Power conversion with external parameter detection

1. A method comprising:

driving an electronic switch in a switched-mode power converter in successive drive cycles, wherein driving the electronic switch in each of the successive drive cycles comprises switching on the electronic switch for an on-period and subsequently switching off the electronic switch for an off-period;

measuring an operation parameter of the switched-mode power converter during the on-periods in the successive drive cycles, and storing the operation parameter measured in the on-period if a duration of the on-period met a predefined criteria; and

forcing the on-period of at least one of the successive drive cycles to meet the predefined criteria if the operation parameter has not been stored for a predefined number of drive cycles of the successive drive cycles, or for a predefined time duration.

2. The method of claim 1 , wherein the on-period meets the predefined criteria if it is longer than a predefined time period.

3. The method of claim 1 , wherein the operation parameter is an input voltage of the switched-mode power converter.

4. The method of claim 3 ,

wherein the switched-mode power converter comprises a transformer with an auxiliary winding, and

wherein evaluating the operation parameter comprises evaluating a voltage across the auxiliary winding.

5. The method of claim 4 , wherein evaluating the voltage across the auxiliary winding comprises:

driving a current into a tap of a voltage divider connected in parallel with the auxiliary winding such that a voltage across a first resistor of the voltage divider has a predefined voltage, and

measuring the current as a representation of the voltage across the auxiliary winding.

6. The method of claim 1 , wherein the one of the predefined number of drive cycles and the predefined time duration is fixed.

7. The method of claim 1 , wherein the one of the predefined number of drive cycles and the predefined time duration is variable such that it is changed after forcing the on-period of a drive cycle to be longer than a duration threshold.

8. The method of claim 1 , further comprising: in at least some of those of the successive drive cycles in which the on-period is not forced to be longer than a duration threshold, adjusting the duration of the on-periods based on an output signal of the switched-mode power converter.

9. The method of claim 8 , wherein adjusting the duration of the on-periods based on an output signal of the switched-mode power converter comprises: switching off the electronic switch when a predefined maximum duration of an on-period has been reached.

10. The method of claim 1 , wherein the switched-mode power converter comprises one of a flyback converter topology, and a boost converter topology.

11. The method of claim 1 ,

wherein the switched-mode power converter comprises a transformer with an auxiliary winding, and

wherein the method further comprises:

evaluating a voltage across the auxiliary winding and starting the on-periods based on this evaluation.

12. A drive circuit for a switched-mode power converter, wherein the drive circuit is configured:

to drive an electronic switch of the switched-mode power converter in successive drive cycles such that in each of the successive drive cycles the electronic switch is switched on for an on-period and subsequently switched off for an off-period,

to measure an operation parameter of the switched-mode power converter during the on-periods in the successive drive cycles, and store the operation parameter measured in an on-period if a duration of the on-period met a predefined criteria, and

to force the on-period of at least one of the successive drive cycles to meet the predefined criteria if the operation parameter has not been stored for a predefined number of drive cycles of the successive drive cycles, or for a predefined time duration.

13. The drive circuit of claim 12 , wherein the on-period meets the predefined criteria if it is longer than a predefined time period.

14. The drive circuit of claim 12 , wherein the operation parameter is an input voltage of the switched-mode power converter.

15. The drive circuit of claim 14 , wherein the drive circuit, in order to evaluate the operation parameter, is configured to evaluate a voltage across an auxiliary winding of a transformer.

16. The drive circuit of claim 15 , wherein the drive circuit is configured:

to drive a current into a tap of a voltage divider connected in parallel with the auxiliary winding such that a voltage across a first resistor of the voltage divider has a predefined voltage, and

to measure the current as a representation of the voltage across the auxiliary winding.

17. The drive circuit of claim 12 , wherein the one of the predefined number of drive cycles and the predefined time duration is fixed.

18. The drive circuit of claim 12 , wherein the drive circuit is configured to vary one of the predefined number of drive cycles and the predefined time duration such that it changes after forcing the on-period of a drive cycle to be longer than a duration threshold.

19. The drive circuit of claim 12 , further configured in at least some of those of the successive drive cycles in which the on-period is not forced to be longer than a duration threshold, to adjust the duration of the on-periods based on an output power of the switched-mode power converter.

20. The drive circuit of claim 19 , further configured to switch off the electronic switch when a predefined maximum duration of an on-period has been reached.

21. The drive circuit of claim 12 , further configured to evaluate a voltage across an auxiliary winding of a transformer and to start the on-periods based on this evaluation.

22. A switched-mode power converter, comprising:

an electronic switch; and

a drive circuit, wherein the drive circuit is configured

to drive the electronic switch of the switched-mode power converter in successive drive cycles such that in each of the successive drive cycles the electronic switch is switched on for an on-period and subsequently switched off for an off-period,

to measure an operation parameter of the switched-mode power converter during the on-periods in the successive drive cycles, and store the operation parameter measured in the on-period if a duration of the on-period met a predefined criteria, and

to force the on-period of at least one of the successive drive cycles to meet the predefined criteria if the operation parameter has not been stored for a predefined number of drive cycles, or for a predefined time duration.

23. The switched-mode power converter of claim 22 , wherein the switched-mode power converter comprises one of a flyback-converter topology, and a boost converter topology.

24. A method comprising:

driving an electronic switch in a switched-mode power converter in successive drive cycles, wherein driving the electronic switch in each of the successive drive cycles comprises switching on the electronic switch for an on-period and subsequently switching off the electronic switch for an off-period;

in a first operation mode, adjusting the on-period based on an output signal of the power converter;

in a second operation mode, forcing the on-period to be higher than a duration threshold level; and

entering the second operation mode for at least one of the successive drive cycles if the on-period in the first operation mode has not met a predefined criteria for a predefined number of drive cycles of the successive drive cycles, or a predefined time period.

25. The method of claim 24 , further comprising: evaluating an operation parameter of the switched-mode power converter in the on-periods.

26. The method of claim 25 , wherein the operation parameter is an input voltage of the switched-mode power converter.