PURPOSE: To trip a magnet contact through Mctt operation circuit when parallel resonance has occurred by providing a main control circuit as overvoltage/ overcurrent protection means and a parallel resonance prevention circuit for detecting a ripple voltage and stopping the output.

CONSTITUTION: DC voltage, ripple voltage and return voltage input to the main control circuit An are compared to reference voltage, overvoltage trip level and overvoltage trip level from a reference power supply VRE, and the main control circuit A1 forms an error signal and a trip signal from the results of comparison. An OSC&con circuit 6, to which the error signal was input, controls the output of inverter based on the error signal and stabilizes the output voltage VO. On the other hand, if a trip signal is input to a Mctt operation circuit CO, then the Mctt operation circuit CO turns the connecting state of a magnet contact Mctt into interrupting state, and the output from a power supply PS to OSC&con circuit 6 is stopped.