table 6-6. The Power Receiver shall not transmit End Power Transfer Packets that contain any of the
values that Table 6-6 lists as reserved.
Table 6-6: End Power Transfer values
Reason
Value
Unknown
0x00
Charge Complete
0x01
Internal Fault
0x02
Over Temperature
0x03
Over Voltage
0x04
Over Current
0x05
Battery Failure
0x06
Reconfigure
0x07
No Response
0x08
Reserved
0x09…0xFF
 
(Informative) It is recommended that the Receiver uses the End Power Transfer values listed in Table 6-6 as
follows:
. 0x00 The Receiver may use this value if it does not have a specific reason for terminating the power
transfer, or if none of the other values listed in Table 6-6 is appropriate.
. 0x01 The Receiver should use this value if it determines that the battery of the Mobile Device is fully
charged. On receipt of an End Power Transfer Packet containing this value, the Transmitter should
set any “charged” indication on its user interface that is associated with the Receiver.
. 0x02 The Receiver may use this value if it has encountered some internal problem, e.g. a software or
logic error.
. 0x03 The Receiver should use this value if it has measured a temperature within the Mobile Device
that exceeds a limit.
. 0x04 The Receiver should use this value if it has measured a voltage within the Mobile Device that
exceeds a limit.
. 0x05 The Receiver should use this value if it has measured a current within the Mobile Device that
exceeds a limit.
. 0x06 The Receiver should use this value if it has determined a problem with the battery of the
Mobile Device.
. 0x07 The Receiver should use this value if it desires to renegotiate a Power Transfer Contract.


. 0x08 The Receiver should use this value if it determines that the Transmitter does not respond to
Control Error Packets as expected (i.e. does not increase/decrease its Primary Cell current
appropriately).

6.3.3 Control Error Packet (0x03)
Table 6-7 defines the format of the message contained in a Control Error Packet.
Table 6-7: Control Error
 
b7
b6
b5
b4
b3
b2
b1
b0
B0
Control Error Value
 
Control Error Value The (two’s complement) signed integer value contained in this field ranges between
–128…+127 (inclusive), and provides input to the Operating Point controller of the Power Transmitter.
See Sections 5.2.3.1 and 5.3.4 for more details. Values outside the indicated range are reserved and shall
not appear in a Control Error Packet.
6.3.4 Rectified Power Packet (0x04)
Table 6-8 defines the format of the message contained in a Rectified Power Packet.
Table 6-8: Rectified Power
 
b7
b6
b5
b4
b3
b2
b1
b0
B0
Rectified Power Value
 
Rectified Power Value The unsigned integer contained in this field contains the amount of power that
the Power Receiver is providing at the output of the rectifier, expressed as a percentage of the Maximum
Power (see Section 6.3.7). For clarity, the value 0 means that the Power Receiver does not provide power
at the output of the rectifier, and the value 100 means that the Power Receiver provides an amount of
power that is equal to the requested Maximum Power. (Informative) It is not an error for the Rectified
Power Value field to contain a value greater than 100. However, this could result in a removal of the Power
Signal.
6.3.5 Charge Status Packet (0x05)
Table 6-9 defines the format of the message contained in a Charge Status Packet.
Table 6-9: Charge Status
 
b7
b6
b5
b4
b3
b2
b1
b0
B0
Charge Status Value
 
Charge Status Value If the Mobile Device contains a rechargeable energy storage device, the unsigned
integer contained in this field indicates the charging level of that energy storage device, as a percentage of
the fully charged level. For clarity, the value 0 means an empty energy storage device, and the value 100
means a fully charged energy storage device. If the Mobile Device does not contain a rechargeable energy
storage device, or if the Power Receiver cannot provide charge status information,14 this field shall contain
the value 0xFF. All other values are reserved and shall not appear in the Charge Status Packet.
14Note that the Charge Status Packet is optional, which means that the Power Receiver may elect not to
send the Charge Status Packet.
6.3.6 Power Control Hold-off Packet (0x06)
Table 6-8 defines the format of the message contained in a Power Control Hold-off Packet.

