Control circuit for controlling the maximum output current of power converter and method thereof

1. A control circuit of a power converter, comprising:

a feedback circuit, generating a feedback signal in accordance with an output of the power converter;

an output circuit, generating a switching signal in accordance with the feedback signal for regulating the output of the power converter; and

an adaptive clamping circuit, limiting the level of the feedback signal under a first level for a first load condition;

wherein the feedback circuit determines a slew rate of the feedback signal for increasing the level of the feedback signal from the first level to a second level; the adaptive clamping circuit is disabled and the level of the feedback signal can be increased to the second level for a second load condition.

2. The control circuit as claimed in claim 1 , wherein the feedback circuit comprises a timing circuit, the timing circuit determines the slew rate of the feedback signal for increasing the level of the feedback signal from the first level to the second level.

3. The control circuit as claimed in claim 2 , wherein the timing circuit is operated by a frequency compensation network of the feedback circuit.

4. The control circuit as claimed in claim 1 , wherein the switching signal is coupled to switch a transformer of the power converter for regulating the output of the power converter.

5. The control circuit as claimed in claim 1 , wherein the second level is higher than the first level, and the second load condition is heavier than the first load condition.

6. The control circuit as claimed in claim 1 , wherein the feedback circuit is a current-feedback circuit, the feedback signal is a current-loop feedback signal.

7. The control circuit as claimed in claim 1 , wherein the enable or disable of the adaptive clamping circuit is determined by the level of a voltage-loop feedback signal.

8. The control circuit as claimed in claim 7 , wherein the adaptive clamping circuit comprises:

a switch, having a first terminal and a second terminal, wherein the first terminal is coupled to the feedback circuit;

a transistor, coupled between the second terminal of the switch and a ground, and controlled by a reference signal, wherein the level of the feedback signal is limited under the first level for the first load condition when the switch is turned on, and the first level is determined by the reference signal; and

a comparator, comparing the level of the voltage-loop feedback signal with a threshold for controlling the on/off of the switch.

9. The control circuit as claimed in claim 1 , wherein the feedback circuit further comprises a current-feedback loop.

10. The control circuit as claimed in claim 1 , further comprising a voltage-feedback loop.

11. The control circuit as claimed in claim 1 , wherein the power converter is a primary-side controlled power converter; in which no opto-coupler is needed for a feedback loop control.

12. A method for controlling a power converter, comprising:

generating a feedback signal in accordance with an output of the power converter;

generating a switching signal in accordance with the feedback signal for regulating the output of the power converter;

clamping the level of the feedback signal under a first level for a first load condition; and

generating a slew rate for increasing the level of the feedback signal from the first level to a second level;

wherein the level of the feedback signal can be increased to the second level for a second load condition.