Switching power supply circuit

A switching power supply circuit comprising:

• switching means formed by half-bridge coupling of two switching devices, for performing switching operation on a direct-current input voltage;
• a converter transformer having a primary winding and a secondary winding formed around a magnetic core having no gap formed therein, for transmitting an output of said switching means obtained in the primary winding to the secondary winding;
• a primary-side series resonant circuit formed at least by a leakage inductance component including the primary winding of said converter transformer and capacitance of a primary-side series resonant capacitor connected in series with said primary winding, for converting the switching operation of said switching devices into current resonance type operation;
• a primary-side partial resonance capacitor connected in parallel with one of said two switching devices, for effecting partial resonance in timing corresponding to a turn-off time of said one of said two switching devices;
• switching driving means for applying a switching driving signal to said two switching devices for switching operation;
• a series circuit connected in parallel with said primary-side series resonant capacitor, and formed at least by a series connection of an auxiliary switching device for performing switching operation so as to have an on period in correspondence with timing of turning off of the switching device with which said primary-side partial resonance capacitor is connected and a capacitor for which more than a predetermined capacitance value is selected;
• and
• direct-current output voltage generating means for receiving an alternating voltage obtained at the secondary winding of said isolation converter transformer as an input and generating a predetermined secondary-side direct-current output voltage.