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From: "Harry Dellamano"
Subject: Re: Core gapping techniques and general SMPS magnetics stuff
Date: Fri, 20 Dec 2002 09:44:46 -0800
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"analog" wrote in message
> Patrick Lawler, Chris Carlen wrote:
> >> 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.
> > I'll avoid the many posts in this thread discussing magnetic principles,
> > and just give the short formulas I use. The formulas are first-order,
> > and do not take into account core non-linearity (not a problem with
> > ferrites, except above 3000 Gauss :) ), fringing flux (possibly an issue
> > with 'large' gaps). Add your own design margins. They should get you
> > 95% of the way to a solution.
> > 1) Determine the number of turns needed to avoid saturation:
> > N = (L*Ipk*10^8)/(Bsat*Ae)
> > 2) Determine the gap necessary to support the peak current:
> > lg = (0.4*pi*N*Ipk/Bsat)-(le/ur)
> > N is inductor turns
> > L is the desired inductance (Henries)
> > Ipk is the peak inductor current (Amperes)
> > Bsat is the core material saturation level (Gauss)
> > Ae is the core cross-sectional area (cm^2)
> > le is the path length of the core (cm)
> > ur is the relative permeability of the core
> > lg is the desired center-leg gap distance (cm)
> Bravo! That's the the same method I also have used and suggested to
> but with all the proper pesky unit conversion factors included as well as
> the (le/ur) term which is important for very small gaps.
> As I recall, there also should be correction factors for fringing effects.
> These are significant with large gaps and with gaps unshrouded by
> such as typically is the case for outer leg gaps. In either case the flux
> path through air is larger than expected, leading to greater energy
> As I dimly recall, the linear dimensions of the cross sectional gap area
> (which starts out the same as core area, Ae) should be increased by one
> half the gap length to account for flux bulging.
> > If you don't want to grind the center leg, split the value of lg in
> > and put each spacer in the outer legs of the core. I don't worry about
> > gluing the shims in place. I just tape the core halves together
> > so the gap is accurate and the shims don't fall out.
> Did you know, when ac flux is significant in an inductor wound with many
> layers, a distributed gap actually can significantly reduce the proximity
> effect losses in the windings by halving their average distance to the
> However, if outer leg gaps are intolerable, don't despair, for there is a
> clever and perhaps little know technique to avoid proximity losses even
> with a center leg only gapped design. In parallel to the normal, multi-
> layered winding, which will continue to carry the bulk of the dc current,
> a single layer of small wire is first wound on the bobbin. Because it is
> nearest to the gap, this winding will carry almost all of the ac
> current for the core and prevent proximity currents from building in the
> multiple layers of the main winding. One caveat, though, the two parallel
> windings must have *exactly* the same number of turns to avoid generating
> huge circulating currents and rendering the whole exercise pointless.
> Fun stuff, huh? -- analog
Hey Analog, you wouldn't have a spare room in you home that I could move in
and just listen and try to absorb.
Chris is spinning his wheels on basic equations which will only get him to
the first layer of understanding. He has about four more layers to go to get
to your level. I admire Chris for all his fine qualities but he is not going
to get that inductor designed from where he is at now.
The technique that you mentioned to avoid proximity losses is not
applicable in Chris's case. This is an inductor and ac magnetizing current
is the total primary current.
My winding method is to make multi single full layers and wire the layers
in series or parallel for desired results. You should separate your currents
into, Peak, AC, and DC and treat each one individually. If there is a lot of
AC current, the bobbin does not get full of copper due to proximity losses.
Single full winding are very important if different outside vender are
used. It guarantees that every bobbin will be wound exactly the same.
We just had a vendor change a design of a three gap core to a single ground
center gap for production ease. The user was getting mucho failures. When I
informed the magnetics house that was not tolerated he said he was doing
this for twenty years and no one ever complained. The spec tells him very
simply where to place every wire on every layer. Single full layers with no
overlap or crossover wires makes it happen.
Hey Analog, let's talk about driving LEDs in constant voltage mode (old
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