PURPOSE: To prevent heat dissipation by detecting load fluctuation of the tertiary and quaternary windings as the fluctuation of induced AC voltage level in a dummy winding.

CONSTITUTION: When a switch S1 is OFF, an output terminal 14 is grounded through a load, so that a transistor 27 becomes OFF, a transistor 33 becomes ON, and dummy detection voltage e2 corresponding to a dummy voltage E1' obtained by rectifying and smoothening an induced AC voltage in a dummy winding 2D is generated; this dummy winding 2D is located at the contact point between a resistor 28 and a comparator 18. At this time, the dummy winding 2D is in the state of dense coupling with a tertiary winding 2T and a quaternary winding 2F. Therefore, the level of an induced AC voltage in the dummy winding 2D fluctuates in response to the load fluctuation of windings 2T and 2F, and then a dummy voltage E1' and dummy detection voltage e2 fluctuate. A dummy voltage detection means DM controls the level of an induced AC voltage in a converter transformer 2 by detecting the fluctuation in the dummy detection voltage e2. As a result, power loss can be eliminated and thus heat dissipation can be prevented.