From: John Woodgate
Subject: Re: Core gapping techniques and general SMPS magnetics stuff
Date: Sat, 14 Dec 2002 22:50:57 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Sun, 15 Dec 2002 07:32:40 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Chris Carlen
wrote (in <6hNK9.982$qA3.email@example.com
od.itd.earthlink.net>) about 'Core gapping techniques and general SMPS
magnetics stuff', on Sat, 14 Dec 2002:
>I realize now that in a transformer the
>only flux is the magnetizing flux, since the fluxes from the primary and
>secondary currents (excluding the primary magnetizing current) cancel.
>Hence high Al values are good to get magnetizing current down. Am I on
>So my ungapped cores are probably great for bridge or push-pull
>converters and GP medium frequency signal and pulse transformers
>(actually I bought a bunch of higher permeability and higher frequency
>3E5 and 3F3 toroids for making pulse transformers for gate drivers.)
>But in a flyback which I will be using for the Nixie clock, there will
>be high peak flux densities associated with energy storage in the core,
>so I need to use a smaller Al, balanced against acceptable I2R losses,
>and winding size. It seems that if there weren't I2R losses, and size
>and EMI issues, an air core would actually be the best for a flyback.
>(A superconducting air core inductor!)
Well, if it's just a single winding, maybe. But a transformer with an
air core would have far too much leakage inductance.
>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.
That's how you do it physically, but how do you work out what gap
thickness you want? It matters A LOT! The traditional method was to use
'Hanna curves', but I don't think I've ever seen a Hanna curve for a
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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