Switching power supply circuit

A switching power supply circuit, comprising:

• a rectifying and smoothing circuit operable to receive an alternating input voltage and to rectify and smooth the alternating input voltage to generate a direct-current input voltage;
• a switching circuit including a switching device operable to receive the direct-current input voltage and to perform a switching operation resulting in a switching output;
• a switching driving unit operable to drive the switching device to perform the switching operation;
• an isolated converter transformer having at least 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 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 circuit 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;
• a secondary side direct-current output voltage generating circuit operable to perform a rectifying operation on the alternating voltage induced in the secondary winding and input to the secondary side direct-current output voltage generating circuit to produce a rectified output, and to smooth the rectified output using a secondary side smoothing capacitor to generate a secondary side direct-current output voltage;
• and
• a constant-voltage control unit operable to perform constant voltage control on the secondary side direct-current output voltage by controlling the switching-driving unit according to a level of the secondary side direct-current output voltage to adjust the switching frequency of the switching circuit, wherein
• a coupling coefficient between the primary side and the secondary side of the isolated converter transform is set so as to obtain proper zero voltage switching in a predetermined load condition range of medium load, and
• a resonant frequency of the primary side parallel resonance circuit and a resonant frequency of the secondary side series resonant circuit are set so as to obtain a certain power conversion efficiency and higher under predetermined load conditions.