Re: [SI-LIST] : Re: approximations for partial self inductance

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From: Sainath Nimmagadda (sainath@lsil.com)
Date: Thu Mar 15 2001 - 16:41:27 PST


Dear Chris,

I would not disregard this statement :-)
However, full-wave field solvers are man-made tools too. What it reveals to
you depends on how well the tool developer(s) understood the equations and
their interplay. We will keep approximations and rest of numerical noise
junk aside for now. As you can see, when I wrote to Dr. Johnson, this was
on my mind: Maxwell's equations and current through inductance.

If you are interested in inductance of XYZ and my so called full-wave field
solver gives you impedance (which, of course, has your inductance 'lumped'
into it), would you buy it?

Thanks,
Sainath

"Reid, Chris" wrote:

> Vinu,
>
> Yes. And as Scott McMorrow pointed out, in real circuit
> boards solving the full-wave equations reveals very
> complex paths.
>
> Chris
>
> -----Original Message-----
> From: Vinu Arumugham [mailto:vinu@cisco.com]
> Sent: Thursday, March 15, 2001 10:01 AM
> To: Reid, Chris
> Cc: 'Sainath Nimmagadda'; Howard Johnson; si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Re: approximations for partial self inductance
>
> How about:
>
> The bulk of the current flows through the path of least impedance.
>
> Thanks,
> Vinu
>
> "Reid, Chris" wrote:
>
> > Hello,
> >
> > Actually, the statement:
> >
> > > It is a principle
> > > of Maxwell's equations that high-speed returning signal
> > > current will flow in whatever path produces the
> > > least overall inductance.
> >
> > is not really correct. Instead of inductance, think of
> > resistance for a moment. If you put two resistors in parallel,
> > one 100 Ohms and the other 1 Ohm the effective resistance is
> > 1/(1/100 + 1/1) =~ 0.99 Ohm. Clearly most of the current
> > flows through the 1 Ohm resistor, but some also flows through
> > the 100 Ohm resistor.
> >
> > Inductance is similar, only now the result depends on frequency.
> >
> > Chris
> >
> > -----Original Message-----
> > From: Sainath Nimmagadda [mailto:sainath@lsil.com]
> > Sent: Wednesday, March 14, 2001 6:19 PM
> > To: Howard Johnson; si-list@silab.eng.sun.com
> > Subject: Re: [SI-LIST] : Re: approximations for partial self inductance
> >
> > Dear Howard,
> >
> > Please see below:
> >
> > Howard Johnson wrote:
> >
> > > Dear Itzhak Hirshtal and Brian Young,
> > >
> > > The difficulties with approximating the inductance
> > > of a via are even worse than you
> > > may have suspected. Both approximations are flawed whether
> > > you use +1 or -3/4, (or, as I have also seen, -1).
> > >
> > > The issue of the exact constant (1, -3/4, or something
> > > else) depends critically on your assumption about
> > > the path of returning signal current. (Current always
> > > makes a loop; when signal current traverses the via,
> > > a returning signal current flows SOMEWHERE in
> > > the opposite direction.). It is a principle
> > > of Maxwell's equations that high-speed returning signal
> > > current will flow in whatever path produces the
> > > least overall inductance.
> >
> > My question is on this last statement. I like to understand which
> > Maxwell's equation suggests this and how? Thanks.
> >
> > Sainath
> >
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