Re: gEDA-user: NGspice and GNUcap Non-Linerar Dependent Sources

[John Doty <jpd@xxxxxxxxx>]

On Nov 10, 2010, at 4:09 PM, clif@xxxxxxxxxxxxx wrote:

> Hi Gang,
> 
> I've been playing with ngspice and gnucap and have gotten good results with the Non-Linerar Dependent Sources eg ASRC Bxxxx devices in ngspice. I mostly use those with VC-Switches to make full wave bridges, SCRs, Triacs, etc...

Why not use the semiconductor models for semiconductor devices? Make full wave bridges from diodes, SCR's from cross-coupled transistors, etc. A great deal of effort has gone into accurately modeling semiconductor physics in these simulators: why not exploit it? A Google search for "spice scr model" turns up quite a few subcircuit models of this sort. If you can't find exactly the model you need, it's probably better to tweak one of these than make one up.

>  Though I see that they are not supported in gnucap. I guess the alternative is to use POLYs. I've looked around and except for a short description in the gnucap manual I haven't found a comprehensive howto on how to use them.

A common way to go from data sheets to model parameters is to model test circuits and then tweak the parameters until there's a decent resemblance.

> I sapose you would want a curve fitting program to help you generate the coeficents like grace, or simfit, or you could use the fit function in gnucap. However this isn't supported in ngspice, so you couldn't use that model in both.

You can configure ngspice to support POLY. See http://www.brorson.com/gEDA/SPICE/x496.html.

> 
> Are POLYs really that much better at solving convergence problems to be worth the extra trouble? Is there a good into or howto on how to use them?

There is a vast literature on the subject of constructing polynomial approximations to functions. Google for "polynomial approximation". You'll never be able to digest it all.

> Are negitive coeficents even legal?

Sure, why not?

John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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