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;
determining if the pulse comparison value is greater than or equal to a threshold or less than a threshold;
determining a correction factor;
if the pulse comparison value is greater than or equal to the threshold, increasing a synchronous rectifier on-time by increasing the correction factor and multiplying the synchronous rectifier on-time by the increased correction factor; and
if the pulse comparison value is less than the threshold, decreasing a synchronous rectifier on-time by decreasing the correction factor and multiplying the synchronous rectifier on-time by the decreased correction factor.
2. The method of claim 1 wherein the threshold is zero.
3. The method of claim 1 wherein the correction factor is a function of the pulse comparison value.
4. The method of claim 1 wherein the correction factor is linearly proportional to the pulse comparison value.
5. The method of claim 1 wherein the correction factor is non-linearly proportional to the pulse comparison value.
6. The method of claim 1 wherein the increase or decrease of the synchronous rectifier on-time is a function of the threshold.
7. The method of claim 1 wherein the time interval corresponds to a periodic interrupt timer time interval.
8. The method of claim 7 wherein the periodic interrupt timer time interval is independent of a switching frequency.
9. The method of claim 7 wherein the periodic interrupt timer time interval is a function of an AC frequency.
10. The method of claim 1 wherein increasing the correction factor comprises adding a modifier to an initial correction factor.
11. The method of claim 1 wherein decreasing the correction factor comprises subtracting a modifier from an initial correction factor.
12. The method of claim 11 wherein determining the pulse comparison value comprises use of a programmable digital-to-analog converter (DAC).
13. The method of claim 12 further comprising rescaling the programmable DAC as a function of input voltage amplitude.
14. The method of claim 1 wherein receiving the number of TCM pulses comprises measuring a time-dependent inductor current value relative to a threshold.
15. The method of claim 14 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.
16. The method of claim 14 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.