A comparator U1 having inputs from the upper out put terminal and source of voltage V’, as depicted, is adapted to compare V01 with reference voltage Vr. Comparator U1 is connected to pulse width modulating (PWM) circuit 1 which is connected to the comparator
U1 as depicted, and is adapted to produce upon receipt of the output from comparator U1, a pulse width signal corresponding to the difference between output V01 and reference voltage V,. The PWM circuit 1 is connected to drive circuit 2 which is adapted toreceive the ouput from PWM circuit 1. A switching transistor Q1 is con nected to drive circuit 2 at its base, and to input source and primary m, at its emitter and collector. Transistor
Q] is turned ON and OFF by drive circuit 2 so as to turn ON and OFF input DC voltage Vin supplied to the primary coil ml of transformer T1.