Patent ID: 7511929

Claim:
A switching power supply, comprising: a switching transformer including a primary winding, a secondary winding and an auxiliary winding; a switching element connected in series with the primary winding of the switching transformer; a rectifier/smoothing circuit connected with the secondary winding of the switching transformer; a wave-shaping circuit connected with the auxiliary winding of the switching transformer; and a control circuit that controls ON/OFF of the switching element, wherein the control circuit comprises: an output voltage detection circuit that generates, from a feedback voltage output from the wave-shaping circuit, a signal for PWM control of the switching element; and a bottom detection circuit that detects, from the feedback voltage, a bottom level of a ringing voltage during a duration of a time when a current does not flow through the secondary winding of the switching transformer, wherein the control circuit switches the switching element from OFF to ON in accordance with a signal output from an oscillator internal to the control circuit or a signal output from the bottom detection circuit.