Method and apparatus for digital, closed-loop control of CrCM switch-mode power converters
A method of setting a synchronous rectifier on-time value includes determining that a time interval has occurred, receiving a number of triangular current mode (TCM) pulses measured during the time interval, and determining a pulse comparison value equal to a number of switching period pulses during the time interval minus the number of TCM pulses during the time interval. The method also includes increasing the synchronous rectifier on-time if the pulse comparison value is greater than or equal to a threshold and decreasing the synchronous rectifier on-time if the pulse comparison value is less than the threshold.
1. A method of setting a synchronous rectifier on-time value, the method comprising:
determining that a time interval has occurred;
receiving a number of triangular current mode (TCM) pulses measured during the time interval;
determining a pulse comparison value equal to a number of switching period pulses during the time interval minus the number of TCM pulses during the time interval;
increasing a synchronous rectifier on-time if the pulse comparison value is greater than or equal to a threshold; and
decreasing the synchronous rectifier on-time if the pulse comparison value is less than the threshold.
2. The method of claim 1 wherein the threshold is zero.
3. The method of claim 1 wherein the increase or decrease in the synchronous rectifier on-time is a function of the pulse comparison value.
4. The method of claim 3 wherein the increase or decrease in the synchronous rectifier on-time increases with the pulse comparison value.
5. The method of claim 1 wherein the increase or decrease in the synchronous rectifier on-time is a function of the threshold.
6. The method of claim 1 wherein the time interval corresponds to a periodic interrupt timer time interval.
7. The method of claim 6 wherein the periodic interrupt timer time interval is independent of a switching frequency.
8. The method of claim 6 wherein the periodic interrupt timer time interval is a function of an AC frequency.
9. The method of claim 1 wherein increasing the synchronous rectifier on-time comprises:
increasing a correction factor; and
multiplying an initial synchronous rectifier on-time by the increased correction factor.
10. The method of claim 9 wherein increasing the correction factor comprises adding a modifier to an initial correction factor.
11. The method of claim 1 wherein decreasing the synchronous rectifier on-time comprises:
decreasing a correction factor; and
multiplying an initial synchronous rectifier on-time by the decreased correction factor.
12. The method of claim 11 wherein decreasing the correction factor comprises subtracting a modifier from an initial correction factor.
13. The method of claim 1 wherein, during a positive AC half-cycle, the number of TCM pulses corresponds to a number of times an inductor current is less than or equal to a negative current threshold.
14. The method of claim 1 wherein, during a negative AC half-cycle, the number of TCM pulses corresponds to a number of times an inductor current is greater than or equal to a positive current threshold.
15. A method for operating a power converter, the method comprising:
comparing, during a first time period of a first AC half-cycle, a first voltage associated with an inductor current to a negative reference value;
generating a first set of pulses corresponding to a number of times that the first voltage becomes less than or equal to the negative reference value;
determining that a number of the first set of pulses is less than a number of switching periods during the first time period; and
increasing a synchronous rectifier conduction time for a first switch;
comparing, during a second time period of a second AC half-cycle, a second voltage associated with the inductor current to the negative reference value;
generating a second set of pulses corresponding to a number of times that the second voltage becomes less than or equal to the negative reference value;
determining that a number of the second set of pulses is equal to the number of switching periods during the first time period; and
decreasing the synchronous rectifier conduction time for the first switch.
16. The method of claim 15 wherein:
the first AC half-cycle comprises a first positive AC half-cycle; and
the second AC half-cycle comprises a second positive AC half-cycle.
17. The method of claim 15 wherein comparing the first voltage comprises using a first programmable digital-to-analog converter (DAC).
18. The method of claim 17 further comprising rescaling the first programmable DAC as a function of input voltage amplitude.
19. The method of claim 15 wherein the first time period is a periodic interrupt timer (PIT) time interval equal to 1/N times a period of the first AC half-cycle.
20. The method of claim 19 wherein N=2 M , wherein M is a positive integer.