Tuesday, October 20, 2009

6. Simulating switching circuits

Simulation with "ideal" components is not a new idea. Spice always had problems with switching circuits: SMPS (switching mode power supplies), switched capacitors, etc. In many cases, detailed and accurate simulation of switching process is not needed: a simple "On/Off" transistor model (i.e. ideal switch) with instant switching is quite appropriate to accurately predict circuit behavior. This is definitely a case when simplifying a component, rather then making it more real, is the only way to obtain a reliable result.

This approach has been described in many papers since 80's. However, there is no any real, reliable, publicly available tool so far. Why? Probably because there is a long way from an idea and a method, usually designed and tested for specific circuits only, to a software with convenient, user-friendly GUI, simulating any arbitrary schematic. Also, there is a mistaken belief that simulating circuits with simplified "ideal" components is easier than "real". This is not true: there are many problems, specific to "ideal" components, and simulation tool should be able to handle all of them.

Although Spice is doing much better now (I'd rather say: faster), it still fails on simple switching circuits. It actually does something, but results are wrong. Of course, there are some "knobs" (simulation and component parameters) you can adjust to make everything work right. However, as I said before, you have to realize a result is wrong, you have to know what to change, and how to change.

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