Power converter with voltage-selective skip mode entry load compensation

1. A method of operating a power converter, the method comprising:

operating, by a controller, the power converter in a discontinuous conduction mode; and then

operating, by the controller, the power converter in frequency foldback operation; and

entering, by the controller, a skip mode when a load current falls below a skip threshold implemented based on an output voltage of the power converter.

2. The method of claim 1 further comprising:

generating a current setpoint, the current setpoint based on a difference between a setpoint voltage and the output voltage of the power converter;

sensing, during the discontinuous conduction mode, that the current setpoint is below a foldback threshold; and

responsive to the current setpoint falling below the foldback threshold

transitioning to the operating the power converter in the frequency foldback operation.

3. The method of claim 2 , wherein the foldback threshold corresponds to a load current of less than thirty percent (30%) of a full power load of the power converter.

4. The method of claim 1 wherein operating the power converter in the frequency foldback operation further comprises:

reading, by the controller, a signal indicative of output voltage;

calculating, by the controller, a current value based on the signal indicative of output voltage; and

ending each charge mode at the current value lower than a foldback threshold at which began the operating the power converter in the frequency foldback operation.

5. The method of claim 4 wherein reading the signal indicative of output voltage further comprises:

receiving a signal indicative of secondary voltage on a secondary winding of a transformer of the power converter during a discharge mode;

sensing demagnetization of the transformer based on the signal indicative of secondary voltage; and

latching a value indicative of output voltage to be the signal indicative of output voltage,

wherein receiving the signal indicative of secondary voltage further comprises receiving the signal indicative of secondary voltage on the secondary winding of the transformer by way of an auxiliary winding of the transformer.

6. The method of claim 4 wherein calculating the current value further comprises:

calculating a ratio of a nameplate output voltage and the signal indicative of output voltage;

setting the current value based on the ratio.

7. The method of claim 1 , further comprising:

adjusting, by the controller, the skip threshold when the output voltage of the power converter changes.

8. A controller for a power converter, the controller comprising:

a gate terminal, a current-sense terminal, a feedback terminal, and a winding-sense terminal; and

a PWM controller coupled to the gate terminal, the current-sense terminal, the feedback terminal, and the winding-sense terminal, the PWM controller configured to:

operate the power converter in discontinuous conduction mode at power levels above a first threshold;

operate the power converter in frequency foldback operation; and

pause assertions of the gate terminal when a load current falls below a skip threshold implemented by the PWM controller based on an output voltage of the converter.

9. The controller of claim 8 wherein the PWM controller is further configured to:

generate a current setpoint, the current setpoint based on a difference between a setpoint voltage and a signal indicative of output voltage of the power converter sensed on the feedback terminal;

sense, during the discontinuous conduction mode, that the current setpoint is below a foldback threshold; and

responsive to the current setpoint falling below the foldback threshold

transitioning to the operating the power converter to the frequency foldback operation.

10. The controller of claim 9 wherein foldback threshold corresponds to a load current of less than thirty percent (30%) of a full power load of the power converter.

11. The controller of claim 8 wherein when the PWM controller operates the power converter in the frequency foldback operation, the PWM controller is further configured to:

read a signal indicative of output voltage;

calculate an current value based on the signal indicative of output voltage; and

end each charge mode at the current value lower than a foldback threshold at which began the operating the power converter in the frequency foldback operation.

12. The controller of claim 11 wherein when the PWM controller reads the signal indicative of output voltage, the PWM controller is further configured to:

receive, by way of the winding-sense terminal, a signal indicative of secondary voltage on a secondary winding of a transformer of the power converter during a discharge mode;

sense demagnetization of the transformer based on the signal indicative of secondary voltage; and

latch a value indicative of output voltage to be the signal indicative of output voltage.

13. The controller of claim 12 wherein when the PWM controller receives the signal indicative of secondary voltage, the PWM controller is further configured to receive the signal indicative of secondary voltage on the secondary winding of the transformer by way of an auxiliary winding of the transformer.

14. The controller of claim 11 wherein when the PWM controller calculates the current value, the PWM controller is further configured to:

calculate a ratio of a nameplate output voltage and the signal indicative of output voltage;

set the current value based on the ratio.

15. A power converter comprising:

a primary side comprising a primary winding of a transformer, and a primary switch coupled to the transformer and defining a control input;

a secondary side comprising a secondary winding arranged for flyback operation and defining an output voltage, and a secondary rectifier coupled to the transformer;

an auxiliary winding of the transformer defining a sense terminal; and

a controller defining a gate terminal coupled to the control input, a current-sense terminal coupled to the primary switch, a feedback terminal coupled to the output voltage, and a winding-sense terminal coupled to the sense terminal, the controller configured to:

operate the power converter in discontinuous conduction mode at power levels above a first threshold;

operate the power converter in frequency foldback operation; and

pause assertions of the control input when a load current falls below a skip threshold implemented based on a magnitude of the output voltage.

16. The controller of claim 15 wherein the controller is further configured to:

generate a current setpoint, the current setpoint based on a difference between a setpoint voltage and a signal indicative of output voltage of the power converter sensed on the feedback terminal;

sense, during the discontinuous conduction mode, that the current setpoint is below a foldback threshold; and

responsive to the current setpoint falling below the foldback threshold

transition to the operating the power converter to the frequency foldback operation.

17. The controller of claim 16 wherein the foldback threshold corresponds to a load current of less than thirty percent (30%) of a full power load of the power converter.

18. The controller of claim 15 wherein when the controller operates the power converter in the frequency foldback operation, the controller is further configured to:

read a signal indicative of output voltage;

calculate a current value based on the signal indicative of output voltage; and

end each charge mode at the current value lower than a foldback threshold at which began the operating the power converter in the frequency foldback operation.

19. The controller of claim 18 wherein when the controller reads the signal indicative of output voltage, the controller is further configured to:

receive, by way of the winding-sense terminal, a signal indicative of secondary voltage on a secondary winding of the transformer of the power converter during a discharge mode;

sense demagnetization of the transformer based on the signal indicative of secondary voltage; and

latch a value indicative of output voltage to be the signal indicative of output voltage.

20. The controller of claim 18 wherein when the controller calculates the current value, the controller is further configured to:

calculate a ratio of a nameplate output voltage and the signal indicative of output voltage;

set the current value based on the ratio.