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Toroidal core for high power buck converter
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Drirr:
Hi guys,

Im trying to find sufficient toroidal ferrite core for my project. What I need is 82uH inductance. Current 25A, frequency 120, kHz

So.. I found for example http://www.farnell.com/datasheets/2168095.pdf

Material N87, Al = 2880nH

Al=L / N2 >  17 turns is optimal ?


To avoid saturation I want to calculate B = (u0*N*I) /( 2PI*R)  > B = (u0*17*25) /( 2PI* 0,04195m) = 2,026mT ?? only? did I miss something?

I would like to use some of theese cores:
https://cz.farnell.com/w/c/pasivni-komponenty/emc-rfi-potlaceni/ferity-prislusenstvi-feritu/prl/vysledky?st=N87

Thanks a lot,

David



TimNJ:
If you want to avoid saturation, you need to look to the core material's datasheet.

https://www.tdk-electronics.tdk.com/download/528882/71e02c7b9384de1331b3f625ce4b2123/pdf-n87.pdf

490mT @ 20C, 390mT @ 100C. Your peak calculated flux density needs to be less than one of those values, depending on what temperature you estimate it will run at. I usually design for 350mT as the peak flux density to avoid saturation at high temperature, and to account for manufacturing variation between cores.

If your calculated Bmax for the core size you selected is too high, you need to move to a larger core. You can also consider using a different core material with higher Bmax, but this probably won't help if you are way off. Also consider changing the properties of the converter, i.e. adjust the inductance, if you cannot fit a physically larger core.
Drirr:
@ TimNJ thnaks for reply.

Is there any formula to calculate flux density of toroidal core? I suspect my equation is incorrect, 2mT is too low.
TimNJ:
By the way, if you are trying to design this inductor with an un-gapped toroid, you're going to have a hard time using MnZn ferrite like N87. If you need to use a toroid, you'll almost definitely have to look at powdered cores, like powered iron (cheapest), Sendust (slightly more expensive), or High Flux / MPP (higher current, lower loss, more expensve).

This is because the initial permeability (ui) of MnZn ferrite is too high. Remember that Bmax = ueff*uo*Hmax. A powdered core has a lower permeability so it reaches saturation at a higher current. Similarly, you can use a a ferrite core, but it will almost always need to have an air gap.
TimNJ:
Bmax = (ueff*uo*N*Ipk)/ lc

ueff = effective permeability
uo = free space permeability
N = turns
Ipk = peak current
lc = mean core length
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