From: Jim Stockton
Subject: Re: Core gapping techniques and general SMPS magnetics stuff
Date: Sat, 14 Dec 2002 16:20:54 -0600
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You can use layers of tape to figure out required gap,
then gap center leg twice that amount.
Chris Carlen wrote:
> I have received a shipment of Ferroxcube cores for experimenting with
> and making a flyback inductor for my Nixie clock project. I wanted a
> 16mm/8/5 E-core but even though they had some in stock at All-Star
> Magnetics, the rep. convinced me that size is an oddball, and that an
> industry standard size is 19mm/8/5. So I ordered 3c90 and 3c94 cores in
> this size and 8-pin bobbins.
> Unfortunatly, the thing is a little larger than I'd like, but I suppose
> it doesn't matter too much. I'll probably design a general purpose
> 90-350V adjustable flyback supply with max. output power of about
> 10-15W, which seems reasonable for that core. Then I can use it for the
> Nixie as well as future projects, or playing with tubes. I like to make
> all my circuits as general purpose as possible, because making PCBs is
> expensive for a hobbyist, and time consuming anyway. Better to make one
> design cover a lot of bases.
> But it looks like I'll have to gap the core to get it to have enough
> amp-turns for the inductance I'd like, without saturating when being
> used as a flyback inductor.
> Things are finally beginning to click in my head about this magnetics
> stuff, thanks to the EM course. 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!)
> 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.
> Comments appreciated.
> Christopher R. Carlen
> Suse 8.1 Linux 2.4.19