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Subject: Re: Core gapping techniques and general SMPS magnetics stuff
Date: 16 Dec 2002 05:42:22 -0600
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On Sat, 14 Dec 2002 21:21:06 GMT, Chris Carlen
>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
>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.
You should have gotten metal clips to hold the core halves together
from the same place that supplied the core halves. I certainly
wouldn't go to all the trouble to glue things together for
prototyping. Just put the shims in and wrap the two core halves with
mylar or kapton tape.
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