Patent ID: 7286374

Claim:
A switching power supply circuit, comprising: a rectifying and smoothing circuit including a rectifying element and a smoothing capacitor, the rectifying element rectifying an AC voltage input from a commercial AC power source to produce a rectified voltage, and the smoothing capacitor smoothing the rectified voltage to generate a rectified and smoothed voltage as a voltage across the smoothing capacitor; a switching element for receiving the rectified and smoothed voltage as a DC input voltage to perform a switching operation; a switching driving section operable to drive the switching element to perform the switching operation; a first inductor disposed in a path through which the rectified and smoothed voltage is input to the switching element; a first primary side series resonance circuit for performing a resonance operation in correspondence with the switching operation performed by the switching element, the first primary side series resonance circuit including an inductance of a second inductor and a capacitance of a primary side series resonance capacitor connected in series with the inductance of the second inductor; a second primary side series resonance circuit for performing a resonance operation in correspondence with the switching operation performed by the switching element, the second primary side series resonance circuit including an inductance of the first inductor and the capacitance of the primary side series resonance capacitor connected in series with the inductance of the first inductor; a first primary side parallel resonance circuit for performing a resonance operation in correspondence with the switching operation performed by the switching element, the first primary side parallel resonance circuit including the inductance of the first inductor, the inductance of the second inductor connected in series with the inductance of the first inductor, and a capacitance of a primary side parallel resonance capacitor connected in parallel with a series connection circuit including the first inductor and the second inductor; a converter transformer having a core with a primary winding on a primary side and a secondary winding on a secondary side, the primary winding being formed by the second inductor, the secondary winding having an AC voltage induced therein by the primary winding, a coupling coefficient between the primary side and the secondary side being set for loose coupling; and a secondary side DC output voltage generating section operable to perform a rectifying operation on the AC voltage induced in the secondary winding of the converter transformer and input to the secondary side DC output voltage generating section to thereby generate a secondary side DC output voltage.