PURPOSE: To prevent occurrence of arc cut phenomena during base current flowing period, by heighten the base voltage arbitrarily and keeping the arc voltage nearly constant as a whole.

CONSTITUTION: A series circuit of a switch S1 and a resistance R1, a series circuit of a switch S2 and resistance R2 and a resistance Rb are connected in parallel between the collector and emitter of a transistor Tr1 for output control. When switches S1, S2 are opened, the base voltage is a reference base voltage, and when the switches S1, S2 are closed, the base voltage becomes high. The output voltage is detected by a controlling circuit 2 and compared with a set value, and the pulse width of pulse voltage is changed by change of base voltage.