Patent ID: 7961481

Claim:
A pulse width modulation (PWM) control circuit, applied to a power converter with a charging capacitor, the PWM control circuit comprising: a PWM signal generator, for generating a PWM signal to control a power switch of the power converter; a first comparator, having a first input terminal and a second input terminal, the first input terminal used for receiving a first reference voltage and the second input terminal used for receiving a sensing voltage, which is proportional to a primary-side current of a transformer in the power converter, wherein when the power switch is turned on and the sensing voltage rises to the level of the first reference voltage, the first comparator outputs a first control signal to the PWM signal generator and then the PWM signal generator stops outputting the PWM signal and the power switch is turned off; and a reference voltage modulator, for outputting the first reference voltage according to a feedback voltage relative to an output voltage of the power converter, wherein the reference voltage modulator comprises: a voltage-to-current converting circuit, for converting the feedback voltage into a first current, the voltage-to-current converting circuit comprising a current mirror circuit for generating a second current to be the same as the first current; a current source, electrically connected to the current mirror circuit; and a first resistor, wherein a third current flows through the first resistor to generate the first reference voltage; wherein the current mirror circuit, the current source, and the first resistor are electrically connected to the first input terminal of the first comparator, and a sum of the second current and a current of the current source equals the third current.