Table 6-10: Power control hold-off
 
b7
b6
b5
b4
b3
b2
b1
b0
B0
Power Control Hold-off Time
 
Power Control Hold-off Time The unsigned integer contained in this field contains the amount of time
in milliseconds, which the Power Transmitter shall wait prior to making adjustments to the Primary Cell
current after receipt of a Control Error Packet.
6.3.7 Configuration Packet (0x51)
Table 6-11 defines the format of the message contained in a Configuration Packet.
Table 6-11: Configuration
 
b7
b6
b5
b4
b3
b2
b1
b0
B0
Power Class
Maximum Power
B1
Reserved
B2
Prop
Reserved
Count
B3
Reserved
B4
Reserved
 
Power Class This field contains an unsigned integer value that indicates the Power Receiver’s Power
Class. Power Receivers that comply with this version 1.0 of the System Description Wireless Power
Transfer, Volume I, Part 1, shall set this field to 0.
Maximum Power Apart from a scaling factor, the unsigned integer value contained in this field indicates
the maximum amount of power, which the Power Receiver expects to provide at the output of the rectifier.
This maximum amount of power is calculated as follows:

Prop If this bit is set to ZERO, the Power Transmitter shall control the power transfer according to the
method defined in Section 5.2.3.1. If this bit is set to ONE, the Power Transmitter may control the power
transfer according to a proprietary method instead of the method defined in Section 5.2.3.1. However, if
this bit is set to ONE, the Power Transmitter shall continue to ensure that the received Control Error
Packets comply with the timings defined in Section 5.2.3. (Informative) This implies that a Power
Transmitter terminates the power transfer if it times out when waiting for a Control Error Packet. Moreover,
this implies that setting the Prop bit to ONE does not relieve the Power Receiver from transmitting Control
Error Packets on a regular basis. Finally, if the Prop bit is set to ZERO, the Power Transmitter could still
decide to abort the power transfer based on information contained in a Proprietary Packet.
Reserved These bits shall be set to ZERO.
Count This field contains an unsigned integer value that indicates the number of optional configuration
Packets that the Power Receiver transmits in the identification & configuration phase.
6.3.8 Identification Packet (0x71)
Table 6-12 defines the format of the message contained in an Identification Packet.

Table 6-12: Identification
 
b7
b6
b5
b4
b3
b2
b1
b0
B0
Major Version
Minor Version
B1
(msb)
Manufacturer Code
 
B2
 
(lsb)
B3
Ext
(msb)
 
 
Basic Device Identifier
B6
 
(lsb)
 
Major Version The combination of this field and the Minor Version field identifies to which revision of
the System Description Wireless Power Transfer the Power Receiver complies. The Major Version field
shall contain the binary coded digit value 0x1.
Minor Version The combination of this field and the Major Version field identifies to which minor
revision of the System Description Wireless Power Transfer the Power Receiver complies. The Minor
Version field shall contain the binary coded digit value 0x0.
Manufacturer Code The bit string contained in this field identifies the manufacturer of the Power
Receiver, as specified in [PRMC].
Ext If this bit is set to ZERO, the bit string
Manufacturer Code || Basic Device Identifier
identifies the Power Receiver. If this bit is set to ONE, the bit string
Manufacturer Code || Basic Device Identifier || Extended Device Identifier
identifies the Power Receiver (see also Section 6.3.9).
Basic Device Identifier The bit string contained in this field contributes to the identification of the
Power Receiver. A Power Receiver manufacturer should ensure that the combination of Basic Device
Identifier and Manufacturer ID is sufficiently unique. Embedding a serial number of at least 20 bits in the
Basic Device Identifier is sufficient. Alternatively, using a (pseudo) random number generator to
dynamically generate part of the Basic Device Identifier is sufficient as well, provided that the generated
part complies with the following requirements:
. The generated part shall comprise at least 20 bits.
. All possible values shall occur with equal probability.
. The Power Receiver shall not change the generated part while the Power Signal is applied.
. The Power Receiver shall retain the generated part for at least 2 s if the Power Signal is
interrupted or removed.

