I'm trying to measure the feedback loop on a isolated flyback SMPS. My goal is to determine the phase margin and 0 dB cross-over frequency.
The most common circuit I have seen is the one recommended in e.g. this article, figure 10:
An isolation transformer is used to apply a small offset voltage to the feedback path. The gain and phase is then measured as the ratio between sine waves at points A and B. This seems to work well for frequencies above the loop cross-over frequency.
However at lower frequencies, the feedback action itself is causing channel B signal to vanish.
Before applying power to the converter, I see a sine wave on point B while point A is kept at 0 volts by the output capacitors:
As soon as I turn on the power, the feedback loop acts to eliminate any wave on point B, while point A now shows the expected nearly-180° phase shift to the wave from previous image:
Question: What am I doing wrong? Should I somehow deactivate the feedback loop before performing the measurement? Should I compare point B with power off against point A with power on?