Patent ID: 7193868

Claim:
A switching power supply circuit, comprising: a switching unit including a main switching element supplied with a direct-current (DC) input voltage and operable to perform a switching operation; a switching drive unit that drives the main switching element to perform the switching operation; an isolation converter transformer having a primary winding on a primary side and a secondary winding on a secondary side, the primary winding being supplied with a switching output obtained by the switching operation of the switching unit, and the secondary winding having an alternating voltage induced therein by the primary winding; a primary-side parallel resonant circuit including a leakage inductance component of the primary winding and a capacitance of a primary-side parallel resonant capacitor, the primary-side parallel resonant circuit converting operation of the switching unit into a voltage resonant operation; a secondary-side parallel resonant circuit formed by a secondary-side parallel resonant capacitor connected in parallel with the secondary winding, the secondary-side parallel resonant circuit including a leakage inductance component of the secondary winding and a capacitance of the secondary-side parallel resonant capacitor; a secondary-side series resonant circuit formed by a secondary-side series resonant capacitor connected in series with the secondary winding, the secondary-side series resonant circuit including a leakage inductance component of the secondary winding and a capacitance of the secondary-side series resonant capacitor; a secondary-side rectifying and smoothing unit supplied with the alternating voltage induced in the secondary winding and performing a rectifying operation on the alternative voltage to produce a secondary-side DC output voltage; 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 adjust the switching frequency of the switching unit, wherein the coupling coefficient between the primary side and the secondary side of the isolation converter transformer is set so as to obtain loose coupling therebetween so that an electromagnetically coupled resonant circuit formed by at least the primary-side parallel resonant circuit and the secondary-side parallel resonant circuit has a unimodal output characteristic with respect to the input of a frequency signal having the switching frequency; and a resonant frequency of the primary-side parallel resonant circuit, a resonant frequency of the secondary-side parallel resonant circuit and a resonant frequency of the secondary-side series resonant circuit are set so as to obtain a power conversion efficiency above a predetermined level under predetermined load conditions.