An electronic switch circuit includes an electronic switch having a first terminal, a second terminal and a third terminal, control means for applying to the first terminal a cyclic drive waveform which causes the electronic switch to conduct between the second and third terminals during a selected portion of each cycle of the waveform, operably coupled to the control means to control synchronisation of the cyclic drive waveform a detector operable to detect a change of direction of current flow at the second terminal of the electronic switch and means for monitoring and, where required, compensating for offset error of the detector. The electronic switch may beneficially be a synchronous rectifier. The synchronous rectifier may be used in DC-DC power converters to improve efficiency.