Patent ID: 7315460

Claim:
A switching power supply device, comprising: a series circuit in which a first switching element and a second switching element are connected in series at both ends of an output of an input rectifier circuit for rectifying an alternating voltage of an alternating current power supply or at both ends of a direct current power supply; a resonant circuit in which a primary winding of a transformer and a current resonant capacitor are connected in series at both ends of any one of the first switching element and the second switching element; at least one pair of rectifying and smoothing circuits for rectifying and smoothing a voltage generated across a secondary winding of the transformer during an on-period of any one of the first switching element and the second switching element; a control circuit for alternately turning on/off the first switching element and the second switching element based on a voltage from the at least one pair of the rectifying and smoothing circuits; and a voltage detecting circuit for outputting a voltage detecting signal when a voltage at both ends of one switching element of the first switching element and the second switching element, which is turned on at a time of transmitting electric power to the at least one pair of the rectifying and smoothing circuits, becomes equal to a predetermined voltage or more, wherein the control circuit turns on the other switching element of the first switching element and the second switching element based on the voltage detecting signal from the voltage detecting circuit.