a switching unit formed of two switching elements at least connected to each other, said switching unit receiving a direct-current (DC) input voltage and performs a switching operation, the switching operation resulting in a switching output;

a switching drive unit that drives the switching elements alternately turning on and off to perform the switching operation with the same cycle;

an isolation 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;

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, said primary-side series resonant circuit connected to a connecting node of said two switching elements, said primary-side series resonant circuit for producing a first resonant frequency for making the switching unit 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 second resonant frequency;

a secondary-side rectifying and smoothing unit that rectifies the alternating voltage induced in the secondary winding to produce a rectified voltage, and that smoothes the rectified voltage using a secondary-side smoothing capacitor to thereby produce a secondary-side DC output voltage; and

a constant-voltage control unit that performs constant-voltage control of the secondary-side DC output voltage by controlling the switching drive unit according to a level of the secondary-side DC output voltage to thereby vary a switching frequency of the switching unit;

the primary-side series resonant circuit and the secondary-side series resonant circuit forming an electro-magnetically coupled resonant circuit, wherein

the core of the isolation converter transformer has a gap formed in a predetermined position between the primary side and the secondary side, the gap having a length selected so that the electro-magnetically coupled resonant circuit has a unimodal output characteristic with respect to the input of a frequency signal having the switching frequency; and

the first resonant frequency and the second resonant frequency are set so that a required phase shift arises between a primary-side series resonant current flowing through the primary-side series resonant circuit and a secondary-side rectified current flowing through the secondary side of the isolation converter transformer.