To stop the operation of a chopper circuit in a moment when the output of the chopper circuit is shorted by locating a first and a second diode so that the anodes and the cathodes of these diodes may be in opposite directions to each other and connecting a connection between the first diode and a second resistor to a self-ripple detecting section of the chopper circuit.

A series combination of a second diode D2 and a sixth resistor R6 is parallelly connected to a series combination of a first diode D1 and a third resistor R3. The first diode D1 and the second diode D2 are so located that their anode terminals and cathode terminals may be in opposite direction to each other. A connection A between the first diode D1 and a second resistor R2 is connected to a self-ripple detecting section 13 of the chopper circuit 10 and the voltage at the connection A is supplied to the chopper circuit 10 as a supply voltage. By this method, the operation of the chopper circuit 10 can be stopped in a moment when the output of the chopper circuit 10 is shorted.