(Informative) These requirements ensure that the scanning procedure of a type B1 Power Transmitter
proceeds correctly; see also Annex C.2.

6.3.9 Extended Identification Packet (0x81)
Table 6-13 defines the format of the message contained in an Extended Identification Packet.
Table 6-13: Extended Identification
 
b7
b6
b5
b4
b3
b2
b1
b0
B0
(msb)
 
 
Extended Device Identifier
B7
 
(lsb)
 
Extended Device Identifier The bit string contained in this field contributes to the identification of the
Power Receiver. See Section 6.3.8

Annex A Example Power Receiver Designs (Informative)
A.1 Power Receiver example 1
The design of Power Receiver example 1 is optimized to directly charge a single cell lithium-ion battery at
constant current or voltage.
A.1.1 Mechanical details
This Section A.1.1 provides the mechanical details of Power Receiver example 1.
A.1.1.1 Secondary Coil
The Secondary Coil of Receiver example 1 is of the wire-wound type, and consists of no. 26 AWG (0.41 mm
diameter) litz wire having 26 strands of no. 40 AWG (0.08 mm diameter). As shown in Figure A-1, the
Secondary Coil has a rectangular shape and consists of a single layer. Table A-1 lists the dimensions of the
Secondary Coil.

dcdoldildowdiw
Figure A-1: Secondary Coil of Power Receiver example 1
Table A-1: Secondary Coil parameters of Power Receiver example 1
Parameter
Symbol
Value
Outer length
 
 mm
Inner length
 
 mm
Outer width
 
 mm
Inner width
 
 mm
Thickness
 
 mm
Number of turns per layer
 
14
Number of layers
1
 
A.1.1.2 Shielding
As shown in Figure A-2, Power Receiver example 1 employs Shielding. This Shielding has a size of
mm2, and has a thickness of mm. The Shielding is centered directly on the
top face of the Secondary Coil (such that the long side of the Secondary Coil and the Shielding are aligned).
The composition of the Shielding consists of any choice from the following list of materials:
. Material 44 — Fair Rite Corporation.
. Material 28 — Steward, Inc.
. CMG22G — Ceramic Magnetics, Inc.


 dcdzInterface
SurfaceSecondary
CoilShieldingMobile
Devicedol, dowdl, dwMagnet
Figure A-2: Secondary Coil and Shielding assembly of Power Receiver example 1
A.1.1.3 Interface Surface
The distance from the Secondary Coil to the Interface Surface of the Mobile Device is mm,
uniform across the bottom face of the Secondary Coil.
A.1.1.4 Alignment aid
Power Receiver example 1 employs a bonded Neodymium magnet, which has its south pole oriented
towards the Interface Surface. The diameter of the magnet is 15 mm, and its thickness is 1.2 mm.
A.1.2 Electrical details
At the secondary resonance frequency kHz, the assembly of Secondary Coil, Shielding and
magnet has inductance values μH and
μH. The capacitance values in the dual
resonant circuit are nF and nF.
As shown in Figure A-3, the rectification circuit consists of four diodes in a full bridge configuration and a
low-pass filtering capacitance μF.
The communications modulator consists of two equal capacitances nF in series with two
switches. The resistance value kΩ.
The subsystem connected to the output of Power Receiver example 1 is expected to consist of a single cell
lithium-ion battery. This Power Receiver example 1 controls the output current and output voltage into
the battery according to the common constant current to constant voltage charging profile. An example
profile is indicated in Figure A-4. The maximum output power to the battery is controlled to a 5 W level.

