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Subject: Re: Core gapping techniques and general SMPS magnetics stuff
Date: Thu, 19 Dec 2002 13:37:08 -0800
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On Sat, 14 Dec 2002 21:21:06 GMT, Chris Carlen
>So I will have to gap my cores. How do you folks gap E-cores for
>prototype/low volume applications? Obviously it's not that hard to
>figure out a way, but I'm curious how others do it too. I'll probably
>cut out some shim material, epoxy it to the outer legs, and hold the
>halves together gently in a vice until it all sets up. Then I can soak
>it in acetone later for a day if I want to disassemble it.
I'll avoid the many posts in this thread discussing magnetic principles, and
just give the short formulas I use.
The formulas are first-order, and do not take into account core non-linearity
(not a problem with ferrites, except above 3000 Gauss :) ), fringing flux
(possibly an issue with 'large' gaps). Add your own design margins.
They should get you 95% of the way to a solution.
1) Determine the number of turns needed to avoid saturation:
N = (L*Ipk*10^8)/(Bsat*Ae)
2) Determine the gap necessary to support the peak current:
lg = (0.4*pi*N*Ipk/Bsat)-(le/ur)
N is inductor turns
L is the desired inductance (Henries)
Ipk is the peak inductor current (Amperes)
Bsat is the core material saturation level (Gauss)
Ae is the core cross-sectional area (cm^2)
le is the path length of the core (cm)
ur is the relative permeability of the core
lg is the desired center-leg gap distance (cm)
If you don't want to grind the center leg, split the value of lg in half, and
put each spacer in the outer legs of the core.
I don't worry about gluing the shims in place. I just tape the core halves
together tightly, so the gap is accurate and the shims don't fall out.
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