Patent ID: 7170761

Claim:
A switching power supply circuit, comprising: a rectifying and smoothing circuit operable to receive an alternating voltage and to generate a rectified and smoothed voltage; a switching circuit including a switching device operable to receive the rectified and smoothed voltage as a direct-current input voltage and to perform a switching operation at a switching frequency, the switching operation resulting in a switching output; a switching driving unit operable to drive the switching device to perform the switching operation; 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 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 resonant circuit including a leakage inductance component of the primary winding and a capacitance of a primary side series resonant capacitor connected in series with the primary winding for producing a predetermined primary side resonant frequency for making the switching circuit operate on a current resonant basis; a secondary side series resonant circuit including a leakage inductance component of the secondary winding and a capacitance of a secondary side series resonant capacitor connected in series with the secondary winding for producing a predetermined secondary side resonant frequency; the primary side series resonant circuit and the secondary side series resonant circuit forming an electromagnetic coupling type resonant circuit; a secondary side direct-current output voltage generating circuit operable to input an oscillation output from the secondary side series resonant circuit and to perform a rectifying operation on the input oscillation output to generate a secondary side direct-current output voltage; a constant-voltage control unit operable to perform constant voltage control on the secondary side direct-current output voltage by controlling the switching driving unit according to a level of the secondary side direct-current output voltage to adjust the switching frequency of the switching circuit; a power factor improving circuit operable to feed back the switching output of the switching operation to a predetermined rectified current path of the rectifying and smoothing circuit, and to interrupt a rectified current according to the fed-back switching output; and a composite coupling coefficient setting unit operable to set a composite coupling coefficient between the primary side and the secondary side of the converter transformer so that the electromagnetic coupling type resonant circuit has a unimodal output characteristic with respect to the input of a frequency signal having the switching frequency.