Prediction of Subharmonic Oscillation in I² Controlled Buck Converters in CCM

A recent paper proposed I2average-current control with constant switching period (CSP) or constant on-time (COT) operation in continuous conduction mcontinuous conduction modeode (CCM). The small-signal analysis was presented, but only for the COT case. No instability or its condition was reported. In this note, unified subharmonic oscillation conditions are derived for both CSP and COT buck converters. The effects of feedback gain, turn-off delay, and stabilizing ramp slope are considered. The results can be extended to other converters or control schemes.