I've done this with PSS + PSTB, and it works well.

Assuming you have some kind of clock pulse for your DC to DC converter, you should be able to run PSS with that period or frequency specified. You might need to give it a tstab time to allow it to get started (although this doesn't normally need to be the time for it to be settled).

Then place an iprobe in the feedback circuit, and setup pstb analysis to point to that and sweep the frequency so you can analyse the loop gain.

If you contact me at Cadence (my email address is my first name followed by the first letter of my surname, at cadence dot com), then I can send you a presentation on this that I gave at a National Microelectronics Institute Power Electronics event last year (in the UK).

Regards,

Andrew.