Abstract

Distributed DC power systems offer many benefits over AC line distribution systems such as improved energy conversion efficiency and reduced mass due to high-frequency isolation. Unfortunately, distributed DC systems with regulated bus voltages suffer from destabilising effects from loading by downstream power electronic converters behaving as constant-power loads. Power electronic constant-power loads present a negative incremental input impedance to the main bus, which may result in negative impedance instability. Avoiding the effects of negative impedance instability is most often achieved by following impedance ratio criteria, such as the Middlebrook stability criterion which has the drawback of imposing conservative constraints on the design of the power system components. Such conservative criteria can also result in the over-design of converter input filters and artificially imposing limits on the bandwidths of the load power electronic converters. Through the use of a current-mode controlled pre-regulator, the input impedance of power electronic constant-power loads can be shaped to interact with the main bus impedance in a stable manner while optimising converter bandwidth and line rejection. A new state space based approach is developed to model peak and valley current-mode control. Following this new approach, models for all basic DC-DC converter topologies are created (Buck, Boost and Flyback). This new model allows for an accurate analysis of a pre-regulator and its straight forward design.