Patent ID: 8213196

Claim:
A power supply circuit comprising: a primary direct current (DC) voltage source configured for providing a primary DC voltage, a filter capacitor configured for converting the primary DC voltage to the first DC voltage, a protecting circuit comprising a first switching element, a second switching element, a first diode, a first bias resistor, and a second bias resistor, the first switching element comprising a control terminal, a first conduction terminal, and a second conduction terminal, the second switching element comprising a control terminal, a first conduction terminal, and a second conduction terminal, and a pulse width modulation (PWM) circuit comprising a first terminal and a second terminal, the first terminal capable of receiving the first DC voltage via the first switching element to thereby enable the PWM circuit, the second terminal capable of receiving a second DC voltage, the second DC voltage capable of being provided to the second terminal of the PWM circuit via the second switching element, the primary DC voltage source capable of being grounded via the first bias resistor and the second bias resistor in series, and also being connected to the controlling terminal of the first switching element and an anode of the first diode, a cathode of the first diode capable of being grounded via the filter capacitor, and also being connected to the first terminal of the PWM circuit via the first conduction terminal and the second conduction terminal of the first switching element, the controlling terminals of the first switching element and the second switching element being connected to a node between the first bias resistor and the second bias resistor, the first conduction terminal of the second switching element capable of receiving the second DC voltage, and the second conduction terminal of the second switching element being connected to the second terminal of the PWM circuit, wherein when the power supply circuit is turned off, the first switching element is switched off.