From: Chris Carlen
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Subject: Core gapping techniques and general SMPS magnetics stuff
Date: Sat, 14 Dec 2002 21:21:06 GMT
NNTP-Posting-Date: Sat, 14 Dec 2002 13:21:06 PST
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.
Christopher R. Carlen
Suse 8.1 Linux 2.4.19