Patent ID: 7683566

Claim:
A fan rotation speed control circuit of a power supply system, applicable for controlling a fan rotation speed according to a loading state outputted by the power supply system, comprising: a waveform generating module, for generating an oscillating waveform signal, wherein the oscillating waveform signal is a triangular wave signal or a saw-tooth wave signal; a voltage buffer module, electrically coupled to the waveform generating module, for buffering the oscillating waveform signal; a current retrieving module, for retrieving a current signal corresponding to the loading state; a rotation speed control signal generating module, electrically coupled to the waveform generating module and the current retrieving module, for comparing the oscillating waveform signal with the current signal, and generating a pulse width modulation (PWM) signal, wherein the rotation speed control signal generating module comprises: an operational amplifier, having a first input terminal, a second input terminal, and an output terminal, wherein the first input terminal receives the current signal, the second input terminal receives the oscillating waveform signal, and the output terminal outputs the PWM signal; a first resistor, electrically coupled between the current retrieving module and the first input terminal; a second resistor, electrically coupled between the first input terminal and a ground terminal; a third resistor, electrically coupled between the voltage buffer module and the second input terminal; a forth resistor, electrically coupled between the second input terminal and the ground terminal; and a first capacitor, electrically coupled between the first resistor and the ground terminal; and a signal amplifying module, electrically coupled to the PWM signal generating module, for amplifying an amplitude of the rotation speed control signal, and outputting the amplified PWM signal to a fan, so as to control the rotation speed of the fan.