P9225-R Datasheet

© 2018 Integrated Device Technology, Inc.

19

November 12, 2018

8.12 Advanced Foreign Object Detection (FOD)

When metallic objects are exposed to an alternating magnetic field, eddy currents cause such objects to heat up. Examples of such parasitic

metal objects are coins, keys, paper clips, etc. The amount of heating depends on the strength of the coupled magnetic field, as well as on the

characteristics of the object, such as its resistivity, size, and shape. In a wireless power transfer system, the heating manifests itself as a power

loss, and therefore a reduction in power-transfer efficiency. Moreover, if no appropriate measures are taken, the heating could be sufficient that

the foreign object could become heated to an undesirable temperature.

During the power transfer phase, the receiver periodically will communicate to the transmitter the amount of power received by means of a

Received Power Packet. The transmitter will compare this power with the amount of power transmitted during the same time period. If there is

a significant unexplained loss of power, then the transmitter will shut off power delivery because a possible foreign object might be absorbing

too much energy.

For a WPC system to perform this function with sufficient accuracy, both the transmitter and receiver must account for and compensate for all

of their known losses. Such losses could be due to resistive losses or nearby metals that are part of the transmitter or receiver, etc. Because

the system accurately measures its power and accounts for all known losses, it can thereby detect foreign objects because they cause an

unknown loss. The WPC specification requires that a power receiver must report to the power transmitter its received power (PPR) in a Received-

Power Packet (RPP). The maximum value of the received power accuracy

������������Δdependsonthemaximumpowerofthepowerreceiverasdefined

in Table 8.

The power receiver must determine its PPR with an accuracy of ±

������������Δ,andreportitsreceivedpowerasPRECEIVED=PPR+������������Δ.Thismeansthat

the reported received power is always greater than or equal to the transmitted power (PPT) if there is no foreign object (FO) present on the

interface surface.

Table 8.

Maximum Estimated Power Loss

Maximum Power [W]

Maximum

������������������������[mW]

5

350

The compensation algorithm includes values that are programmable via either the I2C interface or OTP (one-time programmable) bits.

Programmability is necessary so that the calibration settings can be optimized to match the power transfer characteristics of each particular

WPC system to include the power losses of the transmit and receive coils, battery, shielding, and case materials under no-load to full-load

conditions. The values are based on the comparison of the received power against a reference power curve so that any foreign object can be

sensed when the received power is different from the expected system power.