PURPOSE: To prevent reverse current flow from a diode to a semiconductor switch by providing means for interrupting the current for a predetermined time on the input side of the diode thereby interrupting the input to the diode upon turn ON of the semiconductor switch.

CONSTITUTION: Upon turn ON of a semiconductor switch 4, current of a power supply circuit flows from a reactance 5 to the semiconductor switch 4 side and upon turn OFF of the semiconductor switch 4, the current flows from the reactor 5 to the recovery diode 6 side. Upon turn ON of the semiconductor switch 4, a switch 10 is turned OFF. A reverse current is generated upon turn OFF of the semiconductor switch 4, but since the switch 10 is turned OFF, the reverse current does not flow into the semiconductor switch 4. Since the diode recovery current does not flow into the semiconductor switch 4 the semiconductor switch 4 is protected against damage.