Unified subharmonic oscillation conditions for peak or average current mode control

Dynamics and stability of average current-mode control of buck converters are analyzed by sampleddata and harmonic balance analyses. An exact sampled-data model is derived. A new continuoustime model “lifted” from the sampled-data model is also derived, and has frequency response matched with experimental data reported previously. Orbital stability is studied and it is found unrelated to the ripple size of the current-loop compensator output. An unstable window of the current-loop compensator pole is found by simulations, and it can be accurately predicted by sampled-data and harmonic balance analyses. A new S plot accurately predicting the subharmonic oscillation is proposed. The S plot assists pole assignment and shows the required ramp slope to avoid instability.