Ultralightweight High-Efficiency Buck-Boost DC-DC Converters for Future eVTOL Aircraft With Hybrid Power Supply

The trend toward urbanization intensifies road congestion and thus increases interest in research on urban air mobility. Electric motors enable lightweight distributed propulsion in all-electric vertical take-off and landing (eVTOL) aircraft which combine the advantages of helicopters (low-operating space requirement for take-off and landing) and fixed-wing planes (efficient cruising). When aiming at ranges beyond 300 km, a hybrid power supply architecture leveraging the advantages of fuel-cells (FCs; with high-gravimetric energy density) and batteries (with high-gravimetric power density) is required, as weight is a key performance metric in airborne applications. To regulate the power flow between the two energy sources with their respective dc terminal voltages varying in a wide range, a buck-boost (BB) dc-dc converter is required. Such BB dc-dc converters are, however, understudied in the literature and gravimetric power densities today are limited to 20 kW/kg, whereas a goal of 80 kW/kg is stated for 2040. This article systematically identifies the optimal BB dc-dc converter realization among a set of topologies with respect to weight and mission profile efficiency, considering cutting edge component technology, e.g., SiC power semiconductors and 3-D printed complex geometry liquid-cooled aluminum heatsinks. Two 15-kW demonstrator systems with wide bandgap power semiconductors are built and extensively tested, i.e., an all-SiC two-level four-switch BB dc-dc converter with 44 kW/kg and an all-GaN three-level eight-switch BB dc-dc converter with 86 kW/kg, thus complying with the 2040 power density target and exceeding state-of-the-art BB dc-dc converter systems by a factor of four with respect to the gravimetric power density.