Switching power supply circuit

A switching power supply circuit comprising:

• switching means formed by half-bridge coupling of two switching devices for performing a switching operation on a direct-current input voltage;
• a converter transformer having a primary winding and a secondary winding formed around a magnetic core and having no gap formed therein for transmitting an output of said switching means from the primary winding to the secondary winding;
• a primary-side series resonant circuit having a leakage inductance component including the primary winding of said converter transformer and a 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 a current resonance type operation;
• a primary-side partial resonance capacitor connected in parallel with one of said two switching devices for effecting partial resonance with a 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 causing the switching operation thereof;
• 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 a switching operation so as to have an on period in correspondence with a timing of turning off the switching device with which said primary-side partial resonance capacitor is connected and a capacitor having a capacitance value greater than a predetermined capacitance value;
• 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.