Control methods for over voltage protection and relevant power controllers

1. A control method for over-voltage protection, suitable for a power converter with a rectifier for rectifying an alternative-current voltage to provide a direct-current input voltage, the control method comprising:

determining whether the direct-current input voltage is in a valley where a local minimum of the direct-current input voltage occurs;

providing a detection voltage representing the direct-current input voltage;

comparing the detection voltage with a predetermined reference voltage; and

providing an OVP signal to stop a power conversion of the power converter when the detection voltage exceeds the predetermined reference voltage and the direct-current input voltage is in the valley.

2. The control method as claimed in claim 1 , wherein the power converter includes an inductor, a load capable of emitting light, and a power switch, connected in series, and the step of determining comprises:

providing a current-sense signal, a representative of a current through the power switch; and

comparing the current-sense signal with a predetermined value; and

after the current-sense signal is less than the predetermined value, determining that the direct-current input voltage is in the valley.

3. The control method as claimed in claim 2 , comprising:

after the current-sense signal continues to be less than the predetermined value for a predetermined period of time, determining that the direct-current input voltage is in the valley.

4. The control method as claimed in claim 1 , wherein the power converter includes an inductor, a load capable of emitting light, and a power switch, connected in series, the power converter further includes a wheel diode with two terminals coupled to the direct-current input voltage and the power switch respectively.

5. The control method as claimed in claim 1 , further comprising:

providing a ramp signal with a slope;

sensing and recording a peak value of the detection voltage; and

determining the slope in accordance with the peak value.

6. The control method as claimed in claim 5 , further comprising:

determining an ON time of a power switch based on the ramp signal and a compensation signal.

7. A power controller suitable for a power converter, which comprises a rectifier for rectifying an alternative-current voltage to provide a direct-current input voltage, the power controller comprising:

a valley detector for determining whether the direct-current input voltage is in a valley;

an over-voltage comparator, for comparing a detection voltage with a predetermined reference voltage, wherein the detection voltage is capable of representing the direct-current input voltage; and

a logic coupled to the over-voltage comparator and the valley detector, for providing an OVP signal to stop a power conversion of the power converter when the detection voltage exceeds the predetermined reference voltage and the direct-current input voltage is in the valley.

8. The power controller as claimed in claim 7 , wherein power converter is a buck converter including a power switch and an inductor, and the valley detector comprises:

a valley comparator for comparing a current-sense signal with a predetermined value;

wherein the current-sense signal is a representative of a current through the power switch.

9. The power controller as claimed in claim 8 , wherein the valley detector further comprises:

a delay time generator, coupled to the valley comparator, for providing an output signal indicating that the direct-current input voltage is in a valley after the current-sense signal has been less than the predetermined value for a predetermined period of time.

10. A power converter, comprising:

a rectifier for rectifying an AC mains voltage to provide a direct-current input voltage and a ground line; and

a power controller, comprising:

a valley detector for determining whether the direct-current input voltage is in a valley; and

an over-voltage comparator, for comparing a detection voltage with a predetermined reference voltage, wherein the detection voltage is capable of representing the direct-current input voltage;

wherein the power controller stops a power conversion of the power converter when the detection voltage exceeds the predetermined reference voltage and the direct-current input voltage is in the valley.

11. The power converter as claimed in claim 10 , further comprising:

a buck converter with an inductor and a power switch connected in series between the direct-current input voltage and the ground line;

wherein the valley detector comprises a valley comparator for comparing a current-sense signal with a predetermined value, and the current-sense signal is a representative of a current through the power switch.

12. The power converter as claimed in claim 10 , wherein the valley detector further comprises a delay time generator coupled to the valley comparator for providing an output signal indicating that the direct-current input voltage is in a valley after the current-sense signal has been less than the predetermined value for a predetermined period of time.