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 main switch that is connected between a high potential terminal of an input direct current voltage source and the primary-side main winding;
• a synchronous rectifier circuit including:
• a rectifying switch that is connected to the secondary-side main winding in series to form a rectifying switch series circuit and that is turned ON in synchronization with a turning ON state of the main switch;
• and
• a commutation switch that is connected to the rectifying switch series circuit in parallel and that is turned ON in synchronization with a turning OFF state of the main switch;
• and
• an auxiliary switch circuit that turns the commutation switch OFF, wherein
• when the main switch stops a switching operation while a voltage exists at a first output terminal on a high potential side, the auxiliary switch circuit is configured to turn the commutation switch OFF.