Power controller and control method with functions of soft burst-in and soft burst-out

1. A control method in use of a power converter comprising a power switch and an inductive device, wherein the power switch controls an inductor current through the inductive device, the control method comprising:

providing a current-sense signal representing the inductor current;

determining an ON time of the power switch in response to a feedback signal and a saw-wave signal, to operate the power converter in a non-power-saving mode, wherein the feedback signal is generated according to an output voltage of the power converter; and

operating the power converter in a power-saving mode to perform first steps comprising:

activating pulse-width-modulation for a burst time, wherein the burst time comprises a soft burst-in time and a soft burst-out time;

deactivating the pulse-width-modulation for a pause time;

comparing the current-sense signal and a current-limiting signal to determine the ON time;

increasing the current-limiting signal during the soft burst-in time; and

decreasing the current-limiting signal during the soft burst-out time.

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

making the ON time not less than a minimum ON time when operating in the non-power-saving mode; and

making the ON time not more than the minimum ON time when operating in the power-saving mode.

3. The control method as claimed in claim 1 , wherein a summation of the burst time and the pause time is a predetermined period.

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

stepwise increasing the current-limiting signal during the soft burst-in time.

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

stepwise decreasing the current-limiting signal during the soft burst-out time.

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

checking whether to exit the power-saving mode, to perform second steps comprising:

activating the pulse-width-modulation; and

increasing the current-limiting signal until the current-limiting signal reaches a maximum value.

7. The control method as claimed in claim 6 , wherein the second steps further comprise:

decreasing the current-limiting signal before increasing the current-limiting signal.

8. The control method as claimed in claim 6 , wherein the maximum value corresponds to a count generated from a counter.

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

providing a maximum signal to limit the current-limiting signal when operating the power converter in the power-saving mode.

10. A power controller in use of a power converter comprising an inductive device and a power switch, wherein the power switch controls an inductor current through the inductive device, the power controller comprising:

a voltage-mode controller determining an ON time of the power switch in response to a feedback signal and a saw-wave signal, wherein the feedback signal is generated according to an output voltage of the power converter; and

a current-mode controller determining the ON time of the power switch in response to a current-sense signal and a current-limiting signal, and determining a soft burst-in time and a soft burst-out time in response to the feedback signal, wherein during the soft burst-in time the current-limiting signal increases, during the soft burst-out time the current-limiting signal decreases, and the current-sense signal represents the inductor current,

wherein the current-mode controller starts the soft burst-in time and the soft burst-out time in response to the feedback signal and a triangular-wave signal.

11. The power controller as claimed in claim 10 , wherein the soft burst-in time ends when the current-limiting signal increases to exceed a maximum signal.

12. The power controller as claimed in claim 10 , wherein the current-mode controller comprises:

a hiccup-frequency controller for comparing the feedback signal and the triangular-wave signal to generate a burst-in pulse and a burst-out pulse;

an up/down counter using a gate signal as a clock to provide a count, wherein the gate signal controls the power switch;

a digital-to-analog converter for converting the count into the current-limiting signal; and

an up/down controller controlling the up/down counter in response to the burst-in pulse and the burst-out pulse.

13. The power controller as claimed in claim 12 , wherein the hiccup-frequency controller provides the burst-in pulse when the feedback signal exceeds the triangular-wave signal, and provides the burst-out pulse when the feedback signal is less than the triangular-wave signal.

14. The power controller as claimed in claim 12 , wherein the up/down controller stops the up/down counter counting if the current-limiting signal is not less than a maximum signal.

15. The power controller as claimed in claim 10 , wherein the voltage-mode controller makes the ON time not less than a minimum ON time, and the ON time is not more than the minimum ON time when the power controller operates the power converter in a power-saving mode according to the feedback signal.

16. The power controller as claimed in claim 15 , wherein the power controller determines to exit the power-saving mode according to the feedback signal, and the current-limiting signal increases when exiting the power-saving mode.

17. The power controller as claimed in claim 10 , wherein the power converter has a primary side and a secondary side, the power controller is used in the primary side, and the power controller determines to operate the power converter in a power-saving mode in response to a power saving signal in the secondary side.

18. The power controller as claimed in claim 17 , wherein a burst signal in the primary side is generated in response to the power saving signal, the power controller provides a maximum signal corresponding to the burst signal, and the maximum signal limits the current-limiting signal.