Datasheet

bq500410A

www.ti.com

SLUSB96 – NOVEMBER 2012

Principles of Operation

Fundamentals

The principle of wireless power transfer is simply an open cored transformer consisting of a transmitter and

receiver coils. The transmitter coil and electronics are typically built into a charger pad and 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 once the transmitter coil is

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

voltage is rectified, and power can be transferred 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, or to download a copy of the WPC specification, go to http://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 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 112 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.

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