Conditionally Stable systems
It is quite common in power supply design to encounter loops which are conditionally stable. An example of such a loop is shown in Figure 3. A conditionally stable system is one in which the phase delay of the loop exceeds -180 degrees while there is still gain in the loop. This is a common occurrence with voltage-mode control where the phase dips abruptly around the resonant frequency, then recovers with the effect of real zeros added in the compensation. It also is common in the feedback loop of power factor correction circuits, and is often impossible to avoid.
Figure 3: Loop gain with more than 180 degrees phase delay at low frequencies. The system is still stable.
In the loop of Figure 3, there is between 20 and 40 dB of gain, shown in red, when the phase drops below -180 degrees. There is no problsub with such a system. As long as there is plenty of gain margin and phase margin, the control will be rugged.
In Figure 3, the phase margin is about 50 degrees, and the gain margin above the crossover frequency is about 15 dB.
We must also be concerned with the phase margin to the left of the crossover. This is a measure of how much the gain would need to be reduced due to parameter variations before the system would become unstable. It can be seen that this example has no problsub since it has more than 20 dB gain margin at several kHz.