LSCSCCd+Li-ion
BatteryCcmCcmR
Figure A-3: Electrical details of Power Receiver example 1

 
Figure A-4: Li-ion battery charging profile

A.2 Power Receiver example 2
The design of Power Receiver example 2 uses post-regulation to create a voltage source at the output of
the Power Receiver.
A.2.1 Mechanical details
This Section A.2.1 provides the mechanical details of Power Receiver example 2.
A.2.1.1 Secondary Coil
The Secondary Coil of Power Receiver example 2 is of the wire-wound type, and consists of litz wire
having 24 strands of no. 40 AWG (0.08 mm diameter). As shown in Figure A-5, the Secondary Coil has a
circular shape and consists of multiple layers. All layers are stacked with the same polarity. Table A-2
lists the dimensions of the Secondary Coil.

dcdodi
Figure A-5: Secondary Coil of Power Receiver example 2
Table A-2: Parameters of the Secondary Coil of Power Receiver example 2
Parameter
Symbol
Value
Outer diameter
 
 mm
Inner diameter
 
 mm
Thickness
 
 mm
Number of turns per layer
 
9
Number of layers
2
 
A.2.1.2 Shielding
As shown in Figure A-6, Power Receiver example 2 employs Shielding. The Shielding has a size of
mm2, and is centered directly on the top face of the Secondary Coil. The Shielding
has a thickness of mm and consists of any choice from the materials from the following list:
. Material 78 — Fair Rite Corporation.
. 3C94 — Ferroxcube.
. N87 — Epcos AG.
. PC44 —TDK Corp.


 dsdcdzInterface
SurfaceSecondary
CoilShieldingMobile
Devicedodl, dwMagneticAttractor
Figure A-6: Secondary Coil and Shielding assembly of Power Receiver example 2
A.2.1.3 Interface Surface
The distance from the Secondary Coil to the Interface Surface of the Mobile Device is mm, uniform
across the bottom face of the Secondary Coil.
A.2.1.4 Alignment aid
Power Receiver example 2 employs Shielding material (see Annex A.2.1.2) as an alignment aid (see
Section 4.2.1.2). The diameter of the this Shielding material is 10 mm, and its thickness is 0.8 mm.
A.2.2 Electrical details
At the secondary resonance frequency kHz, the assembly of Secondary Coil and Shielding has an
inductance values μH and
μH. The capacitance values in the dual resonant circuit
are nF and nF.
As shown in Figure A-7, the rectification circuit consists of four diodes in a full bridge configuration and a
low-pass filtering capacitance μF.
The communications modulator consists of a Ω resistance in series with a switch.
The buck converter comprises the post-regulation stage of Power Receiver example 2. The Control and
Communications Unit of the Power Receiver can disable the buck converter. This provides the output
disconnect functionality. In addition, the Control and Communications Unit controls the input voltage to
the buck converter, such that V.
The buck converter has a constant output voltage of 5 V and an output current

,
Where is the output power of the buck converter, and is the power dependent efficiency of the
buck converter.

 RCMLSCSCCDBuck
ConverterVR
Figure A-7: Electrical details of Power Receiver example 2

Annex B Object Detection (Informative)
A Power Transmitter may use a variety of methods to efficiently discover and locate objects on the
Interface Surface. These methods, also known as “analog ping,” do not involve waking up the Power
Receiver and starting digital communications. Typically zero or more analog pings precede the Digital
Ping, which the Power Transmitter executes in the first power transfer phase. This Annex B provides some
analog ping examples.
B.1 Resonance shift
This analog ping method is based on a shift of the Power Transmitter’s resonance frequency, due to the
presence of a (magnetically active) object on the Interface Surface.
For a type A1 Power Transmitter, this method proceeds as follows: The Power Transmitter applies a very
short pulse to its Primary Coil, at an Operating Frequency , which corresponds to the resonance
frequency of the Primary Coil and series resonant capacitance (in case there is no object present on the
Interface Surface). This results in a Primary Coil current . The measured value depends on whether or
not an object is present within the Active Area. It is highest if the resonance frequency has not shifted due
to the presence of an object. Accordingly, if is below a threshold value , the Power Transmitter can
conclude that an object is present. Note that the values of and are implementation dependent.
The Power Transmitter can apply the pulses at regular intervals and have , where each pulse has a
duration of at most μs. Measurement of the Primary Coil current should occur at most μs after
the pulse. See also Figure B-1 and Table B-1.

