  This paper is an extension of the author's recent research in which only buck
converters were analyzed. Similar analysis can be equally applied to other
types of converters. In this paper, a unified model is proposed for buck,
boost, and buck-boost converters under peak or average current mode control to
predict the occurrence of subharmonic oscillation. Based on the unified model,
the associated stability conditions are derived in closed forms. The same
stability condition can be applied to buck, boost, and buck-boost converters.
Based on the closed-form conditions, the effects of various converter
parameters including the compensator poles and zeros on the stability can be
clearly seen, and these parameters can be consolidated into a few ones.
High-order compensators such as type-II and PI compensators are considered.
Some new plots are also proposed for design purpose to avoid the instability.
The instability is found to be associated with large crossover frequency. A
conservative stability condition, agreed with the past research, is derived.
The effect of the voltage loop ripple on the instability is also analyzed.
