Selection of a curved switching surface for buck converters

A general procedure for the selection of a curved switching surface (SS) to control buck-type converters is presented in this letter. The analysis is based on the normalized representation of ideal SSs for different loading conditions. The normalization process leads to a unique representation of the SSs for any possible buck converter. A set of graphics in three dimensions is introduced to give a spatial sense of the behavior of the converter and its control requirements during transients. As a result of the investigation, a switching surface referred to in this letter as the natural unloaded SS is selected, providing excellent transient behavior and no overshoot during startup. For any buck converter with typical parameters, this control scheme produces, in one switching action, a minimum of 99% of the desired output voltage. The general concept of using second-order SS is also geometrically analyzed in this letter to clarify its characteristic features and disadvantages. Experimental results for a typical buck converter are presented to illustrate the transient behavior of the converter during startup and sudden load changes. The results confirm the virtues of the control scheme.