Electronic – Use variable in SPICE parameter list

So to summarize:

• if a precise curve of the resistor vs time doesn't matter .step param, which is supported in PSpice, is a good way. Actually you can remain ignorant of the SPICE dot command syntax in PSpice, because parametric sweeps can be configured entirely via GUI menus (Simulation Settings > Primary Sweep) as explained in this tutorial: http://m.eet.com/media/1179065/chapter (see p. 69)
• for a curve vs time: SPICE3 behavioral expressions. I'm not sure exactly what PSpice supports in this regard... It seems the preferred way of doing this in PSpice is to use its Analog Behavioral Modeling (ABM) extension, which allows abstract components that can use TIME as a variable; see http://www.nordcad.dk/download/Forum/PSpice/abm.pdf for an overview. At least for this problem, it seems ABM is actually the same thing as a SPICE3 behavioral expression, with a (PSpice) syntax like R1 1 0 VALUE={10 + 0.01 * TIME}.

In LTspice, and not only, conditional expressions like if() tend to introduce discontinuities which go against the solver because of the derivative in that point. Some simple discontinuities may be avoided, but, usually, they are to be avoided.

One solution would be to try to smooth out the discontinuity by adding a small capacitor (fF~pF) across the source that has the conditionals, which would force the waveform to follow a more fluid path, thus allowing the solver to "see" all the points and go past it, while, at the same time, not influence the circuit by introducing unwanted poles that could cause wrong results.

Another solution would be to simply use the A-devices that come with LTspice, or, with the newer LTspice XVII, the .machine command. I say this by looking at your schematic, where I see, mostly, cases of if(cond, <one_voltage>, <another_voltage>), which can easily be solved with logic gates.

While we're at it, you could also avoid the positive hysteresis for the switch, which is known for causing trouble, and choosing a negative value, which would make the transitions go smoothly from on to off and vice-versa, instead of abrupt changes.

Another minor note would be to not rely on the default diodes, which are ideal, and either force a "normal" diode by specifying one of the other parameters, like Is=1f, for example. Or, you could keep the faster ideal diode, but it won't hurt adding epsilon=<fraction of Vfwd> and revepsilon=<fraction of Vfwd> -- they add a simple quadratic region to the knee(s) of the transfer function.

And, finally, parasitics in Cs and Ls can go a long way in both avoiding very large switched quantities (currents, mostly), and in making the circuit behave more real.