Patent ID: 6839245

Claim:
A switching power supply circuit, comprising: rectifying smoothing means for rectifying a alternating-current power by means of a bridge rectification circuit to obtain a direct-current input voltage, smoothing the direct-current input voltage by means of a smoothing capacitor and outputting the smoothed direct-current input voltage; an isolating converter transformer for transmitting a primary side output to a secondary side thereof, said isolating converter transformer having a gap formed therein so that a required coupling coefficient with which a primary winding and a secondary winding are coupled loosely may be obtained; switching means for switching the direct-current input voltage on and off by means of a switching element and outputting the resulting voltage to said primary winding of said isolating converter transformer; a primary-side resonance circuit formed at least from a leakage inductance component including said primary winding of said isolating converter transformer and a capacitance of a primary side parallel resonance capacitor for causing said switching means to perform operation of the voltage resonance type; voltage generation means for generating a voltage corresponding to a switching output voltage obtained by said primary-side resonance circuit; power factor improving means including a low speed recovery type diode, a high speed recovery type diode and an inductance for feeding back the voltage obtained by said voltage generation means, said low speed recovery type diode being connected between said bridge rectification circuit and said smoothing capacitor, at least said high speed recovery type diode and said inductance being connected in series, the series connection of said high speed recovery type diode and said inductance being connected in parallel to said low speed recovery type diode; active clamp means formed from a series connection circuit of a clamp capacitor and an auxiliary switching element for clamping the output voltage of said primary-side resonance circuit; a secondary-side resonance circuit formed from a leakage inductance component of said secondary winding of said isolating converter transformer and a capacitance of a secondary side resonance capacitor on the secondary side; and direct-current output voltage production means formed by including said secondary-side resonance circuit for receiving and rectifying an alternating voltage obtained by said secondary winding of said isolating converter transformer to produce a secondary-side direct-current output voltage.