PURPOSE: To vary an output efficiently by simple constitution by mounting a secondary winding to an inductance interposed on the primary side of a DC-AC converter, interposing a diode in series with the secondary winding and regenerating currents to a power supply on the input side.

CONSTITUTION: A DC power supply 13 is connected between power supply terminals 11, 12, and one power supply terminal 11 is connected to an intermediate tap on the primary side of a transformer 15 through the primary side of an inductance 14 with a secondary winding. One tip of a secondary winding for the inductance 14 is connected to one power supply terminal 11 through a diode 20 in series in the forward direction, and the other tip is connected to the other power supply terminal. Transistors 17, 18 are turned ON alternately by an output from a pulse width control oscillator 19. Currents are regenerated to the power supply on the input side through the diode 20 from the secondary side of the inductance 14.