A switching power supply circuit comprising:
- switching means formed with a switching device supplied with a direct-current input voltage and performing switching operation;
- switching-driving means for switching-driving said switching device;
- an isolated converter transformer formed by winding a primary winding supplied with a switching output obtained by the switching operation of said switching means and a first secondary winding and a second secondary winding in which an alternating voltage is induced by the primary winding;
- a primary side series resonant circuit formed by at least a leakage inductance component of the primary winding of said isolated converter transformer and a capacitance of a primary side series resonant capacitor connected in series with said primary winding, a first resonant frequency being set to said primary side series resonant circuit, said primary side series resonant circuit converting operation of said switching means into a current resonant type operation;
- a first secondary side series resonant circuit formed by at least a leakage inductance component of said first secondary winding of said isolated converter transformer and a capacitance of a first secondary side series resonant capacitor connected in series with the first secondary winding, a second resonant frequency being set to said first secondary side series resonant circuit;
- first secondary side direct-current output voltage generating means for generating a first secondary side direct-current output voltage by performing a rectifying operation on the alternating voltage obtained in said first secondary winding, and smoothing a rectification output resulting from the rectifying operation by a first secondary side smoothing capacitor;
- second secondary side direct-current output voltage generating means for generating a second secondary side direct-current output voltage having a same absolute value level as said first secondary side direct-current output voltage and having different polarity from polarity of said first secondary side direct-current output voltage by performing a rectifying operation on the alternating voltage obtained in said second secondary winding, and smoothing a rectification output resulting from the rectifying operation by a second secondary side smoothing capacitor;
- and
- constant-voltage control means for performing constant-voltage control on said first secondary side direct-current output voltage by varying switching frequency of said switching means by controlling said switching-driving means according to level of said first secondary side direct-current output voltage,
- wherein length of a gap formed at a predetermined position of a core of said isolated converter transformer is set such that an output characteristic of an electromagnetic coupling type resonant circuit formed with said primary side series resonant circuit and said first secondary side series resonant circuit with respect to an input of a frequency signal having said switching frequency is a unimodal characteristic.