# [SI-LIST] : Partial inductance....

From: [email protected]
Date: Tue Mar 20 2001 - 07:51:36 PST

Michael, thank you for the very clear discussion.

After many hours (years?) of messing with inductances, I
must agree completely with Michael. Partial inductance
can be useful, but it's very dangerous.

best regards to all

Rich
----------------------------------------------------------------------

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 -----Original Message-----
From: Sainath Nimmagadda [mailto:[email protected]

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