Patent ID: 7480159

Claim:
A pulse-width-modulation (PWM) control circuit, being applied to a switching-mode power converter with a primary-side feedback control; the switching-mode power converter including a transformer, a power switch, a current sensing resistor, and the PWM control circuit; the transformer including a primary-side winding, a secondary-side winding, and an auxiliary winding, where a voltage from the auxiliary winding is rectified for outputting a DC feedback voltage; the power switch, connected to the primary-side winding in series, being turned on/off by a PWM signal generated from the PWM control circuit; the current sensing resistor, positioned between the power switch and a ground level, for transforming a current passing through the primary-side winding into a sensing voltage; the PWM control circuit comprising: a sample and hold circuit, for sampling the sensing voltage for a peak value and maintaining the peak value of the sensing voltage at an output terminal of the sample and hold circuit; a transconductor circuit, coupled to the output terminal of the sample and hold circuit, for transforming the peak value of the sensing voltage into a current signal; an error amplifier having two input terminals, one of the input terminals being coupled to the current signal and the feedback voltage and another of the input terminals being coupled to a fixed reference voltage, the error amplifier being utilized for generating an error signal; and a PWM generator, for determining a duty cycle of the PWM signal according to the error signal.