PURPOSE: To reduce a capacity of pulse transformer by forming a constant DC voltage from a commercial power supply, driving an inverter with said voltage and supplying a high frequency output of said inverter to the primary side of the pulse transformer.

CONSTITUTION: A commercial AC power is converted into a DC voltage by a diode bridge 3 and into a constant DC voltage by a chopper 5. Also, said voltage is converted into AC with a given frequency by an inverter 6 via a capacitor 4 and supplied to the primary side of a transformer 7 and pulse transformers 15∼16. Then, DC power of main power supply 8 and other power supply 9 for system control is obtained from the secondary side winding of the transformer 7 and a voltage of the main power supply 8 is fed back to PWM (pulse width modulation) controller 10 of the chopper 5. This feedback compensates a signal of oscillator 11 and drives and controls a driver 12 so that a constant DC voltage can be obtained. If a system power supply is unnecessary, a voltage between both ends of the capacitor 4 is directly controlled by the PWM controller 10 to control a constant voltage.