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First of all, there are no true square waves for most physical quantities (such as voltage, for example).

That said, you can create a step function by combining the sign and trig functions. For example: f = sign(sin(t)). But this is generally a bad idea to actually use, since time-stepping must be done in finite-time steps. Thus, you can never set a small enough time step to capture an infinitely-rapid transition. And the result in any time-stepping model, if using such an expression, is serious numerical errors.

I find that for real-world problems, it is best to model smoothed-out transitions that allows several time steps during the rapid rise (or rapid fall), to prevent numerical anomalies. There exist various ways to synthesize functions with relatively-flat tops, flat bottoms, and relatively-rapid rises and falls, using the available functions. You'll need to decide what time function shape you really need, based on the details of your voltage source, which (as I said earlier) cannot be a true square wave if it represents anything in the real world.

I hope that helps.

Hi

take a look at the flc2hs step functions of COMSOL, do a search of "heaviside" on the command.pdf (p210 in V3.5a), by combining two of these funtions you will get a nice turn on and off, with a smooth transistion that is compatible with the COMSOL solvers.

in V4 this is "precooked" for you

Have fun Comsoling
Ivar

thank you for your answer! that do help~