We could call the state-space approach of PLECS quasi-symbolic, because it reduces circuits with linear components to state-space equations. As a result, it needs to solve for much fewer variables. The price to pay is that the obtained system is dense. But for power electronics, this is great, because a tailored implementation of RADAU5 can be used (one of the gold standard for very stiff ODEs). For real-time though, the system has to be built differently with companion models for the swtiches, especially to be able to run it on the FPGA.

On the SPICE side of PLECS Spice, we also perform this kind of reduction for some components if we can, using graph algorithms. For instance, a basic SPICE solver would use a zero voltage source as an ammeter, which adds not only a current variable, but also a nodal voltage because the element has two terminals. Currents can be bad for Newton's method, because they have a very different scale. Often, we can completely avoid this by computing the current from surrounding components. But SPICE is very far from anything real-time.