From: John Woodgate
Subject: Re: Core gapping techniques and general SMPS magnetics stuff
Date: Mon, 16 Dec 2002 22:01:44 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Mon, 16 Dec 2002 22:36:12 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Christopher R. Carlen
wrote (in ) about
'Core gapping techniques and general SMPS magnetics stuff', on Mon, 16
>Uh-oh. Now I see the problem. There is no way to know how the
>permeability has changed so as to know what H to shoot for, even though
>I may be able to empirically determine the Al change.
>Wait, the catalog shows a formula for calculating the effective
>permeability for a given gap size, provided the gap isn't too large. No
>criteria are provided for evaluating that, however. One can also back
>out ue from the inductance formula L=N^2 ue u0 A/Le.
I don't know what you mean by 'back out'. The point is that ue isn't any
where near constant, at least for silicon iron. Ferrites may be more
> So one could at
>least do an interative exercise to converge upon a suitable gap size to
>get the flux within bounds.
>Finally, I am interested in these Hanna curves. Where might I find some?
We went through all this here in 1998. Obviously, you weren't paying
Actually, Hanna is really applicable when the AC component of the
induction is small compared with the DC component, and I'm not sure that
applies to your coil.
I once had Hanna's original paper, but I can't find it at present. I
don't know where you will find published Hanna curves for ferrites. But
the paper tells you how to generate your own. IIRC, it's a tedious
process and you need an Owen Bridge or equivalent. No doubt you can do
it with a PIC. (;-) You plot curves of LI^2/V against NI/l for a
succession of gap lengths and then draw a tangent curve (IIRC) to the
original curves. This is your Hanna curve. L = inductance, I = direct
current, V = volume of core, N = number of turns, l = length of magnetic
circuit. To generalize it, you replace the gap length applicable to each
section of the tangent curve by alpha = (gap length/l).
Then you enter the chart with LI^2/V and read off the corresponding NI/l
and the gap length ratio alpha.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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