Gain Margin of a Control Loop
There is more to stability assessment than just the phase margin. The phase margin only addresses one frequency, the crossover point. It does not give information about other frequencies that may cause trouble with variations of parameters in the feedback system. Beyond the crossover of loop, it is important to look at the gain margin. This is defined as the amount the gain is below 0 dB when the phase hits -180 degrees. A gain margin of 10 dB is reasonable. This allows parameter changes which could cause the loop gain to change by a factor of approximately 3 before the system becomes unstable.
The gain margin for the loop gain of Figure 2 is approximately 17 dB, a good value for a rugged and conservatively-designed control system.
Point-of-load converters often push the crossover frequency of a power supply very high in order to minimize the amount of capacitance on the output. In doing so, they often end up with a loop with very small gain margin, and the system may be on the verge of instability even though the phase margin under nominal conditions is reasonable. This is not good design practice.