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Reply-To: "fred bartoli"
From: "fred bartoli"
References: <6hNK9.982$qA3.email@example.com> <3DFC2439.5F2A4EC7@ieee.org> <firstname.lastname@example.org> <3DFC9675.5AE0F26C@ieee.org>
Subject: Re: Core gapping techniques and general SMPS magnetics stuff
Date: Tue, 17 Dec 2002 14:16:15 +0100
X-Newsreader: Microsoft Outlook Express 5.00.2314.1300
Organization: Guest of ProXad - France
NNTP-Posting-Date: 17 Dec 2002 14:16:37 MET
analog a écrit dans le message :
> Fred Bartoli, analog, Mike Engelhardt, Chris Carlen wrote:
> >> 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.
> > Analog,
> > 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?
input = 85-264VAC 50-60Hz
output = 40V, highly pulsed current, the PFC slow voltage loop playing the
role of a huge inductor previously used.
Zero ripple input current sepic structure.
Zero loss turn off snubber commutated by the main mosfet.
80-100KHz (freq modulation over the cycle to spread the EMI spectrum) :
controler = L4981B with some added cycle by cycle MOS peak current managment
because the 4981 scheme that works for boost don't in this structure.
> > 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
> > turn.
> 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.
Of course but this (spacing the foil turn) also allow for more flux leakage.
> > 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. :)
There's no pb.
The worst case temp rise was 35°C. Can't remember where the hot spot were
but the temp was pretty uniform over all the transformer (may a couple of
degrees or so).
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