Patent ID: 7301785

Claim:
A switching power supply circuit, comprising: rectifying and smoothing means for obtaining a direct-current input voltage by rectifying and smoothing an alternating input voltage; switching means including a switching device supplied with the direct-current input voltage and performing a switching operation; switching-driving means for driving the switching device to perform the switching operation; an isolated 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 means 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 means into a voltage resonant type operation; 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; secondary side direct-current output voltage generating means for performing a rectifying operation on the alternating voltage induced in the secondary winding and input to the secondary side direct-current output voltage generating means to produce a rectified output, and for smoothing the rectified output using a secondary side smoothing capacitor to generate a secondary side direct-current output voltage; and constant-voltage control means for performing constant voltage control on the secondary side direct-current output voltage by controlling the switching-driving means according to a level of the secondary side direct-current output voltage to adjust the switching frequency of the switching means, wherein a coupling coefficient between the primary side and the secondary side of the isolated converter transformer is set at a loose coupling where an interaction between the primary side parallel resonant circuit and the secondary side series resonant circuit is weakened, and a resonant frequency of the primary side parallel resonant circuit is set to approximately two times of a resonant frequency of the secondary side series resonant circuit so as to obtain proper zero voltage switching over an entire range of load power.