The operation of FIG. 4 is as follo

The operation of FIG. 4 is as follows. In the normal state, voltage Vd, obtained by rectifying voltages e1, e; of the secondary coils of transformers T1,T2, is below
set voltage VS, so that comparator CP produces a signal of “L” level which indicates that the operation is normal. In this state, control circuit 1, comprising the PWM circuit, performs ON-OFF control of transistors Q1,Q2 so‘ that output voltage V01 obtained through terminals 10,11 takes a constant level corresponding to reference voltage V,.