It may come as a surprise to some that a control loop’s Bode plot does not always depict the loop stability and that the output impedance measurement, obtained non-invasively, always does. Challengers of the non-invasive phase margin measurement approach believe that the method is not always an accurate representation of loop phase margin. While this can be true for transfer functions that are more than first order, the Bode plot does not always accurately depict the control loop stability in these cases either, while the non-invasive output impedance measurement does.
It is important to note, however, that the impedance must be measured at the feedback points, since downstream filters can impact the results. This is not to say that the downstream impedance is unimportant, it certainly is for Power Distribution Network (PDN) evaluations, however; the filters (and trace impedances) can mask the loop stability.
The phase margin and gain margin do not necessarily provide an accurate assessment of the control loop stability. Let’s look at one case study of a five output winding flyback converter, where the Bode plot leads to an incorrect conclusion of stability.