toddtodmtodiIodIodttimecurrent
Figure B-1: Analog ping based on a resonance shift
Table B-1: Analog ping based on a resonance shift
Parameter
Symbol
Value
Unit
Object detection interval
 
500
ms
Object detection duration
 
70
μs
Object detection measurement
 
19.5
μs
 
For type B1 and B2 Power Transmitters, this method proceeds as follows: The Power Transmitter applies
a very short pulse to a set of Primary Coils, which the multiplexer has connected in parallel—note that this
set is not necessarily limited to a Primary Cell. The Operating Frequency of the pulse corresponds to
the resonance frequency of the set of Primary Coils and the capacitance of the impedance matching circuit
(in case there is no object present on the Interface Surface). This results in a current through the
inductance of the impedance matching circuit. The measured value depends on whether or not an object
is present within the Active Area. It is lowest if the resonance frequency has not shifted due to the
presence of an object. Accordingly, if is above a threshold value , the Power Transmitter can
conclude that an object is present. Note that the values of and are implementation dependent.

The Power Transmitter can apply the pulses at regular intervals , where each pulse has a duration of at
most μs. Measurement of the current should occur at most μs after the pulse. See also Figure
B-1 and Table B-1.
B.2 Capacitance change
This analog ping method is based on a change of the capacitance of an electrode on or near the Interface
Surface, due to the placement of an object on the Interface Surface.
This method is particularly suitable for Power Transmitters that use Free Positioning, because it enables
implementations that have a very low stand-by power, and yet exhibit an acceptable response time to a
user. The reason is that (continuously) scanning the Interface Surface for changes in the arrangement of
objects and Power Receivers thereon is a relatively costly operation. In contrast, sensing changes in the
capacitance of an electrode can be very cheap (in terms of power requirements). Note that capacitance
sensing can proceed with substantial parts of the Base Station powered down.
Power Transmitters designs that are based on an array of Primary Coils can use the array of Primary Coils
as the electrode in question. For that purpose, the multiplexer should connect all (or a relevant subset of)
Primary Coils in the array to a capacitance sensing unit—and at the same time disconnect the Primary
Coils from the driving circuit. Power Transmitter designs that are based on a moving Primary Coil can use
the detection coils on the Interface Surface (see Annex C.3) as electrodes.
It is recommended that the capacitance sensing circuit is able to detect changes with a rsolution of 100 fF
or better. If the sensed capacitance change exceeds some implementation defined threshold, the Power
Transmitter can conclude that an object is place onto or removed from the Interface Surface. In that case,
the Power Transmitter should proceed to localize the objects and attempt to identify the Power Receivers
on the Interface Surface, e.g. as discussed in Annex C.

