Analysis of subharmonic oscillation of fixed-frequency current-programming switch mode power converters

Subharmonic oscillation of a generic fixed-frequency current-programming switch mode power converter (SMPC) operating in the continuous-conduction mode (CCM) is analyzed. The conditions for oscillation are derived by the geometric approach as well as by analyzing the converter in the discrete time domain and are shown to be equivalent. In obtaining the state-space averaged differential equation, the forward Euler approximation is invoked. This z→s transformation maps a z-domain pole outside the unit circle that causes the oscillation to a s-domain pole in the left-half plane (LHP), which has previously been overlooked. The analysis is then applied to a current-programming buck converter, which suggests the two correct control equations for both converters employing either trailing- or leading-edge modulation