On Tuesday 19 July 2011, Ramiro Aceves wrote: > I have a convergence problem:
Quick answer, based on experience, so I can give you an immediate response .. Convergence problems are often tied to the model used. You are using the old "modelgen" BJT model, which will be phased out. Try instead to use the Spice BJT model from the spice models tarball. The modelgen models in gnucap (analogous to the spice "code- models") check convergence by recursively checking the subcircuit. This often results in overly critical checking, where really everything is ok, but it indicates non-convergence. Convergence checking in the spice models is less critical, so often just switching to the other model will solve the problem. Try this .. get the plugin tarball http://gnucap.org/devel/gnucap-2009-12-07-models-spice3f5.tar.gz unpack it, compile the "bjt" model .. "load" it as a plugin. More detailed response, for the curious .. The modelgen models use an underlying subcircuit, and check every part of it individually for convergence, the equivalent of checking every state variable and the derivatives separately, then a check pass checks all of the voltages node-to-node. To accept, all must pass. This includes the internal nonlinear capacitances, and all strays. The down side is that occasionally one of the strays can get stuck on amplified numerical noise and never be accepted, when in fact it could be ignored. Spice uses a quick check for convergence, where only a few parameters are checked, as a composite. This method often accepts incorrect values as converged, sometimes leading to believable incorrect results. Gnucap with the spice model is somewhere in the middle .. It first uses the spice method, it's part of the model code, then checks node-to-node voltages. The node-to-node check seems to catch the false convergence problems. Convergence handling is a big issue in simulator and model design. There are trade-offs, impossible to really know what is best in general. _______________________________________________ Help-gnucap mailing list [email protected] https://lists.gnu.org/mailman/listinfo/help-gnucap
