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From: firstname.lastname@example.org (Bill Sloman)
Subject: Re: switching regulator & EMI
Date: 22 Oct 2002 14:52:04 -0700
References: <5XudndodSf5VgjKgXTWcoQ@News.GigaNews.Com> <3DB428CD.22C5FF2A@mmm.com.DELETETHIS>
NNTP-Posting-Date: 22 Oct 2002 21:52:04 GMT
email@example.com (Roy McCammon) wrote in message news:<3DB428CD.22C5FF2A@mmm.com.DELETETHIS>...
> Bill Sloman wrote:
> > 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?
> < ... snip ...>
> > 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.
> Just speculating.
> I'm thinking flyback.
> The primary has a substantial common mode voltage,
> which capacitively couple to what ever it can.
> This can lead to _conducted_ currents in the power
> main, unless they are gathered up and put back
> where they belong. A shield can be a good thing.
> Even a high impedance shield. It has been my limited
> experience that radiated EMI regulations is often
> easier to meet than the conducted EMI.
IIRR Ralph Morrison likes double screens/shields. All the capacitative
current flowing from the coil to the screen can be routed back to the
coil-side ground, and you can ground the other screen to to a
You are stil stuck with the screen-to-screen capacitance between the
grounds, but that is rarely a serious problem - it is typically only a
few tens of pF.
> As for why a toroid would be worse, I guess the
> other forms enforce a physical isolation between
> the primary and the other things that you might
> inadvertently violate with a toroid.
A less subtle trap than the wound loop around the core which you get
with a simple progressive winding, but presumably subtle enough to
trap the unwary.
Bill Sloman, Nijmegen
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