# Another PFC question

Discussion in 'Electronic Design' started by [email protected], Aug 5, 2005.

1. ### Guest

Hi all.
I am working on a inductor for a boost PFC controller.
How do I calculate what the flux density in the core will be , to
make sure I avoid saturation. The frequency will be 100Khz.
Power level would be 300W. AC is 220V.
The formula for flux density is B = (E* 10^8) / 4.44.N.A.F
The value of 4.44 I assume I would change to 4.0 for a square wave , as
the inductor is bieng switched at 100khz.
The core I have is an MPP core (CM400060) with a Ae of 1.07cm^2 which I
assume is A.
What value should I use for E. Would it be 220 * 1.414 ?
Say there are 105 turns on the toroid.
That would give me a value of 690 odd gauss (69mT ?) . Does this look
correct?
According to the litrature I have , the MPP cores saturation flux
density is about 8000 gause(0.8T). If my calculations are correct this
core is WAY over spec , and I should be able to reduce it significantly
, but I'm sure I'm missing something here.I would like to use a powder
iron core as this is cheaper.
Any help appreciated.
Cheers
Rob

You have to look at magnetic losses if there is significant AC flux
density. The equation B=E/(4.44*N*A*f) is more precisely peak AC flux
(Tesla) in the core of cross section, A (meters squared), for sine wave
AC applied peak voltage, E, at frequency f (Hz), and N turns of wire.
You get graphs of core losses vs freq vs peak flux from core
manufacture. Core loss prediction is complicated due to fact the PFC
inductor voltage varies across the mains half cycle. In addition the
waveform is PWM that also varies across mains cycle. Assuming this is a
continuous current mode boost converter, then you also have to consider
DC bias flux. Normally for iron core one aims for maximum DC flux to
cause permeability to decrease no more than %50 of initial permeability.
Again refer to manufacture graphs.