PURPOSE: To feed back the voltage across the ternary winding of a flyback transformer to a linear primary side circuit without using any photocoupler element by exclusively using the ternary winding for constituting an overvoltage protective circuit and insulat ing the primary side of the transformer from the secondary side of the transformer with the winding in a DC manner.

CONSTITUTION: A ternary winding n3 insulated from windings n1 and n2 in DC is exclusively used for an overvoltage protective circuit. As a result, the earthing of the winding n3 and a circuit connected to the winding n3 can be made at the same time when a primary-side circuit is earthed and the winding n3 can be connected to the primary-side circuit without using a photocoupler element. Then a DC voltage V3 is applied across a serial circuit of a Zener diode D2 having a Zener voltage Vz and resistor R1. When the DC output voltages V1 and V2 of power source circuits rise for some reason, the DC voltage V3 across the ternary winding n3 rises and a Zener current flows. As a result, a voltage is generated across the resistor R1 and the OVP 3a of a controller 3 operates to stop power supply generating operations. Since the power supply generating operations are stopped, the circuits are protected from overvoltage.