Wireless power transfer

A power transmitter () provides power to a power receiver () via a power transfer signal generated by a drive signal fed to a transmitter coil (). A first controller () of the power transmitter () controls a parameter of the drive signal in accordance with a first mode which comprises operating a power control loop to control the power transfer signal in response to power control error messages received from the power receiver (). A second controller () controls the parameter of the drive signal in accordance with a second mode which comprises setting the parameter of the drive signal in response to at least one power transfer signal setpoint received from the power receiver (). A circuit () selects between the first mode of operation and the second mode of operation in response to a power transfer control mode request received from the power receiver () and a timing circuit () varies a timing parameter for the communication time intervals between the two modes of operation.