Comm

Rectifier

Receiver Coil

Receiver

Capacitor

Modulation

Resitor

Amax

A(0)

A(1)

F, kHz

Fsw

Operating state at logic “0”

Operating state at logic “1”

a)

b)

bq500210

www.ti.com

SLUSAL8

– JUNE 2011

FUNCTIONAL OVERVIEW

The typical Wireless Power Transfer System consists of primary and secondary coils that are positioned against

each other in a way to maximize mutual coupling of their electromagnetic fields. Both coils have ferrite shields as

part of their structures to even further maximize field coupling. The primary coil is excited with the switching

waveform of the transmitter power driver that gets its power from an AC-DC wall adapter. The secondary coil is

connected to the rectifier that can either directly interface the battery or can have an electronic charger or

post-regulator connected to its output. The capacitors in series with the coils are tuned to create resonance in the

system. The system being in resonance facilitates better energy transfer compared to inductive transfer. Power

transfer in the resonant system can also be easily controlled with the variable frequency control approach. To

limit operating frequency variation the bq500210 uses both frequency and PWM methods to control power

transfer. When the operating frequency approaches a 205kHz limit and the receiver still commands lower power,

the bq500210 will reduce the PWM cycle in discrete steps to maintain the output in regulation.

The rectifier output voltage is monitored by the secondary side microcontroller that generates signals to control

the modulation circuit to pass coded information from the secondary side to the primary side. The coded

information is organized into information packets that have Preamble bytes, Header bytes, message bytes and

Checksum bytes. Per the WPC specification, information packets can be related to Identification, Configuration,

Control Error, Rectified Power, Charge Status, and End of Power Transfer information. For detailed information

on

the

WPC

specification,

visit

the

Wireless

Power

Consortium

website

at

http://www.wirelesspowerconsortium.com/.

There are two ways the coupled electromagnetic field can be manipulated to achieve information transfer from

the secondary side to the primary side. With the resistive modulation approach shown in Figure 3, the

communication resistor periodically loads the rectifier output changing system Q factor, and as a result the value

of the voltage on the primary side coil. With the capacitive modulation approach shown in Figure 4, a pair of

communication capacitors are periodically connected to the receiver coil network. These extra capacitance

application changes slightly the resonance frequency of the system and its response on the current operating

frequency, which in turn leads to coil voltage variation on the primary side.

With both modulation techniques primary side coil waveform variations are detected with a Digital Demodulation

algorithm in the bq500210 to restore the content of the information packets and adjust controls to the transmitter.

Figure 3. Resistive Modulation Circuit

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© 2011, Texas Instruments Incorporated

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