BQ500414Q Datasheet, PDF (17/34 Pages) Texas Instruments – Qi Compliant Wireless Power Transmitter Manager

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bq500414Q
SLUSBE4B – JANUARY 2014 – REVISED JUNE 2014
8.4 Device Functional Modes
8.4.1 Power Transfer
Power transfer depends on coil coupling. Coupling is dependant on the distance between coils, alignment, coil
dimensions, coil materials, number of turns, magnetic shielding, impedance matching, frequency, and duty cycle.
Most importantly, the receiver and transmitter coils must be aligned for best coupling and efficient power transfer.
The smaller the space between the coils is, the better the coupling; however, the practical distance is set to be
less than 5-mm, as defined within the WPC specification, to account for housing and interface surfaces.
Shielding is added as a backing to both the transmitter and receiver coils to direct the magnetic field to the
coupled zone. Magnetic fields outside the coupled zone do not transfer power. Thus, shielding also serves to
contain the fields to avoid coupling to other adjacent system components.
Regulation can be achieved by controlling any one of the coil coupling parameters. However, for WPC
compatibility, the transmitter-side coils and capacitance are specified and the resonant frequency point is fixed.
Power transfer is thus regulated by changing the frequency along the resonance curve from 120-kHz to 205-kHz,
(that is the higher the frequency is, the lower the power). Duty cycle remains constant at 50% throughout the
power band and is reduced only once 205-kHz is reached.
The WPC standard describes the dimensions, materials of the coils, and information regarding the tuning of the
coils to resonance. The value of the inductor and resonant capacitor are critical to proper operation and system
efficiency.
8.4.2 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 bq500414Q
features internal digital demodulation circuitry.
The modulated impedance network on the receiver can either be resistive or capacitive. Figure 7 shows the
resistive modulation approach, where a resistor is periodically added to the load, and the resulting amplitude
change in the transmitter voltage. Figure 8 shows the capacitive modulation approach, where a capacitor is
periodically added to the load and the resulting amplitude change in the transmitter voltage.
Rectifier
Receiver Coil
Receiver
Capacitor
Modulation
Resitor
Amax
A(0)
Operating state at logic “0”
Comm
A(1)
Operating state at logic “1”
Fsw
F, kHz
a)
b)
Figure 7. Receiver Resistive Modulation Circuit
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