From: "Bill Sloman"
Subject: Re: Core gapping techniques and general SMPS magnetics stuff
Date: Sun, 15 Dec 2002 13:02:11 +0100
Organization: Planet Internet
NNTP-Posting-Date: 15 Dec 2002 12:02:16 GMT
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
"John Woodgate" wrote in message
> I read in sci.electronics.design that Chris Carlen
> wrote (in <6hNK9.982$qA3.firstname.lastname@example.org
> 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
> >track here?
> >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
The *only* time I've seen a Hanna curve was in the Philips book on ferrite
Bill Sloman, Nijmegen