Abstract: A power adaptor is disclosed, which comprises an input for connection to an AC power supply, a resonant circuit coupled to the input that provides an output suitable for driving a load, at least one half-bridge drive circuit for providing a drive signal to the resonant circuit, and a switch controller for the half-bridge drive circuit. The switch controller is adapted to provide one or more of the following, in at least one mode: (i) to provide the high-side switch and the low-side switch with on-times of different durations, (ii) to provide the high-side switch and the low-side switch with on-times that overlap, and (iii) to provide the high-side switch and the low-side switch with on-times that are synchronous. This may be utilised to control the current delivered to the output without any need to change the frequency at which the resonant circuit is driven.