PURPOSE: To make it possible to use a switching power supply even in a region below the shoulder of a regulator by a method wherein an electronic switch is turned OFF upon dropping the output voltage below a predetermined level (shoulder of the regulator), DC component from a detecting circuit is interrupted and the output voltage is controlled only by the AC fluctuation of output voltage fed from an AC fluctuation applying circuit.

CONSTITUTION: When an output voltage Vout drops below a predetermined level (shoulder of a regulator), an electronic switching diode D1 is turned OFF to interrupt current flow from a variable resistor R2 to a transistor Tr. But a bias resistor R4 provides a bias and the transistor Tr functions in the active region. A capacitor C1 and a resistor R5 apply an AC fluctuation component of the output voltage Vout to the base of the transistor Tr. The AC fluctuation component is amplified through the transistor Tr and outputted as a collector voltage Vc which is employed as a control signal for controlling the operation of a pulse generating circuit 10 and a switching circuit 20.