**Antonio Lamantia 1, Francesco Giuliani <sup>1</sup> and Alberto Castellazzi 2,\***


Received: 1 September 2020; Accepted: 9 October 2020; Published: 19 October 2020

**Abstract:** With the introduction of the more electric aircraft, there is growing emphasis on improving overall efficiency and thus gravimetric and volumetric power density, as well as smart functionalities and safety of an aircraft. In future on-board power distribution networks, so-called high voltage DC (HVDC, typically +/−270VDC) supplies will be introduced to facilitate distribution and reduce the associated mass and volume, including harness. Future aircraft power distribution systems will also very likely include energy storage devices (probably, batteries) for emergency back up and engine starting. Correspondingly, novel DC-DC conversion solutions are required, which can interface the traditional low voltage (28 V) DC bus with the new 270 V one. Such solutions presently need to cater for a significant degree of flexibility in their power ratings, power transfer capability and number of inputs/outputs. Specifically, multi-port power-scalable bi-directional converters are required. This paper presents the design and testing of such a solution, addressing the use of leading edge wide-band-gap (WBG) solid state technology, especially silicon carbide (SiC), for use as high-frequency switches within the bi-directional converter on the high-voltage side.

**Keywords:** DC-DC converters; multi-port dual-active bridge (DAB) converter; wide-band-gap (WBG) semiconductors; silicon carbide (SiC) MOSFETs; power converter

## **1. Introduction**

The use of 115VAC 400 Hz and 28VDC power networks is a historical feature of avionic electrical power generation and distribution systems. The AC power is used directly for high power loads, such as starting, and is then rectified and conditioned to supply the bus power of +28VDC distributed to aircraft control systems, flight decks, and entertainment systems. However, there has been a pronounced movement within the aerospace industry to shift towards cleaner, more efficient and lower maintenance aircraft design as a result of the emergence of high fuel prices, the global warming problem, and high operating costs. The electrification of the aircraft to replace hydraulic or pneumatic functions with electrical ones is one of the prime movers in this field, as in the concepts of the *More* and *All* electric aircrafts [1–3]. This refers in particular to the replacement by electric actuators of complex aircraft hydraulic actuator systems, thereby dramatically reducing weight, maintenance costs, fuel consumption, footprint of carbon dioxide, and operating costs. Furthermore, the removal of pneumatic engine bleed systems makes it possible to run the engine more effectively and thus to save additional fuel.

The replacement of hydraulic and pneumatic systems with electric actuators and systems greatly increases the total electrical power requirements for an aircraft, requiring new approaches to the safe and intelligent delivery of aircraft power. To achieve the higher power ratings in a feasible way, novel enhanced aircraft Electrical Power Distribution System (EPDS) architectures investigated over

recent years include smart power management systems, characterized by the presence of at least one high-voltage dc bus (HVDC, +/−270 V) and a low-voltage DC bus (LVDC, typically, 28 V), as illustrated very summarily, for instance, in Figure 1. Such architectures require the presence of HVDC/LVDC bi-directional DC-DC converters to interconnect and manage power transfer between the two bus levels, also enabling the addition of energy storage devices. Due to the abundance and variety of loads and still partly undefined power ratings (e.g., batteries) here, the focus is on a solution which enables a high degree of flexibility in relation to power scaling. In particular, a solution is pursued, in which a basic DC-DC converter power cell can be paralleled a number of times with control and supervision functionalities carried out by a unique central board. The prime drivers of the design are of course efficiency and power density, with an eye also to solutions meeting the single-fault tolerance expectations typical of avionic solutions. The design and test results of both single power cell and parallel operation are presented.

**Figure 1.** Generic illustration of future aircraft electrical network, including an high voltage dc (HVDC) power distribution bus: dedicated bi-directional DC-DC converters interface the 270 and 28 V buses; inverters cater for the AC loads (e.g., motors in pumps, actuators). Additional ports in the DC-DC converters may also be used to interconnect storage devices, with different voltage levels.
