Predicting Subharmonic Oscillation of Voltage-Mode Switching Converters Using a Circuit-Oriented Geometrical Approach

A circuit-oriented geometrical approach in predicting subharmonic oscillation (SHO) of voltage-mode switching converters is presented. It is based on a straightforward graphical analysis of the operation of the converter without using advanced mathematical techniques. It is shown that the slope of the output of the compensator at the trip point is an important factor in determining the onset of SHO. The critical condition of SHO of the buck converter working in continuous conduction mode (CCM) is derived and expressed in terms of circuit parameters that serve as the design guideline for avoiding SHO. The proposed approach is applied to analyze three schemes, namely, proportional, dominant-pole and Type-III compensation. The analysis of SHO in discontinuous conduction mode (DCM) for both the buck and the boost converters are also included to demonstrate the universality of the proposed approach. Crucial Cadence simulation results and measurement results are presented to verify the analysis.