FOREIGN OBJECT DETECTION IN A WIRELESS POWER TRANSFER SYSTEM

A power transmitter (101) comprises a driver (201) generating a drive signal for a transmitter coil to generate a power transfer signal during a power transfer time interval and an electromagnetic test signal during a foreign object detection time interval. A set of balanced detection coils (207, 209) comprise two detection coils arranged such that signals induced in the two detection coils by an electromagnetic field generated by the transmitter coil compensate each other. A foreign object detector (205) is coupled to the detection coils and performs foreign object detection during the foreign object detection time interval. The foreign object detector (205) is arranged to detect a foreign object in response to a foreign object detection criterion requiring that at least one signal from a set of balanced detection coils (207, 209) exceeds a first threshold and not more than a given threshold number of at least two of the signals exceed a second threshold.