Switching power supply circuit

A switching power supply device, comprising:

• a main transformer that has a primary-side main winding and a secondary-side main winding;
• a first main switch that is connected between a high potential terminal of an input direct current voltage source and the primary-side main winding;
• a second main switch that is connected between a low potential terminal of the input direct current voltage source and the primary-side main winding;
• a control circuit configured to output a driving pulse signal from a driving pulse signal output terminal that is configured to synchronize the first and second main switches to perform an ON and OFF driving operation;
• a first rectifying device having a first cathode and a first anode, the first cathode being connected to a first node connected between the first main switch and the primary-side main winding, the first anode being connected to the low potential terminal of the input direct current voltage source;
• a second rectifying device having a second cathode and a second anode, the second anode being connected to a second node connected between the second main switch and the primary-side main winding, the second cathode being connected to the high potential terminal of the input direct current voltage source;
• a synchronous rectifier circuit;
• and
• a positive bias circuit configured to apply a positive bias to a control terminal of one of the first and second main switches so as to turn the one of the first and second main switches ON, wherein
• when the first and second main switches stop a switching operation while an output voltage exists between output terminals, the positive bias circuit applies the positive bias to the one of the first and second main switches.