1 Introduction
The control characteristic of a power converter is that the input signals are discrete, since they are the ON/OFF state of the switching devices, while the rest of the signals, including the signals to be controlled, are continuous. Most methods used to power converters were averaging models in which discrete signals are averaged over each sampling period and are therefore treated as continuous signals [1]. During the recent decades, the control scholars paid their attentions on discussion of dynamic behavior while tapping new control techniques because of the applications a number of power converters.