**1. Introduction**

The modern aerospace industry is facing challenges in terms of improving emissions, fuel economy, and also cost. The US National Aeronautics and Space Administration (NASA) has laid out ambitious goals for NO*<sup>x</sup>* emissions and fuel consumption for the next three generations of subsonic aircraft, namely N + 1, N + 2, and N + 3 [1,2]. These goals are given in Table 1. The more electric aircraft (MEA) architecture is the state-of-the-art technology that was developed to address those challenges. The reduction of the total full consumption is about 3% [3]. However, such performance is far behind the targets. The essential reason is that, in one typical large commercial aircraft such as Boeing 777, 95% of jet fuel is used for generating propulsion thrust [4]. Hence, a radical propulsion system design is required to meet the abovementioned goals.

The so-called turboelectric distributed propulsion (TeDP) concept, which is now intensely investigated by research agents and companies such as NASA and Airbus [1,5], is considered as the key to meeting those goals. Under this concept, electric generators driven by gas turbine engines supply electrical power to multiple electric fans, which are distributed across the airframe to provide

propulsion [2]. The major benefit of this concept is that the separation of gas turbine engines and propulsion fans enables both components to be designed in their optimal points and operated at optimum speeds. It is anticipated that, with the application of this technology coordinated with advanced body design, the fuel consumption can be reduced by 70%.

**Table 1.** National Aeronautics and Space Administration (NASA)'s goals for future-generation subsonic aircraft [2].


Under the TeDP concept, the total power capacity of the electrical system onboard expands to the megawatt (MW) level. Such high power raises lots of challenges to electrical system designs. Weight is the most critical design factor for all aerospace components, which is usually evaluated with a specific power (unit: kW/kg). It was reported that 1 kg saving will save roughly 1700 t of fuel and 5400 t of CO2 per year for all air traffic and decrease system costs by US\$1000 [6]. Compared to traditional aircraft propulsion systems, TeDP systems require additional high-power generators, distribution cables, and motor drive systems. All these components should achieve extremely high specific power, before TeDP systems can show its significant benefits. The research summarized in [7] showed that the traditional voltage standard of ±270 V for MEA is no longer suitable for this power rating and a higher voltage rating is preferred. In addition, the total weight of the overall electrical system can be optimized [8].

The power electronic converters are the key to enabling components in a TeDP architecture. They realize the power conversion between AC and DC powers. The designs of power electronic converters are quite essential in aerospace electric power systems. For motor drive systems onboard, power electronic parts include two converters: one rectifier and one inverter.

In a traditional MEA architecture, a rectifier usually is realized by a passive equipment called autotransformer rectifier unit (ATRU). Schematics of typical ATRUs can be found in [9,10]. They essentially consist of multiwinding autotransformers plus diode rectifiers. Although ATRUs are proved to be practical in MEA architectures, its power density is rather low because of heavy autotransformers. According to the survey of some commercial products, the specific power of an ATRU is around 1.6–3.3 kW/kg, depending on the power rating [11,12]. In an industrial motor drive system, active front end (AFE) rectifiers are used to achieve a better-quality AC input. By eliminating the use of autotransformers, the total weight of AFE rectifiers can be potentially lower than that of ATRUs. Furthermore, AFE rectifiers can bring potential energy-back capability and recover kinetic energy in some aerospace applications such as E-taxiing [13,14]. Therefore, evaluation and comparison should be carried out between an AFE-rectifier-based power converter and a converter with an ATRU rectifier to show which solution is better for TeDP architectures.

On the other side, two-level (2L) converters are the classic and also the only commercial solution for motor controllers in aerospace, before the TeDP concept emerged. The major reason is that, due to low DC link voltages, 2L converters are sufficient to realize high conversion efficiency. However, following the increase of the DC link voltage rating, 2L converters are no longer suitable for TeDP applications because of their low efficiency and low power density. The comparison in [15] showed that multilevel converters can achieve better efficiency and better power density in higher-power applications. Research activities related to topologies with three-level (3L) [16,17], five-level (5L) [18], and even higher-voltage-level [19] output were investigated. However, these researches did not compare

different multilevel converter topologies with different voltage levels to show which topology is more favorable in certain applications. In addition, they did not investigate back-to-back configurations, which has more flexibility in selecting topologies on both rectifier and inverter stages. As a result, comparative work could be done to find the optimal solution based on a certain specification for aerospace applications. With the specification, an optimal design is also worth being investigated to show potential improvements that can be made compared to conventional designs.

In this paper, comparisons are made between different multilevel converter topologies. Based on the comparison results in terms of potential power density and power loss, a high-power-density power converter design for aerospace TeDP applications is demonstrated, which is named as the asymmetrical back-to-back converter topology. It utilizes a 5L converter in a rectifier stage and a 3L converter in an inverter stage. A compact triple-surface-mounted heatsink structure is designed to realize high density and manufacturable thermal management. This paper is organized as follows. In Section 2, the specification of a motor drive system for comparison and design is given and analyzed. In Sections 3 and 4, the topology evaluation and selection as well as the design for thermal solutions are demonstrated. In Section 5, the design is verified by electrical and thermal testing, where the results of the testing are shown. A final evaluation and comparison session are given in Section 6. Section 7 concludes the paper.
