Analysis of Boundary Control for Buck Converters With Instantaneous Constant-Power Loads

This paper examines a boundary control for dc-dc buck converters subject to instantaneous constant-power loads. These loads introduce a destabilizing nonlinear effect on the converter through an inverse voltage term that leads to significant oscillations in the main bus voltage or its collapse. Converter dynamics are analyzed in both switching states and the various operating regions of switch interaction with a first-order switching surface (boundary) are identified. Furthermore, sufficient conditions for large-signal stability of the closed-loop system are established. The analysis indicates that a linear switching surface with a negative slope passing through the desired operation point yields a stable operation. It is also shown that instability as well as system-stalling, which we term the invariant-set problem, may still occur in reflective mode. However, a hysteresis band that contains the designed boundary may be used to prevent system-stalling. This hysteresis band also allows for a practical implementation of the controller by avoiding chattering. Design considerations are included and recommendations are given. The theoretical analysis is verified by simulations and experimental results.