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Subject: Re: Core gapping techniques and general SMPS magnetics stuff
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NNTP-Posting-Date: Fri, 20 Dec 2002 07:16:40 GMT
Organization: AT&T Broadband
Date: Fri, 20 Dec 2002 07:16:40 GMT
Patrick Lawler, Chris Carlen wrote:
>> 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)
Bravo! That's the the same method I also have used and suggested to Chris,
but with all the proper pesky unit conversion factors included as well as
the (le/ur) term which is important for very small gaps.
As I recall, there also should be correction factors for fringing effects.
These are significant with large gaps and with gaps unshrouded by windings,
such as typically is the case for outer leg gaps. In either case the flux
path through air is larger than expected, leading to greater energy storage.
As I dimly recall, the linear dimensions of the cross sectional gap area
(which starts out the same as core area, Ae) should be increased by one
half the gap length to account for flux bulging.
> 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.
Did you know, when ac flux is significant in an inductor wound with many
layers, a distributed gap actually can significantly reduce the proximity
effect losses in the windings by halving their average distance to the gap?
However, if outer leg gaps are intolerable, don't despair, for there is a
clever and perhaps little know technique to avoid proximity losses even
with a center leg only gapped design. In parallel to the normal, multi-
layered winding, which will continue to carry the bulk of the dc current,
a single layer of small wire is first wound on the bobbin. Because it is
nearest to the gap, this winding will carry almost all of the ac magnetizing
current for the core and prevent proximity currents from building in the
multiple layers of the main winding. One caveat, though, the two parallel
windings must have *exactly* the same number of turns to avoid generating
huge circulating currents and rendering the whole exercise pointless.
Fun stuff, huh? -- analog