Bidirectional power flow with constant power load in electric vehicles: A non-linear strategy for Buck+Boost cascade converters

The constant power behavior of tight-speed controllers in the vehicle's traction system and tightly regulated DC-DC converters connected to the HV-DC bus produces instability effects. This paper proposes a simple and practical non-linear control, using Circular Switching Surfaces, to address constant power load instability in electric vehicle's power systems. The proposed control technique provides a solution in the geometrical domain to constant power loading conditions while achieving outstanding dynamic response. The controller is implemented in a bidirectional Buck+Boost cascade converter as a battery charge/discharge unit and ensures reliable system operation. The predictable and consistent behavior of the converter with constant power load is presented by analyzing the system curves in the normalized phase plane with the switching surfaces employed. Simulation and experimental results on a scaled 2kW Buck+Boost cascade converter validate the proposed switching surfaces and predictions regarding the converter's behavior under constant power loading conditions.