FOREIGN OBJECT DETECTION IN A WIRELESS POWER TRANSFER SYSTEM

A power transmitter () for a wireless power transfer system comprises a transmitter coil () and a driver () generates a drive signal for the transmitter coil () employing a repeating time frame with a power transfer time interval and a foreign object detection time interval. A test generator () generates a test drive signal for a test coil () during the foreign object detection time interval. A foreign object detector () performs a foreign object detection test based on a measured parameter for the test drive signal. Prior to entering a power transfer phase, an adapter () controls the power transmitter () to operate in a foreign object detection initialization mode in which a preferred value of a signal parameter for the test drive signal is determined in response to at least a first message received from the power receiver (). During the foreign object detection time interval the signal parameter of is set to the preferred value.