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From: "Harry Dellamano"
Subject: Re: Core gapping techniques and general SMPS magnetics stuff
Date: Mon, 16 Dec 2002 09:29:04 -0800
Organization: Posted via Supernews, http://www.supernews.com
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"Christopher R. Carlen" wrote in message
> Phantom wrote:
> > 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.
> >>Comments appreciated.
> For some strange reason, this core doesn't have clips. Should I get
> clips even if none are shown as available accessories with this core in
> the Ferroxcube catalog?
> Christopher R. Carlen
> Principal Laser/Optical Technologist
> Sandia National Laboratories CA USA
Kiss off the clips and the center gap. Use tape, as mentended in this
thread, to wrap the cores together for testing and pre production. This
allows you to fine tune the gap with spacers on all three legs. One gap
size fits all with ungapped cores with spaces. Why buy specially gapped
cores? In production the decision is made weather to use clips or epoxy the
gapped cores together.
Remember that in flyback topology the magnetics is an inductor and if it
has a secondary it is a coupled inductor, never a transformer therefore the
secondary current does not cancel primary current because they never flow at
the same time. The primary will saturate as a function of the L*I product.
The gap is introduced to reduce the inductance so that the current can be
increased to meet your L*I product needs which is equal to volt seconds
across the primary winding. You will calculate L and I by circuit
requirements and then control it by varying turns and gap to meet your L*I
requirement. L=K*N^2 so it is a intuitive process to get an optimum design
It turns out that spreading the gap across three legs instead of (x2) on
the center leg, that the AC resistance of the winding is greatly reduced and
the inductor is more efficient. 95% of the time the external fringing flux
will not cause EMI to the rest of the system. If it does, rotate thre core
or use the external shield as pointed out elsewhere in the thread.
good luck and don't let the smoke out
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