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

From: JuYoung Lee ([email protected])
Date: Tue Mar 20 2001 - 14:42:01 PST

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