Patent ID: 7339799

Claim:
A switching power supply comprising: a transformer configured to include a primary winding and a secondary winding; a first series circuit formed by connecting first and second switching elements in series at both ends of a direct-current power source; a second series circuit formed by connecting a resonant capacitor, a resonant reactor, and the primary winding of the transformer in series at both ends of the first switching element; a rectifying-smoothing circuit configured to rectify and smooth a voltage generated on the secondary winding of the transformer when the second switching element turns on, and to supply an output voltage to a load, the rectifying-smoothing circuit including a diode and a smoothing capacitor; and a control circuit configured to turn the first and second switching elements on and off alternately based on the output voltage from the rectifying-smoothing circuit, wherein the control circuit sets an on-period of the second switching element to a predetermined time period longer than a half value of a cycle of a resonant current attributable to the resonant reactor and the resonant capacitor, and controls an on-period of the first switching element based on the output voltage from the rectifying-smoothing circuit.