Some standard SPICE dc algorithms revisited: why does SPICE still not converge?

The application of continuation methods to circuit simulation is discussed. Although many commercial circuit simulators use continuation methods, they are not globally convergent-that is, it is not guaranteed that a dc solution to the circuit will be found. Two continuation methods, "Gmin-stepping" and "source-stepping", that are used in most circuit simulation programs are discussed and it is shown why they sometimes fail. Simple changes to these algorithms are proposed and it is demonstrated that these changes significantly improve convergence of the continuation algorithms.