Annex C Power Receiver Localization (Informative)
This Annex C discusses several aspects that relate to the discovery of Power Receivers amongst the
objects that the Power Transmitter has discovered on its Interface Surface.
C.1 Guided Positioning
In the case of Guided Positioning, discovery and localization of a Power Receiver is straightforward: The
Power Transmitter should simply execute a Digital Ping, as defined in Section 5.2.1. If the Power
Transmitter receives a Signal Strength Packet or an End Power Transfer Packet, it has discovered and
located a Power Receiver. Otherwise, the object is not a Power Receiver.
C.2 Primary Coil array based Free Positioning
In the case of Free Positioning, discovery and localization of a Power Receiver is less straightforward. This
Annex C.2 discusses one example approach, which is particularly suited to a Primary Coil array based
Power Transmitter. In this approach, the Power Transmitter first discovers and locates the objects that
are present on its Interface Surface (e.g. using any of the methods discussed in Annex B). This results in a
set of Primary Cells, which represents the locations of potential Power Receivers. For each of the Primary
Cells in this set, the Power Transmitter executes a Digital Ping (Section 5.2.1), removing the Power Signal
after receipt of a Signal Strength Packet (or an End Power Transfer Packet, or after a time out).15 This
yields a new set of Primary Cells, namely those which report a Signal Strength Value that exceeds a certain
threshold—which the Power Transmitter chooses. Finally, the Power Transmitter executes an extended
Digital Ping (Sections 5.2.1 and 5.2.2) for each of the Primary Cells in this new set in order to identify the
discovered Power Receivers. In order to select the most appropriate Primary Cells for power transfer
from the set, the Power Transmitter should take the situations discussed in Annex C.2.1, C.2.2, and C.2.3
into account.
15Note that the Power Transmitter should ensure that after terminating a Digital Ping using a particular
Primary Cell, it waits sufficiently long—for example (see Table 5-5 in Section 5.3)—prior to executing
a Digital Ping to that same Primary Cell or any of its neighboring Primary Cells. This ensures that any
Power Receiver that is present on the Interface Surface at the location of the Primary Cell can return to a
well-defined state.
C.2.1 A single Power Receiver covering multiple Primary Cells
Figure C-1 shows a situation in which the final set contains 12 Primary Cells. In order to select the most
appropriate Primary Cell from this set, the Power Transmitter compares all Basic Device Identifiers that is
has obtained. In this case, these are all identical. Accordingly, the Power Transmitter concludes that all
Primary Cells in the set correspond to one and the same Power Receiver. Therefore, the Power
Transmitter selects the Primary Cell that has the highest Signal Strength Value as the most appropriate
Primary Cell to use for power transfer. In the specific example shown in Figure C-1, this could be Primary
Cell 2, 3, 4, 5, 8, 9, 10, or 11.

 121049511378612
Figure C-1: Single Power Receiver covering multiple Primary Cells
C.2.2 Two Power Receivers covering two adjacent Primary Cells
Figure C-2 shows a situation in which the final set contains 12 Primary Cells—the same set as in the
situation discussed in Annex C.2.1. In order to select the most appropriate Primary Cell from this set, the
Power Transmitter compares all Basic Device Identifiers that is has obtained. In this case, the Power
Transmitter determines that there are two subsets of identical Basic Device Identifiers. Accordingly, the
Power Transmitter concludes that it is dealing with two distinct Power Receivers. Therefore, the Power
Transmitter selects the most appropriate Primary Cell from each subset. In the specific example shown in
Figure C-2, this could be Primary Cell 2, or 8 for the left-hand Power Receiver, and Primary Cell 5, or 11 for
the right-hand Power Receiver. Note that due to interference, the Power Transmitter most likely cannot
communicate reliably using Primary Cells 3, 4, 9, and 10.

121049511378612
Figure C-2: Two Power Receivers covering two adjacent Primary Cells
C.2.3 Two Power Receivers covering a single Primary Cell
Figure C-3 shows a situation in which the final set contains 2 Primary Cells. Here, the underlying
assumption is that the two Power Receivers have widely different response times ( , see Section 5.3.1)
to a Digital Ping. For example, the left-hand Power Receiver responds very fast (close to
), whereas
the right-hand Power Receiver responds very slow (close to
). This enables the Power Transmitter to
receive the Signal Strength Packet from the fast Power Receiver, but not from the slow one. However, the
Power Transmitter cannot reliably receive any further communications—from either Power Receiver—
due to collisions between transmissions from the two Power Receivers. Accordingly, the Power
Transmitter cannot select a Primary Cell for power transfer.

