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From: "Bill Sloman"
Subject: Re: switching regulator & EMI
Date: Sat, 19 Oct 2002 00:20:40 +0200
Organization: Planet Internet
NNTP-Posting-Date: 18 Oct 2002 22:20:32 GMT
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
"Harry Dellamano" wrote in message
> "Dave VanHorn" wrote in message
> > Not likely.. Even if you try HARD to screw it up.
> > Switchers are much better behaved than that.
> > Toroid inductors have almost all their field enclosed, so it's truly
> > imagine how you'd get there.
> > Do pay attention to where the high currents are flowing though, minimize
> > those loops, and don't let anything connect inside the loop, even though
> > the same net.
> Toroids have their flux (B field) in a closed loop as do most cores (E,
> Pot, PQ, RM, EPC, EP, EFD) but the other cores listed do a much better job
> shielding the near field (E Field) which will be a large part of the EMI.
> Pot cores do the best job of shielding both E and B fields. Toroids do a
> lousy job with leakage inductance unless you Bi Filar wind, are difficult
> wind and gap. Other than that they are ok.
> Just don't use an unshielded rod or drum core.
> that should shake up the troops,
I'm shaken. Here we seem to have a communication from a universe where the
physical laws are distinctly different from the ones that apply in the
Since when has an electric field (presumably what was meant by E-field) been
a "near field" and why would a toroid be noticeably worse at "shielding" it?
You have to be a bit careful in winding a toroidal coil to avoid the loop in
the plane of the toroid. Rayner and Kibble' s "Coaxial AC Bridges" (ISBN
0-85274-389-0) discusses this in detail in section 4.2.1 - you can minimise
this by looping the end of simple progressive winding back around the
toroid, or by using the Ayrton-Perry (otherwise "bootlace") winding
technique, or by winding a two layer coils in three stages - from the start
to half-way around the toroid, then back over the first layer all the way
around the toroid, then back again to the start.
It is a fairly obvious problem and was solved a long time ago
but if you miss it you may think that toroids do a lousy job with leakage
Pot cores are the topological inverse of the toroid, but they have slots to
let you connect your coils to the outside world, so they aren't quite as
good at minimising external magnetic fields. RM cores are squared off pot
cores with biger slots. E-cores are even worse - and IIRR most of Harry's
alphabet soup are variant E-cores.
The E-field - otherwise the capacitative coupling - from the coils is rarely
a problem. There is usually enough grounded metal in the vicinity to
minimise any radiation. Picky people screen their coils with over-lapped
electrostatic screens (avoiding the dreaded shorted turn). For low frequency
transformers the cores may be conductive enough to contribute useful
shielding, but ferrite materials are pretty resistive.
Oddly enough, Harry is right about unshielded rod and drum cores - they do
generate significant external magnetic fields.
Bill Sloman, Nijmegen
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