[SI-LIST] Re: Convergence problems with Hspice

This is one of the big pitfalls of partial inductance models:  they're
only valid if hooked up properly in SPICE (since they rely on SPICE to
make sure that all the loops are closed).  It's really easy to have a
node 0 sneak in where it's not expected.  You're lucky if it fails to
converge in that case; it could easily converge and give you a bogus
answer.

The fact that the partial inductance models isolate large portions of
your circuit from node 0 makes convergence much more difficult for
SPICE, it seems, even when used properly.

Ever since I've given up partial inductance models in favor of loop
inductance models, I've been much happier. :-)

--=20
Michael Tsuk
Compaq AlphaServer Platform Group
(508) 467-4621



-----Original Message-----
From: Ray Anderson [mailto:Raymond.Anderson@xxxxxxx]
Sent: Wednesday, September 26, 2001 2:31 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Convergence problems with Hspice



Good point Scott. I remember a case many years ago ( before I
understood the folly in such endeavors) where I created a huge=20
PEEC model of a power plane (with tens of thousands of mutually=20
coupled inductors) where I just couldn't get the simulation to
converge. Found a single node (out of thousands and thousands)
in the model subcircuit that was shorted to ground. Turns out that
the error message emitted by Hspice (version 95 or 96 I think) were
worse than useless in that they were misleading.

-Ray



>One more possibility:
>
>Sometimes convergence problems will occur when
>Ansoft package models do not have the correct connections
>made to power and ground.
>
>Be sure that power and ground pass through the model
>and have different names on the die side of the model.
>Many people mistakenly (or inadvertently) tie both the
>die side and the pin side of the model to ground.  This can
>cause all sorts of convergence problems due to the shorted
>loop.
>
>
>regards,
>
>scott

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