PURPOSE: To prevent the drop of efficiency caused by the capacitance between the electrodes of switching element, and enable the operation of high frequency by clamping the voltage added to the switching element below the power source voltage, and also, keeping the condition of Z.V.S, making use of accumulated energy.

CONSTITUTION: Voltage is added to the series circuit of a rectifier D2 for clamping, a transformer 5 parallel with a switching element S2, and a capacitor C2 by a switching element S1 parallel with a clamp rectifier D1. The voltage added to the switching elements S1 and S2 is clamped below the voltage of the power source 1 by the rectifier D2 for clamping by turning on or turning off the switching elements S1 and S2 at a proper delay time. By making use of the energy accumulated in the primary winding of the transformer 5 and the capacitor C2, the condition of Z.V.S(zero volt switching) is always kept, covering a wide output control range.