# Measuring flyback SMPS feedback loop response

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?

• Power off measurements are not relevant. Where is the plot of CHa/CHb? It'll be fun to see what CHa/CHb looks like when channel B is momentarily 0 volts. That article I note was written by someone in marketing (just saying). Commented Oct 11, 2023 at 13:37
• @Andyaka Yeah, I guess it is not literally A/B, but rather as the division of complex numbers / phasors representing the sine waves. But as you'd expect, even that has rather random values when B is very small. In this siglent appnote they seem to have reasonable results by adjusting the stimulus amplitude, but I don't seem to get reasonable signal at channel B no matter what I do.
– jpa
Commented Oct 11, 2023 at 13:50
• The trouble is... does anyone know what it is and does everybody have that Sigilent scope? My method is simpler; a sudden load discontinuity and look for ringing and the length of time ringing occurs for and, compare with a 2nd order filter response to estimate phase margin. Commented Oct 11, 2023 at 14:18