**From:** JuYoung Lee (*jylee@lsil.com*)

**Date:** Tue Mar 20 2001 - 14:42:01 PST

**Next message:**Bill Chen: "RE: [SI-LIST] : specctraquest timing question"**Previous message:**Sampson, Scot: "[SI-LIST] : specctraquest timing question"**In reply to:**Tsuk, Michael: "RE: [SI-LIST] : Re: approximations for partial self inductance - WHY"**Next in thread:**Sainath Nimmagadda: "Re: [SI-LIST] : Re: approximations for partial self inductance - WHY"**Reply:**Sainath Nimmagadda: "Re: [SI-LIST] : Re: approximations for partial self inductance - WHY"

Michael,

Good point.

In Physics community, people often say about gauge attached to vector

potential A. A is defined by curl A = B. You can attach any gradiant of

function X (gauge) to A without affecting E and B field according to

Maxwell equation form. So A' = A + gradient X and there are infinitely

many options of A'.

One interpretation is that A' is partial inductance per unit length.

Field solvers often use A'. As long as one sticks to a particular choice

of X consistently, the potential error in using partial inductance can

be avoided.

And X is supposed to mean "nonexistence" in electromagnetics. In quantum

physcis it means something related to phase of photon.

Regards,

JuYoung

"Tsuk, Michael" wrote:

*> Sainath,I used to believe as you do, that partial inductances are
*

*> useful to obtain some first-cut answers. Over the years, I've changed
*

*> my mind. I believe that the potential for misuse from partial
*

*> inductances outweighs their benefits, and I'm now doing all my signal
*

*> integrity modeling with loop inductances. I'm much happier. :-)Here
*

*> are some of the problems I see with partial inductances:1.) They are
*

*> arbitrary; as Brian Young points out in his wonderful new book, you
*

*> can add any constant you want to the partial inductance matrix without
*

*> changing the physical result. Different techniques for calculating
*

*> partial inductance give different answers --- witness the discussion
*

*> we've just had on this point.2.) When you use partial inductances in
*

*> SPICE simulations, they give you things that look like "ground
*

*> bounce": voltage differences across large sections of your circuit,
*

*> where it is impossible to make a unique physical measurement of
*

*> voltage because of linked flux. Brian Young again points out that
*

*> ground bounce is not unique; it depends on your definition of partial
*

*> inductance. You can be mislead by how chip ground is bouncing with
*

*> respect to module ground in your simulation --- it looks like
*

*> something real, but it's not. When you use loop inductances, and use
*

*> SPICE node 0 to represent local reference everywhere, you can't be
*

*> mislead; there's no node voltage in your simulation that looks like
*

*> ground bounce.3.) If you use partial inductances in your SPICE
*

*> simulations, you have to make sure that all the current in your
*

*> simulation moves from one side of your circuit to the other only
*

*> through the partial inductances. If you have node 0 on both sides,
*

*> for example, you've violated the assumptions under which partial
*

*> inductance is valid. And it can be very hard to avoid node 0
*

*> sometimes, and it appears that having large sections of your circuit
*

*> isolated from node 0 makes convergence more difficult.4.) Partial
*

*> inductances are completely invalid without mutual inductances, but
*

*> there's a great tendency to ignore them as a "first-pass engineering
*

*> assumption". This is natural; all of engineering is about ignoring
*

*> things. :-) But it just doesn't work with partial inductances. At
*

*> best, you're making assumptions about where the return path is (and
*

*> different ways of calculating partial inductances make different
*

*> assumptions); at worst, you miss the entire point of the exercise.
*

*> Without partial mutual inductances, there's no reason to put power and
*

*> ground planes close to each other.Basically, my feeling now is that
*

*> partial inductances are a wonderful tool for calculating inductance in
*

*> the standard signal integrity situation where the full loop is not
*

*> completely known (package without chip or board, for example). But I
*

*> think now they should remain a computational tool, and that the models
*

*> that are eventually generated should be based on loop inductances.I'm
*

*> working on a paper explaining these points in more detail and talking
*

*> about how we've been using loop inductance rather than partial
*

*> inductance for package modeling here at Compaq. I hope to present the
*

*> paper at EPEP'01 here in Massachusetts. I would appreciate any
*

*> comments people might have.--
*

*> Michael Tsuk
*

*> Compaq AlphaServer Product Development
*

*> (508) 467-4621
*

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**Next message:**Bill Chen: "RE: [SI-LIST] : specctraquest timing question"**Previous message:**Sampson, Scot: "[SI-LIST] : specctraquest timing question"**In reply to:**Tsuk, Michael: "RE: [SI-LIST] : Re: approximations for partial self inductance - WHY"**Next in thread:**Sainath Nimmagadda: "Re: [SI-LIST] : Re: approximations for partial self inductance - WHY"**Reply:**Sainath Nimmagadda: "Re: [SI-LIST] : Re: approximations for partial self inductance - WHY"

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