Comm

Rectifier

Receiver Coil

Receiver

Capacitor

Modulation

Capacitors

Amax

A(0)

A(1)

F, kHz

Fo(1) < Fo(0)

Operating state at logic “ 0”

Operating state at logic “ 1”

Fsw

a)

b)

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)

bq500410A

SLUSB96 – NOVEMBER 2012

www.ti.com

Principles of Operation (continued)

Communication

Communication within the WPC is from the receiver to the transmitter, where the receiver tells the transmitter to

send power and how much. In order to regulate, the receiver must communicate with the transmitter whether to

increase or decrease frequency. The receiver monitors the rectifier output and using Amplitude Modulation (AM),

sends packets of information to the transmitter. A packet is comprised of a preamble, a header, the actual

message and a checksum, as defined by the WPC standard.

The receiver sends a packet by modulating an impedance network. This AM signal reflects back as a change in

the voltage amplitude on the transmitter coil. The signal is demodulated and decoded by the transmitter-side

electronics and the frequency of its coil-drive output is adjusted to close the regulation loop. The bq500410A

features internal digital demodulation circuitry.

The modulated impedance network on the receiver can either be resistive or capacitive. Figure 1 shows the

resistive modulation approach, where a resistor is periodically added to the load, Figure 2 shows the resulting

amplitude change in the transmitter voltage. Figure 2 shows the capacitive modulation approach, where a

capacitor is periodically added to the load and the resulting amplitude change in the transmitter voltage.

Figure 1. Receiver Resistive Modulation Circuit

Figure 2. Receiver Capacitive Modulation Circuit

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