  A general critical condition of subharmonic oscillation in terms of the loop
gain is derived. Many closed-form critical conditions for various control
schemes in terms of converter parameters are also derived. Some previously
known critical conditions become special cases in the generalized framework.
Given an arbitrary control scheme, a systematic procedure is proposed to derive
the critical condition for that control scheme. Different control schemes share
similar forms of critical conditions. For example, both V2 control and voltage
mode control have the same form of critical condition. A peculiar phenomenon in
average current mode control where subharmonic oscillation occurs in a window
value of pole can be explained by the derived critical condition. A ripple
amplitude index to predict subharmonic oscillation proposed in the past
research has limited application and is shown invalid for a converter with a
large pole.
