an induction heating apparatus of the invention is provided with a pair of inversely parallel connected thyristors which receives power from a low frequency energy source . depending on the polarity of the source voltage , one of the thyristors is triggered into conduction by a control pulse generated in delayed response to a sensed forward bias potential at its anode for the purpose of supplying a forward current to a commutating circuit which is tuned to a high frequency in the ultrasonic range , whereupon a backward current is generated . the backward current is commutated through the other thyristor to complete a high frequency oscillation . a detector is provided to detect when the magnitude of the backward commutating current reaches a predetermined value for the purpose of terminating the trigger control pulse . the control pulses are supplied simultaneously to both thyristors through a pulse transformer which upon termination of the control pulse generates a counterelectromotive force which negatively biases the thyristor which has been conducting during the first half - cycle of each complete oscillation to reduce its turn - off time .