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

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From: Reid, Chris (chris_reid@mentorg.com)
Date: Thu Mar 15 2001 - 15:51:35 PST


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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