BCM boost PFC is very poor due to high F(sw) at lighter loads

[treez]

Hi
Do you know when the BCM Boost PFC is going to be shown up for its poorness?

The big problem, not discussed in the datasheets, is the increase in switching frequency as load lightens. The attached shows the frequency at mains peak for 200W load, and 25W load…39kHz to 314kHz !!!

At the higher frequencies, the BCM Boost PFC wont be able to valley switch as well as at max load, so it will suffer high turn off losses at these high frequencies.
So the BCM Boost PFC is OK if you are going to be on near maximum load most of the time…otherwise…..bad news.
None of the OffTheShelf BCM Boost PFC controllers limit the switching frequency below 400kHz.

 

[Easy peasy]

this is a bit of an issue with some chips - the designers have no idea about reasonable frequencies at light loads - many chips go to 500kHz - which is stupid - designing one's own discrete solutions always works best ....

 

[treez]

I think it speaks volumes, that the PFS7623 PFC controller, is meant for PFC'd converters of 70W to 100W, and is in Continuous Conduction Mode.

HiperPFS-4 Data Sheet

PFC Controller with Integrated 600 V MOSFET Op

led-driver.power.com

do you think the BCM PFC bubble is bursting.

 

[Easy peasy]

CCM is fine - until you have to do the RFI ...

 

[treez]

Thanks yes, i think we would all tend to agree with that....except of course at light load where the CCM PFC would be in DCM anyway.....and the BCM would be switching at a few hundred kHz, and probably not getting nice "valley" switching like that.
...Though actually, the PFC is a non isolated stage, so there is less scope for Common mode emissions to wreak havoc with Boost PFC. (Compared to an isolated transformer based stage).
In my view, the best thing that can be done with a BCM PFC controller is convert it to a constant off time (C.O.T) controller, and just arrange the boost inductor etc so that you are just inside DCM at max load, minimum vin......then you will be in DCM during the RFI test....and since you have chosen C.O.T, you will not get such ridiculously high fsw at light load.

The L6564 BCM PFC controller's literature actually gives instructions on how to convert it to a C.O.T. controller (!).....they can see the bubble is bursting i think.
Of course, if you're on Maximum load all the time...then i would admit BCM PFC is best.