Analysis of boundary control for boost and buck-boost converters in distributed power architectures with constant-power loads

Cascade distributed power architectures include tightly-regulated point-of-load converters that exhibit instantaneous-constant-power-load (CPLs) characteristics. Boundary control is investigated for dc-dc boost and buck-boost converters that feed these instantaneous (CPLs). Without adequate controls, the destabilizing nonlinear effect of the CPL inverse-voltage term leads to significant oscillations in output voltage of these converters, and possible voltage collapse. The analysis presented in this paper reveals important characteristics of CPL effects on these two basic converter topologies. In order to avoid issues related with the fact that the state-dependent switching function is undefined on the switching surface, in reflective mode solutions to both converter systems are defined in the sense of Filippov. Consequently, Lyapunov's direct method is used to identify stable reflective regions that guarantee sliding-mode operation towards the desired operating point for both converters. It is shown that first-order switching surfaces with negative slopes achieve large signal stability for both converter systems, while positive slopes lead to instability. For the boost converter, it is illustrated via simulation and experiment that positive slopes may lead to another closed-loop limit cycle. Design considerations are included and recommendations are given. Simulations and experimental results verify the analysis.