Electronic – Active Power Factor Correction Operation Mode


I'm trying to design a SMPS power supply with potentially widely varying loads, from say a watt or two to ~150W, and I want to include Active PFC because of the medium power load, as well as to use this as a learning experience for getting familiar with active PFC design.

Looking through the literature, I found the general pros/cons list for different active PFC operation modes:

Discontinuous – Limited in max power output of ~300W, which isn't a significant factor here. The peak current through the inductor is larger, though, so better EMI filtering and more expensive inductors must be used. There's no significant reverse recovery current so the boost diode can be "smaller". In general the actual controller chip is simpler/cheaper.

Continuous – Can achieve high power outputs. The peak currents through the inductor are smaller, so smaller EMI filters and inductors can be used. However, the boost diode needs to have relatively fast reverse recovery times. In general the actual controller chip is more complicated and expensive. As far as I can tell, though, this last point of cost/complexity doesn't seem large (for low-quantities at least).

There are also various sub-modes like critical conduction mode (discontinuous), continuous conduction mode (continuous), average current (continuous)…

I haven't found any reasons why continuous (in particular, average current) modes can't be used effectively at lower powers, or would not be as good (performance wise) compared with a discontinuous mode. Are there any reasons/data why?

Which operation mode should I pursue and why?

Best Answer

My experience with PFC supplies is mostly theoretical, but as general comments about SMPS, I can say a few things:

Continuous mode sometimes have "prettier" waveforms. This might translate to lower harmonics, if you care. That's why discontinuous mode requires better filtering.

"Correct" selection of modes may be less of a concern. The bigger concern is often staying in one mode, because your controls are all designed around it. Mode transitions tend to be more difficult for controllers to handle.

Boost converters (which share a lot with PFC supplies as I understand them) will enter discontinuous mode at low loads. Which implies that a well-designed continuous-mode controller can probably handle discontinuous mode as well. I'd be surprised if anyone marketed a controller that failed at open load!

If it was me, I'd try the average current continuous mode control (which you seem to be interested in). I'd be surprised if it didn't do what you want. The tradeoffs seem mostly to be about cost in components, which seems to not concern you.