Patent ID: 7145785

Claim:
A switching power supply circuit, comprising: switching means including a switching element for receiving a DC input voltage as an input thereto to perform switching; switching driving means for switching driving said switching element operation; an insulating converter transformer formed by winding a primary winding to which a switching output obtained by the switching operation of said switching means is supplied and a plurality of secondary windings in each of which an alternating voltage is induced by said primary winding; a primary side series resonance circuit formed from at least a leakage inductance component of said primary winding of said insulating converter transformer and the capacitance of a primary side series resonance capacitor connected in series to said primary winding such that a first resonance frequency is set for making the operation of said switching means that of a current resonance type; a plurality of secondary side series resonance circuits each formed from at least a leakage inductance component of one of said plural secondary windings of said insulating converter transformer and the capacitance of a secondary side series resonance capacitor connected in series to the plural secondary winding such that a second resonance frequency is set; secondary side DC output voltage production means for performing a rectification operation for the AC voltages obtained by said secondary windings and smoothing the rectification outputs of said plural secondary windings by means of a common secondary side smoothing capacitor to produce a secondary side DC output voltage; and constant voltage control means for controlling said switching driving means in response to the level of the secondary side DC output voltage to adjust the switching frequency of said switching means to perform constant voltage control for the secondary side DC output voltage, wherein said insulating converter transformer further includes a core which has a gap formed at a predetermined position thereof, the gap having a gap length set so that an output characteristic of an electromagnetic coupling type resonance circuit formed from said primary side and secondary side series resonance circuits when a frequency signal having the switching frequency is inputted to said electromagnetic coupling type resonance circuit becomes a single-humped characteristic.