Thank you guys,
I have one more related problem.
I'm calculating the inductor and have some troubles with it.
As the PFC is supposed to be 1200W (400V/3A output) I used the supplied EXCEL tool and got the value for the inductor - L=350uH, IdcMax=10A. The frequency selected is 65kHz.
For the inductor core, I'm going to use E-E ferrites 3C90 from FERROXCUBE.
As I calcuated the needed core size, it shows up that if the core is E42/20/21 set, then I would need about 3,5-4mm air gap, to prevent it from saturation. For the needed 350uH - N=49;AL=147nH;Bmax=308mT;Igap=3,1mm. If I increase a little the airgap, the Bmax would fall to about 280mT and N~51. That would lower the losses in the core, and bring them to about 0,5W/cm3.
Ok, this gap seems way to long.
If I choose E55/28/21, the gap can go down to 1,3mm, but the N=30, is forcing me to use only one turn on the aux.winding that would feed the PFC IC. If I increase the gap, to lower Bmax, as it would be about 330mT(too high for 65kHz), N goes to 35+, and 390V/35turns < 11V per turn, 2turns are >20V and one is not enough. So what do ou suggest?
The question is, does 3-4mm a tremendously long air gap? Is only one turn in the aux winding healthy? What is the reasonable upper limit for an air gap anyway?
Thank you in advance.