Datasheet

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
SLUSAL8C –JUNE 2011–REVISED SEPTEMBER 2012 www.ti.com
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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