Comparative Evaluation of Ultra-Lightweight Buck-Boost DC-DC Converter Topologies for Future eVTOL Aircraft

Abstract

Hybrid battery/fuel cell power supplies for all-electric Vertical Takeoff and Landing (eVTOL) aircraft enable a high peak power capability as well as long-range operation. However, the wide and overlapping voltage ranges of the battery and the fuel cell require an interconnecting bidirectional DC-DC converter with buck-boost capability. For such a converter system, the gravimetric power density is a key metric. Therefore, this paper systematically investigates the weight limits of several converter topology concepts: Starting from a standard two-level four-switch non-inverting buck-boost converter benchmark system, fully soft-switched operation, as well as flying-capacitor multilevel and partial-power-processing variants are compared by means of a comprehensive Pareto optimization (mission-profile efficiency vs. gravimetric power density). Aiming at a mission profile efficiency of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\boldsymbol{\overline{\eta} &gt; 98.5}$</tex> %, the results indicate the feasibility of converter realizations with gravimetric power densities of up to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\gamma &gt; 62\text{kW}/$</tex> kg with a three-level flying capacitor multilevel converter. Further, a virtual prototype of a two-level four-switch non-inverting buck-boost benchmark system is presented with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\gamma=46\text{kW}/$</tex> kg, hence illustrating the trade-off between converter performance and complexity.