A switching power circuit comprising:
- a switching unit provided with a plurality of switching devices and performing switching by turning ON and OFF a DC input voltage inputted thereto;
- a primary-side drive unit for performing switching driving so that said plurality of switching devices are alternately turned ON and OFF;
- an insulated converter transformer for transmitting a switching output fed from said switching unit from the primary side to the secondary side thereof, said insulated converter transformer including a primary winding, and a secondary winding having a center-tapped tap output, with a gap length set to be not less than a predetermined value to thereby set the coupling coefficient of said primary winding and said secondary winding to be not more than a predetermined value;
- a primary-side resonance capacitor for forming a primary-side resonance circuit for causing the operation of said switching unit to be of a resonance type, at least by the leakage inductance component of said primary winding of said insulated converter transformer and the capacitance thereof;
- and
- a synchronous rectification circuit having a secondary-side smoothing capacitor connected to the tap output of said secondary winding for obtaining a secondary-side DC output voltage as an end-to-end voltage of said secondary-side smoothing capacitor by performing full-wave rectification of an alternating voltage induced in said secondary winding of said insulated converter transformer and charging said secondary-side smoothing capacitor with the rectified current, wherein
- numbers of turns of said primary winding and said secondary winding are so set that a secondary-side rectified current caused to flow in said synchronous rectification circuit by said full-wave rectification is in a continuous mode, irrespective of variations in the conditions of a load connected to said secondary-side DC output voltage, and wherein
- said synchronous rectification circuit comprises:
- a first field effect transistor connected in series to a point between one of end portions divided by the tap output of said secondary winding and a secondary-side reference potential;
- a second field effect transistor connected in series to a point between the other of said end portions divided by the tap output of said secondary winding and said secondary-side reference potential;
- a first drive circuit for outputting a gate voltage for turning ON said first field effect transistor by detecting, through a resistance device, a secondary winding voltage corresponding to the period of a half wave in which said first field effect transistor should flow a rectified current;
- a second drive circuit for outputting a gate voltage for turning ON said second field effect transistor by detecting, through a resistance device, a secondary winding voltage corresponding to the period of a half wave in which said second field effect transistor should flow a rectified current;
- and
- a first inductor device having a predetermined inductance inserted in series respectively between said one of said end portions divided by the tap output of said secondary winding and said first field effect transistor and between said other of said end portions divided by the tap output of said secondary winding and said secondary field effect transistor.