I was going through this Operational Amplifiers, Theory and Practice by James Roberge. On Page 146, it is mentioned that for systems that have negative feedback at low or mid frequencies and that have no right-half-plane singularities in their loop transmission, we can utilize a simple criterion that the loop transmission(af) i.e.
- If the magnitude of af is 1 at only one frequency, the system is stable if the angle of af is between + 180 deg and - 180 deg at the unity-gain frequency.
- If the angle of af passes through +180 deg or - 180 deg at only one frequency, the system is stable if the magnitude of af is less than 1 at this frequency.
which later evolves into the widely used gain and phase margin parameters. However, for SMPC systems, except the buck (and buck derived topologies), most other topologies (say flyback) have a right hand zero in the transfer function like below
But I see that parameters like gain and phase margin are widely used to design compensators in this case. It looks like a contradiction as the condition that the phase of loop transmission being above -180 deg at crossover is valid only when right half-plane singularities are not present, which is not the case in above. Is this a valid way to design compensators? If yes, can someone resolve this contradiction (or any points I am missing) and if not, what are possible alternatives?