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sekhar 26th May 2011 07:19 AM

I am in the process of making a 400W pfc with a smps for amp...
But being a newbie it is hard for me get the calculation right...... help needed
i am in a bit of trouble........ i have calculated the details for the pfc inductor but it gives me 875microhenry and 238 turn .......... which seems absurd..... for a 400w pfc
here is my calculation....

vin(min)= 90V
fsw = 100khz
Bmax = 0.25T
Ae = 0.97cm square
Core used ETD34 ferrocube...
ipk = 1.414 X pout/ 0.95 X vin(min) = 6.615A
ripple current =15% of ipk = 0.99A
Lcritical = 1.414 Xvin(min){(1-(1.414 X vin(min))/vout} / ripple current X fsw = 875 microhenry
no of turns = lcritical X ipk x 10000 / Bmax X Ae X 1000000 = 238

i have been refering to a pdf by metglass and some other online documents.....
kindly tell me what am i doing wrong .............


sekhar 26th May 2011 07:19 AM

pfc chip used UC3854


ChocoHolic 26th May 2011 08:22 AM

Hi Sekhar,
your numbers do not look absurd.
There might be some chances for optimization, but such optimization should based on real measurements. (I.e. if the circuit does not show overshoot effects in the inductor current, during ramp up and during load steps, you may allow higher flux densities, ie. 300mT. But don't move to this direction without carefull analysis of the real beast)
As a starting point your numbers should work.
If you go for continuous 400W from 90V, the inductor heat might become an issue....
You also may consider a SiC diode, because this system is doing hard switching of high voltage devices with 100kHz (Or check with Eva for her magnetic snubbers - well it's easier to choose a SiC diode....)

sekhar 26th May 2011 08:29 AM

u think my calculations r all right . Dont u think 238 turns is way to much...... although poweresims also gives similar results.....


luka 26th May 2011 08:52 AM

also, do you really need from 90v to 265v? I mean if your are doing it for yourself, that range will be far smalled, which makes things more simple

and yes it will be far less then 238 turns

ChocoHolic 26th May 2011 10:44 AM


Originally Posted by sekhar (
u think my calculations r all right . Dont u think 238 turns is way to much...... although poweresims also gives similar results.....


...well, you didn't tell us your output voltage, but for an output voltage slightly above 400V - your calculations would fit.

High number of turns:
Well, high inductance, high current, small Ae... yes, you will need such high number of turns and a large gap.
With respect to the fact that most of the flux in your application will be low frequency and just the ripple is 100kHz, you may consider other core materials like cool Mu, in order to optimize the inductor size.
But don't bother to much. If you need the SMPS for an audio amp, then the average load is much less than the peak load. Thermal design is sufficient if it can handle full power in a rythm of ON/OFF 1sec/2sec.

sekhar 26th May 2011 12:05 PM

in every design i am seeing the pfc inductor does not have more than 50-60 turns ...... then hw am i getting 238 turns what am i doing wrong........??????????


sekhar 26th May 2011 02:16 PM

hey guys help me out here.........................


luka 26th May 2011 02:50 PM

what is your normal input AC?

sekhar 26th May 2011 03:20 PM

200V to 240V


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