PURPOSE: To eliminate the need of a circuit for controlling an external control signal while reducing the loss by providing a Zener diode for protecting the gate electrode of an auxiliary semiconductor switch element against overvoltage.

CONSTITUTION: Since the charging circuit for a snubber capacitor 4 has a one- directional path passing through a snubber diode 5, the snubber diode 4 is charged continuously. But since an auxiliary semiconductor switch element 6 does not function when the capacitor voltage (ec) is lower than a set level, the capacitor voltage (ec) increases up to the set level of a Zener diode 7 and the surge energy is absorbed entirely by the snubber capacitor 4. When the capacitor voltage (ec) reaches the set level, the gate voltage of the auxiliary semiconductor switch element 6 is held at the Zener voltage. Consequently, the auxiliary semiconductor switch element 6 is turned ON and the snubber capacitor 4 is regenerated to a DC power supply 1. This circuitry reduces the loss and eliminates the need of any external control signal.