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Power Management
Texas Instruments Incorporated
An introduction to the Wireless Power
Consortium standard and TI’s
compliant solutions
By Bill Johns
Senior Applications Engineer
Introduction
Wireless power is beginning to show great potential in the
consumer market. The ability to power an electronic
device without the use of wires provides a convenient
solution for the users of portable devices and also gives
designers the ability to develop more creative answers to
problems. This technology’s benefits can be seen in the
many portable devices, from cell phones to electric cars,
that normally operate on battery power.
Inductive coupling is the method by which efficient and
versatile wireless power can be achieved. For ease of use
and the benefit of both designers and consumers, the
Wireless Power Consortium (WPC) has developed a
standard (see Reference 1) that creates interoperability
between the device providing power (power transmitter,
charging station) and the device receiving power (power
receiver,portabledevice).Establishedin2008,theWPCis
a group of Asian, European, and American companies in
diverse industries, including electronics manufacturers
andoriginalequipmentmanufacturers(OEMs).TheWPC
standard defines the type of inductive coupling (coil con-
figuration) and the communications protocol to be used
for low-power wireless devices. Any device operating
under this standard will be able to pair with any other
WPC-compliant device. One key benefit to this approach is
that it makes use of the coils for communications between
the power transmitter and the power receiver. See Figure 1
for a typical application diagram.
WPC standard for wireless power
UndertheWPCstandard,“lowpower”forwirelesstransfer
means a draw of 0 to 5 W. Systems that fall within the
scope of this standard are those that use inductive coupling
between two planar coils to transfer power from the power
transmitter to the power receiver. The distance between
the two coils is typically 5 mm. Regulation of the output
voltage is provided by a global digital control loop where
the power receiver communicates with the power trans-
mitter and requests more or less power. Communication is
unidirectional from the power receiver to the power trans-
mitter via backscatter modulation. In backscatter modula-
tion, the power-receiver coil is loaded, changing the current
draw at the power transmitter. These current changes are
monitored and demodulated into the information required
for the two devices to work together.
The WPC standard defines the three key areas of the
system—the power transmitter that will supply power, the
power receiver that will use the power, and the communi-
cations protocol between the two devices. These three
areas are explored next.
Figure 1. Typical wireless-power functional diagram
Transmitter
Receiver
Power
Load
(Phone or
Battery)
Voltage
Conditioning
AC to DC
Drivers
Rectification
Communication
V/I
Sense
Controller
Controller
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Texas Instruments Incorporated
Power Management
Power transmitter
The direction of power transfer is always from the power
transmitter to the power receiver. The key circuits of the
power transmitter are the primary coil, used to transfer
power to the power-receiver coil; the control unit for
driving the primary coil; and the communications circuit
for demodulating the voltage or current from the primary
coil. Flexibility of the power-transmitter design is limited
to provide consistent power and voltage levels to the
power receiver.
The power receiver identifies itself to the power trans-
mitter as a compliant device and also provides configura-
tion information. Once the transmitter initiates power
transfer, the power receiver sends error packets to the
power transmitter requesting more or less power. The
power transmitter stops supplying power upon receiving
an “End Power” message, or if no packets are received
for more than 1.25 seconds. While no power is being
transmitted, the power transmitter enters a low-power
standby mode.
The WPC specification allows for both fixed- and moving-
coil configurations. A single fixed coil, referred to as type
A1, is the solution that Texas Instruments (TI) supports.
The power transmitter, typically a flat surface upon
which the user places the power receiver, is connected to
the power source. The coils of a WPC-compliant device
operateasaresonanthalfbridgeona50%dutycycle,
witha19-VDCinput(±1V).Ifmoreorlesspoweris
needed at the power receiver, the frequency in the coil
changes but stays between 110 and 205 kHz, depending
on power demands.
Power receiver
The power receiver is typically a portable device. The key
circuits of the power receiver are the secondary coil, used
to receive power from the power-transmitter coil; the rec-
tificationcircuit,usedtoconvertACtoDC;thepower-
conditioningcircuit,whichbufferstheunregulatedDC
intoregulatedDC;andthecommunicationscircuit,which
modulates the signal to the secondary coil. The power
receiver is responsible for all communications of its
authentication and power requirements, as the power
transmitter is only a “listener.”
While design of the power transmitter is restricted to
keep it WPC-compliant, much more freedom is permitted
for designing the power receiver. The coil dimension of the
power receiver can be adjusted to meet the device’s form
factor. The coil voltage at the power receiver is full-wave
rectified,withatypicalefficiencyof70%fora5-V,500-mA
output. Because communication between the two devices
is unidirectional, the WPC selected the power receiver to be
the “talker.” Inductive power transfer works by coupling a
magneticfieldfromprimarytosecondarycoils.Uncoupled
field lines rotate around the primary coil and do not repre-
sent loss as long as the field lines don’t couple a parasitic
load (for example, eddy-current loss in metal).
