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SLUSBB1 – DECEMBER 2012

Principles of Operation

Fundamentals

The principle of wireless power transfer is simply an open cored transformer consisting of primary and secondary

coils and associated electronics. The primary coil and electronics are also referred to as the transmitter, and the

secondary side the receiver. The transmitter coil and electronics are typically built into a charger pad. The

receiver coil and electronics are typically built into a portable device, such as a cell-phone.

When the receiver coil is positioned on the transmitter coil, magnetic coupling occurs when the transmitter coil is

driven. The flux is coupled into the secondary coil which induces a voltage, current flows, it is rectified and power

can be transferred quite effectively to a load - wirelessly. Power transfer can be managed via any of various

familiar closed-loop control schemes.

Wireless Power Consortium (WPC)

The Wireless Power Consortium (WPC) is an international group of companies from diverse industries. The WPC

standard was developed to facilitate cross compatibility of compliant transmitters and receivers. The standard

defines the physical parameters and the communication protocol to be used in wireless power. For more

information, go to www.wirelesspowerconsortium.com.

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 closer the space between the coils, the better the coupling, but 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. For WPC compatibility, the

transmitter coils and capacitance are specified and the resonant frequency point is fixed at 100 kHz. Power

transfer is regulated by changing the operating frequency between 112 kHz to 205 kHz. The higher the

frequency, the further from resonance and 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 dimension and materials of the coils. It also has information on tuning the coils

to resonance. The value of the inductor and resonant capacitor are critical to proper operation and system

efficiency.

Copyright © 2012, Texas Instruments Incorporated

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