ACTIVE-CLAMP CIRCUIT FOR QUASI-RESONANT FLYBACK POWER CONVERTER

1 . An active clamp circuit for a QR flyback power converter, comprising:

an active-damper connected to a primary winding of a power transformer of the QR flyback power converter in parallel;

a high-side transistor driver coupled to drive the active-clamper;

a charge-pump circuit coupled to the high-side transistor driver to provide a power supply to the high-side transistor driver in accordance with a voltage source; and

a control circuit generating a control signal coupled to control the high-side transistor driver;

wherein the control signal is generated in response to a PWM signal and an input voltage of the QR flyback power converter.

2 . The active clamp circuit as claimed in claim 1 , wherein the active-clamper comprises:

a capacitor coupled to a first terminal of the primary winding of the power transformer; and

a power transistor coupled to a second terminal of the primary winding of the power transformer and connected to the capacitor in series.

3 . The active clamp circuit as claimed in claim 1 , wherein the PWM signal is coupled to control a main-power transistor of the QR flyback power converter for the regulation; the main-power transistor is coupled to switch the primary winding of the power transformer.

4 . The active clamp circuit as claimed in claim 1 , wherein the control signal is turned on once the PWM signal is turned off.

5 . The active clamp circuit as claimed in claim 1 , wherein the pulse width of the control signal is generated in response to the pulse width of the PWM signal and the amplitude of the input voltage.

6 . The active clamp circuit as claimed in claim 1 , wherein the charge-pump circuit comprises:

a diode coupled to the voltage source; and

a charge-pump capacitor coupled to the diode in series;

wherein the charge-pump capacitor is connected to the high-side transistor driver.

7 . The active clamp circuit as claimed in claim 1 , wherein the control circuit is a linear-predict circuit that generates the control signal in accordance with the pulse width of the PWM signal and the amplitude of the input voltage; the pulse width of the control signal is proportional to the pulse width of the PWM signal and the amplitude of the input voltage.

8 . The active clamp circuit as claimed in claim 1 , wherein the control signal is turned off before the power transformer is fully demagnetized.

9 . The active clamp circuit as claimed in claim 1 , wherein the voltage source is generated by an auxiliary winding of the power transformer.

10 . The active clamp circuit as claimed in claim 1 , wherein the control signal is generated after a delay time when the PWM signal is turned off.

11 . The active clamp circuit as claimed in claim 1 , wherein the control circuit comprises:

an input voltage-detection circuit coupled to receive a sense signal for generating a voltage signal, in which the sense signal is correlated to a high voltage signal of a main-power transistor of the QR flyback power converter, the main-power transistor is coupled to switch the primary winding of the power transformer;

a voltage-to-current converter receiving the voltage signal to generate a charge current;

a capacitor charged by the charge current for generating a charge signal when the PWM signal is on-state;

a discharge current discharging the capacitor when the PWM signal is off-state; and

a comparator receiving the charge signal to compare with a threshold voltage for switching off the control signal when the charge signal is lower than the threshold voltage.