Re: [SI-LIST] : **error**: internal timestep too small

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From: Kim Helliwell (khelliwe@acuson.com)
Date: Thu Jan 20 2000 - 13:41:00 PST


"Zabinski, Patrick" wrote:
>
> I cannot claim to know why this works, but if RELV, RELI, IC,
> or NODESET helps convergence or if there are problems
> with any form of transmission line (T, U, or W), I
> simply insert 500K ohm resistors from various nodes to
> node 0 (ground). Generally, the value is high enough
> to not effect the results, but I've found it not only helps SPICE
> find its way, but it can shave off 50-80% off run-time
> as well (depending upon the circuit).

Yep, that would work alright, and would help both DC and
transient convergence (as long as the transient convergence
problem is not due to model discontinuities). You might
want to try a couple of values for these resistors to get
a feeling for how much the solution is affected by this approach.

"GMIN" stepping uses exactly this approach, putting in 1-ohm resistors
to get a solution, then using that solution as a starting point for
a new set of iterations with a larger resistor, and so on. Eventually
the resistors are removed entirely. This is obviously a DC convergence
aid and wouldn't help timestep problems, but leaving them in permanently
would help timestep problems except as I already noted.
]

-- 
Kim Helliwell
Senior CAE Engineer
Acuson Corporation
Phone: 650 694 5030  FAX: 650 943 7260

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