Communications protocol
The communications protocol includes analog and digital
pinging; identification and configuration; and power trans-
fer. A typical start-up sequence that occurs when a power
receiver is placed on a power transmitter proceeds as
follows:
1. An analog ping from the power transmitter detects the
presence of an object.
2. A digital ping from the power transmitter is a longer
version of the analog ping and gives the power receiver
time to reply with a signal-strength packet. If the signal-
strength packet is valid, the power transmitter keeps
power on the coil and proceeds to the next phase.
3.Duringtheidentificationandconfigurationphase,the
power receiver sends packets that identify it and that
provide configuration and setup information to the
power transmitter.
4. In the power-transfer phase, the power receiver sends
control error packets to the power transmitter to
increase or decrease the power supply. These packets
are sent approximately every 250 ms during normal
operation or every 32 ms during large signal changes.
Also during normal operation, the power transmitter
sends power packets every 5 seconds.
5. To end the power transfer, the power receiver sends an
“End Power” message or sends no communications for
1.25 seconds. Either of these events places the power
transmitter in a low-power state.
TI’s WPC-compliant solutions
TI is a founding member of the WPC and has taken an
active role in developing a robust wireless-power specifica-
tion. TI has developed reliable solutions for both a power
receiver and a power transmitter in the form of three newly
developedICs.ThepowerreceiverusestheMSP430bq1010
and bq25046 devices. The power transmitter is based on
the bq500110, which supports type A1 (single-coil) config-
urations. Both receiver and transmitter ICs are designed
to be interoperable with other WPC-compliant solutions.
TheMSP430bq1010inthepowerreceiverhandlesall
of the logic functions and communications. The onboard
analog-to-digital converters monitor the levels of voltage
into and current out of the bq25046. The bq25046 provides
load-currentinformationtotheMSP430bq1010,whichthen
uses this information to control the power transmitter’s
operating point. The bq25046 provides a low-current, 3.3-V
low-dropoutregulator(LDO)topowertheMSP430bq1010
andlogiccircuit,whilealarger5.0-VLDOiscapableof
providing up to 1 A of current to the main output.
The power-transmitter solution is provided with the
bq500110. This device demodulates and decodes serial
data from the power receiver. The control circuits first
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Power Management
Texas Instruments Incorporated
certify that the power receiver is indeed a WPC-compliant
device, then configure the power transmitter accordingly.
TI’sBQTESLA100LPEVMkitcombinesseparatetrans-
mitter and receiver designs into a single kit that includes
mechanical packaging. This kit can be used for evaluation
of the ICs or as a design example. The WPC has certified
that both the power-transmitter and the power-receiver
solutions meet the Version 1.0 specification. No software
isrequiredtooperatetheEVM,whichneedsonlya19-V
input.TheEVMkit’soutputwillbe5Vatupto1A.The
transmitterEVMincludesmultipleLEDoptionsforvisual
indication of power-transmission status. Also, two buzzer
options provide audio indication of the start of power
transfer.
Conclusion
The WPC standard is a set of guidelines that allows manu-
facturers to develop solutions with the confidence that
their components will mesh with a variety of other WPC-
certified components designed for inductive power transfer.
References
For more information related to this article, you can down-
load an Acrobat ® Reader ® file at www.ti.com/lit/ litnumber
and replace “ litnumber ” with the TI Lit. # for the
materials listed below.
Document Title
TI Lit. #
2.“WirelessPowerTransmitterManager,”
bq500110DataSheet...................... slusae0
3. “1.1A, Single-Input 5-V Power Supply IC
for Wireless Power Applications,” bq25046
DataSheet .............................. slusa83
4. “Wireless Receiver-Side Communication and
PowerMonitoringICforWirelessPower,”
MSP430bq1010DataSheet................. slas696
5.“bq500110EVM-688EvaluationModule,”
User’sGuide ............................. slvu429a
6.“bq25046EVM-687EvaluationModule,”
User’sGuide ............................. slvu420
Related Web sites
www.ti.com/sc/device/ partnumber
Replace partnumber with bq25046, bq500110 , or
Document Title
TI Lit. #
1. Wireless Power Consortium. “System
DescriptionWirelessPowerTransfer,Vol.1,
Part 1,” Version 1.0 [Online]. Available:
—
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SLYT401
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