Novel Modulation Concepts for a Drive-Integrated Auxiliary Dc-Dc Converter for Hybrid Vehicles

Hybrid vehicles are a characteristic feature of the daily traffic yet, with growing fuel costs and other factors increasing their importance in the nearest future even more. Nevertheless, hybrid vehicles are not a mass product up to now. Although the life cycle costs of the hybrid cars are usually smaller than the costs for comparable fuel vehicles, the main purchase criteria for vehicles are still the acquisition costs, leading to a preference for conventional fuel cars. So as to increase the number of hybrid vehicles, the production costs have to be lowered. In hybrid vehicles, the energy distribution system causes a significant share of volume and costs. One part of this system is the dc-dc converter that transfers power between the low- and high-voltage buses. In order to reduce the costs and the volume of this converter, two new concepts for integrating the dc-dc converter functionality into the traction drive system have been presented in [1]. This paper will present different switching strategies to improve the efficiency of the total system. Starting from the phase-shift modulation known from the conventional Dual Active Bridge, the switching instants are optimized regarding efficiency to obtain the optimal modulation. In addition, a concept with multiple switching during the inverter's switching period is analyzed as well as a slightly modified system, where one leg of the dc-dc converter is implemented as a three-level switch. These four switching strategies are analyzed and compared. In addition, measurement results are shown to verify the calculations.