Patent ID: 7110268

Claim:
A switching power supply circuit, comprising: a switching circuit including a switching device operable to perform a switching operation at a switching frequency based on a DC input voltage, the switching operation resulting in a switching output; a switching driving unit operable to drive the switching device to perform the switching operation; an insulating 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 supplied with the switching output of the switching operation, and the secondary winding having an alternating voltage induced therein by the primary winding, the core having a gap formed at a predetermined position between the primary side and the secondary side, the gap having a length selected to produce a predetermined coupling coefficient between the primary side and the secondary side; a primary side series resonance circuit including a leakage inductance component of the primary winding and a capacitance of a primary side series resonance capacitor connected in series with the primary winding for producing a predetermined primary side resonance frequency for making the switching circuit operate on a current resonance basis; a secondary side series resonance circuit including a leakage inductance component of the secondary winding and a capacitance of a secondary side series resonance capacitor connected in series with the secondary winding for producing a predetermined secondary side resonance frequency; the primary side series resonance circuit and the secondary side series resonant circuit forming an electromagnetic coupling type resonance circuit; a secondary side DC output voltage production unit operable to input a resonance output from the secondary side series resonance circuit and to perform a rectification operation on the input resonance output to produce a secondary side DC output voltage; a constant voltage control unit operable to control the switching driving unit in response to a level of the secondary side DC output voltage to adjust the switching frequency of the switching circuit to perform constant voltage control for the secondary side DC output voltage; and a composite coupling coefficient setting unit operable to set a composite coupling coefficient between the primary side and the secondary side of the insulating converter transformer so that the electromagnetic coupling type resonance circuit has a unimodal output characteristic with respect to an input of a frequency signal having the switching frequency.