| 摘要 |
For many applications an SMPS is designed to operate in boundary conduction mode. As the load decreases the switching frequency increases, and so the concept of valley skipping may be used in which the switching frequency is clamped, by delaying to turn on the time of the active switch, for an integral number of cycles of a resonant circuit in the SMPS. With further reduction of the load, additional valleys may be skipped. However, each change in the number of valleys skipped results in a step in the input current that is drawn, distorting the ideal mains sine wave, thereby increasing unwanted harmonics. A control method is disclosed, which reduces the steps: instead of a constant on-time for the switch, the duration of the on-time is increased each time an additional valley to be skipped. The predetermined increase may be either a fixed fractional increase or a further additional increment; it may be determined by a small regulation loop that multiplies the on-time from the main loop with a factor equal to the ratio between measured period time and the sum of primary and secondary stroke times. |
