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Subject: Re: Core gapping techniques and general SMPS magnetics stuff
References: <6hNK9.982$qA3.email@example.com> <3DFC2439.5F2A4EC7@ieee.org> <firstname.lastname@example.org>
NNTP-Posting-Date: Sun, 15 Dec 2002 14:49:31 GMT
Organization: AT&T Broadband
Date: Sun, 15 Dec 2002 14:49:31 GMT
Fred Bartoli, analog, Mike Engelhardt, Chris Carlen wrote:
>>>> So I will have to gap my cores. How do you folks gap E-cores for
>>>> prototype/low volume applications?
>>> Shims work and even, unforturnately, get into production sometimes.
>>> I had a Lambda SMPS with shims in the outer legs. But the fringe
>>> fields interfered with the the scanning electron microscope it was
>>> powering so I swapping out that power supply with one that had used
>>> properly gapped magnetics, i.e., a gap only in the center.
>> A distributed gap (which is better approximated by gapping all three
>> legs) will fringe less into the windings and will generally lead to
>> better performance in flyback applications. Usually a shorted turn
>> around the outside of the core in the form of a band of copper tape
>> will suffice to contain external leakage fields if that is a problem.
>> However, if you do this, be sure to space the band at least two or
>> three gap lengths away from the core in the vicinity of the gap in
>> order to avoid inducing excessive eddy currents in the copper band.
> how far is excessive ?
That would depend on your design goals. Since you went to the trouble
of using the band in the first place, obviously minimizing the magnetic
component's external field is important enough to justify the cost of
adding the band. How much more would it cost to space it away from the
> I've just release a 200W isolated SEPIC PFC.
Nice. What were the design parameters?
> I don't have the figures at hand but IIRC the gap was 1mm inner and
> outer legs with the shorted turn directly wound on the core.
Uh-oh, I once had a problem with an inductor in a design operating in
the low hundreds of kilohertz, but the ac fields were quite high and
the gap was bigger.
> Efficiency was 90/91% and I didn't notice any heating of the shorted
Conductive foil cutting through a normal magnetic field component may
be subject to problematic induced eddy currents regardless of whether
the foil is outside of the core or whether it is on the inside as a
winding or shield (assuming foil thickness is more than a sub fraction
of a skin depth, of course). Spacing foils away from the zones of
heavy field fringing subjects them to smaller normal components of
field. Usually the bobbin does this for inside windings without any
effort/awareness on the part of the designer.
> Maybe I'll check this if I have some time.
With accurate and sensitive enough power measurements you could
carefully compare efficiencies before and after removing the external
foil while leaving everything else the same. If efficiency is not an
issue then I would only confirm that the temperature in the vicinity
of the gap does not rise high enough to interfere with the core's
magnetic properties. If it is getting really hot you might notice gap
specific discoloration of the foil. Alternatively, with the power
supply running, you could carefully press the moistened end of a Q-tip
directly to the foil opposite the gap and see if it sizzles. :)
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