Patent ID: 7696739

Claim:
An electronic switch circuit comprising: 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 repetitively monitoring for offset error of the detector and, where required, compensating for the offset error, wherein the detector comprises: a voltage comparator operable to compare a measured voltage between the second and third terminals of the electronic switch with a reference voltage and to determine when the measured voltage changes sign with respect to the reference voltage and wherein the means for monitoring and compensating for offset error of the detector includes: (i) a state sensor operable to sense a polarity of the second terminal at a selected point in the cycle of the cyclic drive waveform when conduction of the electronic switch is turned off by the waveform and to produce a corresponding output signal; and (ii) means for adding to or withdrawing from the detector in response to the output signal produced by the state sensor a compensating current which adjusts a reference voltage of the detector.