July 18, 2008 -- The performance of a switching regulator for the given external passive components is largely determined by how well the closed loop is being compensated. An excellent loop-compensation design is essential to high performance switchers. It is also often the most difficult block to design, partly due to the switching activity. In the compensation design, we often linearize the loop to simulate the feedback stability using the average switching model. However, you can simulate the phase margin and gain margin directly from the switching feedback loop. This is achieved through the PSS (periodic steady-state) and PAC (periodic small-signal AC) analyses. PSS and PAC are often used for RF simulations, where the carrier is a periodic signal. PSS and PAC analyses are available in commercial simulators such as Cadence SpectreRF.

PSS analysis uses the shooting method to find the periodic-steady-state solution. The shooting method solves a boundary value problem by reducing it to the solution of an initial value problem. PAC analysis is used to compute transfer functions for circuits that exhibit frequency translation. It is a small signal analysis like AC analysis, except the circuit is first linearized about a periodically varying operating point as opposed to a simple DC operating point. A PAC analysis cannot be used alone; it must follow a PSS analysis.

By Yushan Li. (Li is principal design engineer in the Power Management Group at National Semiconductor Corp.)