Averaged Modeling and Switching Instability Prediction for Peak Current Control

This paper attempts to resolve some fundamental issues related to averaged modeling of peak-current controlled PWM converters. One issue to be addressed is the ability of averaged models to predict switching instability (also referred to as subharmonic oscillation) under peak-current control. It will be shown that, contrary to the widely held belief, such instability can be predicted by properly defined averaged models, and that incorporation of an additional second-order transfer function representing the so-called sample-and-hold effect is not necessary for this purpose. The averaged models capable of predicting such instability are developed by combining standard state-space averaged models with a properly defined duty-ratio constraint. A circuit approach based on general topological properties of PWM converters and a mathematical approach based on KBM ripple estimation algorithm to the derivation of the duty ratio constraint are reviewed and are shown to yield identical results. Implementation of the averaged models in circuit simulation programs using iterative method is also presented