Patent ID: 7609038

Claim:
A switching power supply with protection function, comprising: a transistor having an input terminal for receiving a direct current voltage, an output terminal, and a control terminal; a switch device having an input terminal, an output terminal for connecting to a load, and a control terminal; a pulse width modulation (PWM) controller having a PWM signal output terminal for outputting a PWM signal to the control terminal of the transistor to turn on or off the transistor so that a square wave signal is produced at the output terminal of the transistor, a feedback signal input terminal connected to the input terminal of the switch device, and a control signal output terminal connected to the control terminal of the switch device; and a filter connected between the output terminal of the transistor and the input terminal of the switch device for filtering the square wave signal from the transistor and outputting an output voltage signal to the input terminal of the switch device, the output voltage signal being fed back to the PWM controller to be compared with a predetermined output voltage; when the voltage of the output voltage signal from the filter is not equal to the predetermined output voltage, the PWM controller does not output a signal at the control signal output terminal thereof and the switch device is in an off state, the PWM controller adjusts the duty ratio of the PWM signal until the voltage of the output voltage signal from the filter is equal to the predetermined output voltage such that the PWM controller outputs a control signal to turn on the switch device.