Analysis of switching regulator input
I'm revising my question.  I think I have a better idea but still not sure.  Below is the schematic of the circuit I want to assess.

The switch represents an on switching regulator with a 5A load through 3 ohms.  I want to analyze the transients at the input capacitor when the regulator mosfet switches off.  My idea is to determine the initial conditions with the switch closed, then do a Laplace and f(t) analysis with the switch open.  I think the initial conditions are a voltage on the capacitor and a 5A initial current through the trace inductance.
On the correct path?