The SIMPLIS Periodic Operating Point (POP) analysis followed by an AC Analysis has a proven record of quickly and accurately measuring the AC loop response of switching power supplies. This analysis works very well for stable systems where the feedback signals are represented by double precision numbers.
Characterizing Digital Control Loops
In digitally controlled systems where the signals in the feedback loop are quantized into discrete levels, the POP / AC analysis is typically not applicable. The extremely small perturbation of the injected signal during the AC analysis is not large enough to produce a control loop response. The Multi-Tone AC analysis, available in the Design Verification Module (DVM) starting with SIMetrix/SIMPLIS version 7.1, is able to provide an AC analysis of such digitally controlled systems, including closed loop response, input and output impedance as well as conducted susceptibility.
Here is a comparison of closed loop gain and phase results from a digitally controlled DC/DC converter.
Characterizing PFC Control Loops
For PFC converters using an AC input and variable frequency control, the POP / AC analysis is not appropriate. In these cases, the Multi-Tone AC analysis offers a way to characterize virtually any control loop.
Here are the loop gain and phase results from a single phase continuous conduction mode PFC converter. The red curves represent the POP / AC data using a PWM multiplier, while the blue curves show the Multi-Tone AC results using the full NLB multiplier.
Both linear and decade frequency sweep modes are available with the four Multi-Tone tests objectives that are built in to DVM: Bode Plot , Output Impedance, Input Impedance and Conducted Susceptibility. Using the built-in testplans for DC/DC and AC/DC converters, a working design can be converted to run the Multi-Tone AC analysis in 5 minutes or less.