EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: ocset on November 14, 2017, 09:38:30 am
-
Please confirm that the reason that most <150W Offline Boost PFC designs are done with Boundary conduction mode (BCM) PFC controllers rather than continuous mode (CCM) boost PFC controllers is because the BCM controllers more easily facilitate a faster transient response of the boost PFC stage......and yet still slow enough voltage feedback loop to get a good power factor.
(aside from the obvious advantage of BCM vs CCM which is the lack of reverse recovery in the boost diode)
Its noteable, that CCM PFC stages can be done with a plain inductor, rather than the BCM boost inductors which need the extra winding for the zero cross detection (to tell when the inductor has discharged)......and yet you don’t see CCM PFC stages being used below 150W... in spite of the potentially cheaper inductor......which could even be offtheshelf. A BCM boost inductor must be a more expensive custom wound part.
8)
-
Cost, economy of scale for various components.
Tim
-
Thats about the size of it, a custom inductor is not particularly expensive if you are taking 10-100k quantities, and particularly if you are trying for an overall power system meeting 97% or so efficiency, saving a half watt or so in that diode matters.
There is a really interesting BOM optimisation problem in small switchers as they often have to meet mandatory efficiency standards and where to distribute such losses as you can tolerate has an impact on total BOM cost that is quite distinct from the impact on the PFC or flyback taken in isolation.
Regards, Dan.
-
Power electronics is tradeoff city.
DCM tends to have (compared to CCM)
- reduced switch turn on loss
- no diode reverse recovery loss
- increased switch turn off loss
- increased switch RMS current
- increased capacitor ripple current
- possibly reduced EMC filter size
- smaller inductor
- increased inductor RMS current
- increased inductor core losses
- (and we haven’t even talked about thermal considerations)
So you can see that here are a lot of considerations. It happens that the balance of the pros and cons change with power level. And I think it’s fair to say that CCM might be a good choice for lower power designs but becomes less favourable as power level increases.
Hope this helps explain things.
-
Thanks, i believe you meant "CCM" is better for higer power
-
Oops
Yes. DCM for lower power, CCM for higher power. Sorry.