Dummy load controllers and control methods for power supplies capable of quickly lowering output voltage at output node

1. A control method suitable for a power converter with a primary winding and a secondary winding isolated from each other, wherein the secondary winding is capable of de-energizing to provide an output voltage at an output node to power a load, the control method comprising:

sensing a winding voltage across the secondary winding;

providing a non-switching time based upon the winding voltage;

checking if the non-switching time exceeds a predetermined reference time;

comparing the output voltage with a predetermined voltage; and

providing a discharge current to drain from the output node and to lower the output voltage if the non-switching time exceeds the predetermined reference time and the output voltage exceeds the predetermined voltage.

2. The control method of claim 1 , wherein the predetermined reference time is a second reference time and the discharge current is a second discharge current, the control method further comprising:

providing a first discharge current to drain from the output node and to lower the output voltage if the non-switching time exceeds a first reference time and is less than the second reference time.

3. The control method of claim 1 , wherein the second discharge current is larger than the first discharge current.

4. The control method of claim 1 , wherein, when the load is a light load or no load, the output voltage is regulated to be an output rating voltage less than the predetermined voltage.

5. The control method of claim 4 , wherein the primary winding is at a primary side and the secondary winding is at a secondary side, the control method comprising:

regulating the output voltage at the output rating voltage;

regulating the output voltage at a charging voltage higher than the output rating voltage;

providing a PWM signal to control a power switch connected in series with the primary winding; and

making a cycle time of the PWM signal longer than the predetermined reference time if the load is determined to be a light load or no load.

6. The control method of claim 5 , wherein the predetermined reference time is a second reference time and the discharge current is a second discharge current, the control method further comprising:

providing a first discharge current to drain from the output node and to lower the output voltage if the non-switching time exceeds a first reference time and is less than the second reference time; and

making another cycle time of the PWM signal longer than the first reference time and less than the second reference time when the output voltage is regulated at the output rating voltage and the load is at light load or no load.

7. The control method of claim 1 , further comprising:

comparing the winding voltage with a reference voltage to provide a comparison result;

wherein the non-switching time is a duration when the comparison result stays at a first logic value.

8. The control method of claim 7 , comprising:

continuing providing the discharge current for a predetermined discharge time.

9. The control method of claim 8 , wherein the predetermined discharge time is counted by using the comparison result as a clock.

10. The control method of claim 1 , further comprising:

stopping the discharge current if the output voltage is less than the predetermined voltage.

11. A dummy-load control unit, suitable for a power supply with a primary winding and a secondary winding isolated from each other, wherein the secondary winding is configured to de-energize and to build an output voltage at an output node for powering a load, the dummy-load control unit comprising:

a detection node coupled to the secondary winding;

a first comparator with a first input coupled to the detection node, a second input coupled to a reference voltage and an output providing a comparison result;

a debounce circuit coupled to the first comparator for turning on a timeout signal when the comparison result has stayed in a first predetermined value for a predetermined reference time;

a second comparator for comparing the output voltage with a predetermined voltage; and

a logic coupled to the debounce circuit and the second comparator;

wherein when the output voltage exceeds the predetermined voltage and the timeout signal is turned on, the logic provides a discharge path for discharging the output node to lower the output voltage.

12. The dummy-load control unit of claim 11 , wherein the predetermined reference time is a second predetermined reference time, and the timeout signal is a second timeout signal, the dummy-load control unit further comprising:

a timeout detector coupled between the debounce circuit and the first comparator, for turning on a first timeout signal when the comparison result has stayed in the first predetermined value for a first predetermined reference time;

wherein when the first timeout signal is turned on, a first discharge current is provided to discharge the output node.

13. The dummy-load control unit of claim 12 , wherein the discharge path provides a second discharge current for discharging the output node, and the second discharge current is larger than the first discharge current.

14. The dummy-load control unit of claim 12 , wherein the debounce circuit provides the second time output signal based on the first timeout signal.

15. The dummy-load control unit of claim 12 , further comprising:

a counter with a clock input receiving the comparison result;

wherein when the comparison result has stayed in the first predetermined value for the first predetermined reference time, the counter is reset.

16. The dummy-load control unit of claim 15 , wherein when a count result of the counter exceeds a predetermined number, the first timeout signal is turned off.

17. The dummy-load control unit of claim 11 , wherein when the output voltage is less than the predetermined voltage, the logic disconnects the discharge path that discharges the output node.