 21
Figure C-3: Two Power Receivers covering a single Primary Cell
C.3 Moving Primary Coil based Free Positioning
In the case of moving Primary Coil based Free Positioning, typically a special Detection Unit provides
discovery and localization of a Power Receiver. This Annex C.3 discusses an example of such a Detection
Unit, which makes use of the resonance in the Power Receiver at the detection frequency . In this
example Detection Unit, detection coils are printed on the Interface Surface of the Base Station. The top
right-hand part of Figure C-4 shows a single rectangular detection coil, which consists of two windings.
The width of the detection coil is 22 mm, and its length depends on the size of the Interface Surface. As
shown in the bottom part of Figure C-4, a first set of these detection coils is laid out in parallel to cover the
entire Interface Surface, in such a way that that the areas of two adjacent detection coils overlap for 60%.
A second set of these detection coils is laid out similarly, but orthogonal to the detection coils in the first
set.

 30 mm0.2 mm0.2 mm0.2 mm375 ns3.75 ms5.0 V22 mm8.8 mm
Figure C-4: Detection Coil
Detection of a Power Receiver proceeds as follows: In first instance, the Power Transmitter uses the
detection coils as an electrostatic sensor to detect the placement or removal of objects on the Interface
Surface; see Annex B.2. Once the Power Transmitter has detected an object, it uses the detection coils to
determine the position of that object on the Interface Surface. For this purpose, the Power Transmitter
applies a short pulse train to each of the detection coils—one by one. This pulse train consists of 8 pulses,
and is shaped to trigger the resonance in the Power Receiver at the frequency . See the top left-hand
part of Figure C-4. As a result, a minute amount of energy is transferred to the resonant circuit in the
Power Receiver. Immediately after the pulse train terminates, this energy is re-radiated, which the Power
Transmitter can detect using the detection coils. By analyzing the responses from each of the detection
coils, the Power Transmitter can determine the location of the Power Receiver on the Interface Surface.
Subsequently, the Power Transmitter can move its coil underneath the Power Receiver, and can start to
transfer power as defined in Section 5. During power transfer, the Power Transmitter can adjust the
position of the Primary Coil in order to optimize its coupling to the Secondary Coil, e.g. by maximizing the
system efficiency—the Power Transmitter can calculate the system efficiency from its input power and
the Actual Power Value contained in the Actual Power Packets, which it receives from the Power Receiver.
An advantage of this detection method is that it is not sensitive to foreign object that do not exhibit a
resonance near the detection frequency . The reason is that such objects do not store and re-radiate
energy picked up from the pulse train. As a result a Power Transmitter does not need to move the
Primary Coil to attempt power transfer to such objects.

Annex D Metal Object Detection (Informative)
When metallic objects are exposed to an alternating magnetic field, eddy currents cause such objects to
heat up. The amount of heating depends on the amplitude and frequency of the magnetic field, as well as
on the characteristics of the object such, as its resistivity, size, and shape. In a wireless power transfer
system, such heating is undesired as it manifests itself as a power loss, and therefore a reduced power
tranfer efficiency. Moreover, if no appropriate measures are taken, such heating could lead to unsafe
situations if the heated objects would reach high temperatures. This Annex D discusses the recommended
mechanism for Power Transmitters and Power Receivers to deal with the power loss due to parasitic
metals—i.e. metals that are neither part of the Power Transmitter, nor of the Power Receiver, but which
are dissipating power from the magnetic field during power transfer. Examples of such parasitic metals
are coins, keys, paperclips, etc.
It is recommended that a Power Receiver performs metal object detection by monitoring the temperature
near its Interface Surface. If the measured temperature exceeds an internal safety threshold, the Power
Receiver should terminate power transfer by communicating an End Power Transfer Packet—with End
Power Transfer Code set to 0x03—to the Power Transmitter.
It is recommended that a Power Transmitter monitors the temperature near its Interface Surface. If the
measured temperature exceeds an internal safety threshold, the Power Transmitter should terminate the
power transfer. In addition, it is recommended that a Power Transmitter estimates the power that is
transmitted through its Interface Surface, and monitors the Rectified Power Values that are communicated
by the Power Receiver. If the combination of the estimate and these values indicate an unexpected power
loss, the Power Transmitter should terminate the power transfer.

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