An exemplary switching power supply circuit (2) includes a power source (20), a pulse width modulation (PWM) circuit (21), a first switching circuit (22), a second switching circuit (23), a first transformer (25), and a second transformer (26). The first switching circuit includes a first transistor (221) and a second transistor (222). The second switching circuit includes a third transistor (231) and a fourth transistor (232). The first transformer includes a first primary winding (251) and a second primary winding (252), and the second transformer includes a third primary winding (261) and a fourth primary winding (262). The switching circuits are controlled by pulse signals from the PWM circuit. When the first and third transistors are switched on, the power source, the first primary winding, and the first transistor form a first closed loop. The power source, the third primary winding, and the third transistor form a second closed loop.