Patent ID: 7522430

Claim:
A switching power supply comprising: a resonant transformer having a primary winding, a secondary winding, and an auxiliary winding tightly coupled to the secondary winding; a first series circuit in which a first switching element and a second switching element connected in parallel to an output of a non-AC power source are serially connected; a voltage resonant capacitor connected to opposite ends of the first switching element or the second switching element; a second series circuit in which a current resonant capacitor, a resonant reactor and the primary winding of the resonant transformer are serially connected; a rectifying and smoothing circuit that rectifies and smoothes a voltage of the secondary winding of the resonant transformer; a control circuit turning on and off alternately the first switching element and the second switching element according to an output voltage of the rectifying and smoothing circuit; and a resonant period detector detecting a period during which energy is transmitted from a primary side to a secondary side of the resonant transformer based on a voltage of the auxiliary winding of the resonant transformer, and outputting a resonant period detection signal.