Sliding-mode control of a boost converter feeding a buck converter operating as a constant power load

Multi-stage switching converters are necessary in DC distributed power system applications featuring many individually regulated converters. The individual converters in a distributed power system are usually designed for general usage and their controllers are synthesized to provide a suitable transient response in stand-alone operation. Often, a tight regulation is required for the output voltage of both the source and the load converters. Tightly regulated power electronic load converters sink constant power from the intermediate bus and show a strong nonlinear voltage-current characteristic that can be approximated by a static constant power load (CPL) when connected to the output of the source converter. At the slow time-scale, CPLs exhibit negative impedance behavior making interconnected converters prone to a high risk of instability and therefore the mitigation of this problem is an important issue in the multiple-stage switched mode power supply design. In this paper, a discussion on the validity of the CPL approximation is presented and then a sliding-mode approach is used to stabilize a source boost converter feeding a regulated buck converter. The resulting sliding-regime for linear switching surfaces in both the source and the load converters is analyzed and the conditions for the existence of sliding-mode are derived. A study of the system is performed in terms of the converter start-up with the limitation of the inrush current and the output voltage regulation in presence of external perturbations. PSIM simulations are in perfect agreement with the theoretical predictions.