Optimization of GaN-based Series-Parallel Multilevel Three-Phase Inverter for Aircraft applications

The electrification of means of transport is a challenging reality that has taken over the scene over the years. The socio-environmental impact of burning fossil fuels and their future scarcity guided sustainable and versatile efforts of energy consumption development. The aerospace domain has set ambitious goals regarding gas emissions limitation, leading to new concepts and system architectures in several sectors. The replacement of heavy systems, such as mechanical, pneumatic, and hydraulic systems, reduces aircraft weight and fuel consumption with their electrical equivalents. Several projects have emerged in the last decades aiming to replace these systems using new technologies.This thesis presents an optimized design of a DC – AC converter (filters included) with gravimetric power density superior to 8 kW/kg and higher efficiency than 98.5 % for a 70 kVA MEA power drive system. Among different inverter topologies and semiconductors technologies, a 7-level flying capacitor three-phase inverter, composed of three paralleled legs of 3-level flying capacitor topology per phase with 4 GaNs in parallel per switch, was chosen to be built. The prototype presented around 14 liters and 10.04 kg (it comprises filters, metallic structure, and control board). The measured weight is less than 4 % superior considering the solution proposed in simulation, which corroborates the precision of the models used in the optimization procedure. A traditional 2-level converter using 1200 V for the same operating conditions shall be 74 % heavier than the 7-level inverter, with a power density 43.4 % smaller than the multilevel topology. On three-level flying capacitor leg with 4 GaNs in parallel per switch was extensively explored, and the solution reached almost 99.2 % of measured efficiency, for 38 A / 5.8 kW at the output of the converter (DC-DC Buck test) and 98.8 % in the monophase inverter test (42 A / 6.6 kW at the output of the converter) , both at 40 kHz with an RL load (resistor + inductor). The experimental verification of the entire converter led to 98.6 % efficiency at 24.6 kVA (flf = 240 Hz, fsw = 40 kHz and Mi = 0.76). Due to circulating currents between paralleled legs, inverter safe operation was limited to 500 V/38 kVA/87 A (70 kVA/130 A nominal operation) with open-loop control. Regarding EMI modeling, this thesis proposed a generic model for multilevel converter equivalent circuit with good correspondence until 5 to 6 MHz (supported by experimental results) and can assist predictive design of EMI filter.