Designing a Bidirectional DC-DC Power Converter for use in Electric Aircraft

Abstract

The ever-increasing need for power and efficient methods of its storage, delivery, and conversion across many industries led to environmental concerns regarding the emission of greenhouse gases and noise pollution due to the current trajectory of global climate change. This is especially true of the aviation industry where traffic exponentially grew over the past few decades until the COVID-19 pandemic, with current forecasts predicting full recovery or even growth compared to pre-pandemic levels by 2024. Thus, the International Civil Aviation Organization (ICAO) and the National Aeronautics and Space Administration (NASA) in the US set standards and targets to reduce noise and greenhouse gas (NOx and CO2) emissions by the 2020s and 2030s respectively, with the latter being more aggressive, needing significant leaps in aircraft technologies to achieve as they are unfeasible with current ones. The most promising leap to achieve such goals is the electrification of aircraft and their systems, needing significant upgrades in power and energy densities of electrical systems and thus the power electronic devices they need. This thesis focuses on a bidirectional DC-DC converter to deliver power to and from an energy storage system, critical in electric aircraft, though power devices, their cryogenic performance, and testing methods at room and cryogenic temperatures are also covered.

During the literature review, key requirements were identified for converters to be used in electric aircraft: enabling bidirectional power flow; high power density and efficiency; high switching frequency to minimise filters and transformer size; galvanic isolation; and reduced electromagnetic interference. Due to the flexibility of Dual Active Bridge (DAB) DC-DC converters in addition to satisfying the identified requirements, they were chosen. Similarly for energy storage, Superconducting Magnetic Energy Storage (SMES) systems were chosen for integration with a DAB converter for a few reasons. They offer very fast response times (in ms) and can discharge a lot of power within a very short time, meaning they can quickly respond to major power losses, or satisfy high power demand as expected from take-off or landing electric aircraft. Furthermore, previous studies used SMES either in hybrid energy storage systems where a chopper was enough, or large power grids where preferred topologies included multilevel converters, giving space for a novel design methodology to size, operate and control both a DAB converter and SMES in one system. Finally in terms of simulation software, Plexim’s PLECS was selected due to its focus on power electronic systems, ease of use, flexibility, breadth and support of simulation and analysis tools, and good support from manufacturers in terms of device models.
The combined DAB converter with SMES system presented in this thesis used the Single Phase-Shift (SPS) control scheme and was successfully implemented as a model built in PLECS, with both the charging and discharging modes delivering power to the SMES and from it as designed and expected. Design challenges like the range of Zero-Voltage-Switching (ZVS) operation inherent to the DAB converter and how various parameters affect RMS currents were discussed, as were control challenges including considerations for integration into an electric aircraft’s power system, effects of dead-times, and practical implementation of the DAB converter and the SMES chopper. The control system’s operation of both the SPS DAB converter and SMES chopper were detailed, with the SPS DAB converter mathematically modelled and the resulting bode plot shown alongside one generated by the PLECS model. Additionally, possibilities of modification and improvement in terms of the system’s control scheme were presented for exploration in future works, with the Dual Phase-Shift scheme implemented in PLECS as an example.

In terms of power devices and improvements, studies concerning their role in electric aircraft suggested better switching, lower losses, and other improvements at cryogenic temperatures. The literature review on this aspect focused on Power Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs), Insulated Gate Bipolar Transistors (IGBTs), and Gallium Nitride (GaN) High-Electron-Mobility Transistors (HEMTs), with more focus on Silicon (Si) and Silicon Carbide (SiC) devices. The literature showed Si and GaN devices consistently improving when cryogenically operated in most criteria, with Si devices suffering from carrier freeze-out at very low temperatures, while SiC devices offered inconsistent results showing improvements were possible but not always.

Static and dynamic tests to characterise semiconductor devices were carried out at room temperature to attain graphs and values similar to datasheet ones as per industry standards. Static characteristics included forward I-V and on-state resistance curves, while dynamic ones included double pulse test waveforms and derived values such as switching times, delays, and losses. Additionally, the static test was attempted at cryogenic temperatures using an IGBT module, though it was unsuccessful as its encapsulant failed at the tested temperatures. The main purpose of the tests was familiarisation with the testing methods and to obtain results at room temperature as well as cryogenic ones if possible. While the first two were met, valuable insight into failure modes and a review of cryogenic testing methods and requirements were gained from the third. Consequently, future works can improve these tests using lessons learned here while minimising the identified limitations.

Date of Award	13 Sept 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Xiaoze Pei (Supervisor), Vincent Zeng (Supervisor) & Peter Wilson (Supervisor)