Patent ID: 7388762

Claim:
A switching power supply circuit for converting primary side direct-current voltage to secondary side direct-current voltage, said switching power supply circuit comprising: a choke coil supplied with said primary side direct-current voltage; a converter transformer having a primary winding on a primary side, the primary winding being connected to said choke coil, and a secondary winding on a secondary side; a switching element for switching said primary side direct-current voltage supplied via said choke coil and the primary winding of said converter transformer; a primary side series resonant capacitor having one terminal connected to a point of connection between said choke coil and said primary winding, the primary side series resonant capacitor forming a primary side first series resonant circuit with an inductance of said choke coil and forming a primary side second series resonant circuit with a leakage inductance occurring at said primary winding; a primary side parallel resonant capacitor connected in parallel with said switching element, the primary side parallel resonant capacitor forming a primary side parallel resonant circuit with the inductance of said choke coil and the leakage inductance occurring at said primary winding; an oscillating and driving circuit for on-off driving of said switching element; a secondary side rectifier circuit for generating said secondary side direct-current voltage, the secondary side rectifier circuit being connected to the secondary winding of said converter transformer, a switching output of said switching element being transmitted to the secondary winding of the converter transformer; and a control circuit for supplying a control signal to make a value of said secondary side direct-current voltage output by said secondary side rectifier circuit a predetermined value to said oscillating and driving circuit; wherein resonance frequency of said primary side first series resonant circuit is set at a frequency substantially twice resonance frequency of said primary side second series resonant circuit, and resonance frequency of said primary side parallel resonant circuit is set at a frequency substantially equal to or higher than 1.5 times the resonance frequency of said primary side first series resonant circuit.