Switched-mode power controller with multi-mode startup

1. A switched-mode power converter, comprising:

a primary side controller circuit configured in a startup ramp mode of operation to generate a fixed switching frequency pulse width modulation (PWM) signal with incrementing duty-ratio value to drive a main-switch that charges an inductive device with stored energy and discharges the stored energy into a capacitor on a secondary side to generate a power converter output voltage;

wherein, based on a comparison of the power converter output voltage with a reference voltage, the primary side controller circuit is configured to stop the incrementing of the duty-ratio value of the PWM signal by exiting the startup ramp mode of operation and to begin a quasi-resonant valley reduction mode of operation during which the primary side controller circuit reduces a number of valleys detected in one or more off-times of the main-switch in one or more respective main-switch switching periods; and

wherein the primary side controller circuit is configured to stop reducing the number of valleys detected per main-switch switching period when a difference between the power converter output voltage and the reference voltage is not greater than a predefined threshold voltage.

2. The switched-mode power converter of claim 1 , wherein for each main-switch switching period the primary side controller circuit reduces the number of valleys detected by one when the primary side controller circuit is configured in the valley reduction mode of operation.

3. The switched-mode power converter of claim 1 , wherein the predefined threshold voltage is is about 150 mV.

4. The switched-mode power converter of claim 1 , wherein the primary side controller circuit is configured to enter a normal mode of operation when the difference between the power converter output voltage and the reference voltage is not greater than the predefined threshold voltage.

5. The switched-mode power converter of claim 1 , further comprising a secondary side controller circuit configured to generate representations of the power converter output voltage and the reference voltage and send the representations to the primary side controller circuit over a communications link.

6. The switched-mode power converter of claim 5 , wherein the secondary side controller circuit is configured to turn-on before the power converter output voltage reaches the reference voltage.

7. The switched-mode power converter of claim 5 , wherein the secondary side controller circuit drives the communications link.

8. The switched-mode power converter of claim 5 , wherein the secondary side controller circuit is configured to generate an encoded digital data stream with digital representations of the power converter output voltage and the reference voltage, and the communications link is a uni-directional high-speed digital data link.

9. The switched-mode power converter of claim 1 , wherein the switched-mode power converter is a quasi-resonant flyback converter.

10. A method of operating a switched-mode power converter, comprising:

a primary side controller circuit generating a fixed switching frequency pulse width modulation (PWM) signal with incrementing duty-ratio value to drive a main-switch when the primary side controller circuit is configured in a startup ramp mode of operation;

the main-switch charging an inductive device with stored energy;

the main-switch discharging the stored energy into a capacitor on a secondary side to generate a power converter output voltage;

based on a comparison of the power converter output voltage with a reference voltage, the primary side controller circuit stopping the incrementing of the duty-ratio value of the PWM signal by exiting the startup ramp mode of operation and beginning a quasi-resonant valley reduction mode of operation;

during the quasi-resonant valley reduction mode of operation, the primary side controller circuit reducing a number of valleys detected in one or more off-times of the main-switch in one or more respective main-switch switching periods; and

stopping the reducing of the number of valleys detected per main-switch switching period by exiting the valley reduction mode of operation when a difference between the power converter output voltage and the reference voltage is not greater than a predefined threshold voltage.

11. The method of claim 10 , comprising, for each main-switch switching period of the primary side controller circuit, the primary side controller circuit reducing the number of valleys detected by one when the primary side controller circuit is configured in the valley reduction mode of operation.

12. The method of claim 10 , wherein the predefined threshold voltage is about 150 mV.

13. The method of claim 10 , comprising the primary side controller circuit entering a normal mode of operation when the difference between the power converter output voltage and the reference voltage is not greater than the predefined threshold voltage.

14. The method of claim 10 , further comprising a secondary side controller circuit generating representations of the power converter output voltage and the reference voltage, and sending the representations to the primary side controller circuit over a communications link.

15. The method of claim 14 , comprising the secondary side controller circuit turning-on before the power converter output voltage reaches the reference voltage.

16. The method of claim 14 , comprising the secondary side controller circuit driving the communications link.

17. The method of claim 14 , comprising the secondary side controller circuit generating an encoded digital data stream with digital representations of the power converter output voltage and the reference voltage, and wherein the communications link is a uni-directional high-speed digital data link.

18. The method of claim 10 , wherein the switched-mode power converter is a quasi-resonant flyback converter.