DC-to-DC Power Conversion: Building Block Strategies Break Through the Complexity
Like most homes, the power distribution within aircraft has traditionally been AC. However, technology developments have enabled power systems designs to turn towards higher voltage DC distribution systems that can provide greater overall efficiency while simultaneously being more compatible with energy storage (batteries) and load requirements. Yet reliance on AC-DC conversion and distribution is a design approach that inherently multiplies the number of power electronics required – at odds with the principles of Size, Weight, Power, and Cost (SWAP-C) that are so critical to success. The SWAP-C challenge is compounded by exponential growth in the amount of electrical power required onboard aircraft.
More systems are increasingly electric, from more sophisticated cockpit avionics and more electrically actuated systems to creature comforts such as coffee, food service, and entertainment. It’s an upward trend, not only in the amount of power but also in the complexity of overall systems. This generational trend applies to both military and civil aircraft, but the rise of all-electric aircraft promises even greater disruption on the power landscape. Many of the urban air mobility systems in development are fundamentally DC platforms – powered by batteries storing DC power – eliminating the need for AC-DC power conversion at point of load. Designers of traditional aircraft, with gas turbines driving auxiliary power units that are naturally AC, must embrace the shift and recognise the critical value of DC power distribution strategies.
Real-world design strategies are complex and diverse
When power is both generated and distributed in DC form, voltage conversions are required, dependent on how and where the power will be used onboard the aircraft. Is power delivered and then stepped up or down at the device? Or is it converted and then distributed? There is no ‘right’ answer to this not-so-simple question. Strategies vary widely and consider the systems involved, the type of aircraft and how it is used, system weight, reliability, scalability, longevity, and more. Power needed for a well-defined area of the aircraft, for instance the 28 Volts typical to cockpit avionics, could be achieved with only power conditioning from a local battery source. Alternatively, the required 28Vdc could be converted centrally and distributed as needed.
This approach applies best to smaller, less complex architectures, as wider distribution options can result in heavier cabling wherein power is passed to a variety of devices. A smarter design might increase the voltage as a trade-off to reduce cabling requirements, for example creating a primary distribution at 540 Volts supported by a localised secondary distribution at 28 Volts. In this design, step down is not required at every single load, and a localised 28 Volt network could be designed to support multiple devices with one conversion. This type of design is also advantageous for electronics performing in more remote areas of the aircraft, such as a heating system or controls on the wing. Cabling weight is a more significant concern over long distances, so power is ideally distributed in a single higher voltage option and then stepped to its required voltage locally near the device. Complexity increases when systems have secondary loads that require multiple voltages, such as 28, five, and three volts.
New modularity holds great industry promise
The aerospace industry dreams of a day when its needs are met by thousands of parts churning off a production line, where quality can be measured statistically rather than through validation. Unlike the automotive industry, aircraft volumes are not typically consistent with this vision. The industry response is to deliver modular, building block components that offer both performance and scalability in a much quicker time to market. Modularity applies on two levels, the first of which is the device itself. Instead of development from the ground up, with full modeling and analysis of each new design, design models are being parametrically defined so they can be applied to a range of applications.
These can be readily tailored into bespoke designs suitable for the full spectrum of customer secondary power needs, whether a low-power control unit of perhaps 100 Watts or a heavier end load that could require 6000 Watts. By collecting a superset of requirements and working to meet many of them in a single device, custom designs are minimised. This scenario requires a close eye on the efficiency of the device, likely a point of trade-off for simplicity and accelerated design. To solve the efficiency challenge, a family of building blocks is needed to allow interchangeably at different points in the design range. Each has its own performance window, guiding the requirement to scale up or scale down for maximum efficiency. Performance is protected, timelines are accelerated, and proven engineering is reused as much as possible. The second level of modularity lies with the end unit itself. Is it possible to design for multiple reuse of a 2kW converter, one that has been designed for a particular application, in the power distribution strategy for a 4kW or 6kW application?
Yes, it is. With modular, proven systems, this approach not only delivers custom power levels but also aligns with the power redundancy needs of all-electric aircraft. For example, a system could be designed using five separate but coordinated 2kW converters to deliver nominally 10kW. This can be engineered for flexibility and redundancy, with automatic routing into the system such that the loss of one converter can be managed to retain overall integrity. This strategy is analogous to having four engines on a traditional aircraft; in the unlikely event one fails, the others compensate to keep the aircraft safe and operational. It’s also an approach that can be more weight efficient than traditional “A lane/B lane” duplication. While power control systems traditionally tend to be isolated, engineering advancements underway demonstrate this is the path forward for electric aircraft power.
More power… but smaller, lighter, lower cost, and faster
Technology advancements from the massive global investment in energy storage technology (cells/batteries) have moved the once science fiction vision of small electric flying vehicles into a nearfuture reality. For engineers, holistic thinking is integral to the evolving challenge of aviation power system design, particularly as DC-DC power conversion and distribution earn greater recognition as an enabling technology.
SMART POWER DISTRIBUTION AT ALTITUDE
Every electronic system requires DC power. Traditionally, aircraft electrical power is generated from an engine and AC generator set of some form. AC power is distributed and then converted to DC at or near the point of load - yet existing designs and solutions must continually evolve in the face of SWAP-C requirements. The automotive industry provides an example and is further along its electrification journey as legislation and commercial pressures have forced designers to innovate and solve similar challenges. The need to store large amounts of energy for the electrical motors required for new automotive propulsion systems has naturally created a DC source for secondary loads.
This design approach is familiar for automotive engineers as electrical power has traditionally been provided from a battery, although today’s trend is towards higher voltage solutions both for end loads and distribution. High-end automotive platforms such as the Porsche Taycan have emerged with 800Vdc systems as a standard, due to faster charging times as well as the need for reduced cabling size and weight. At the same time, managing the complexity of power system design in aviation is an entirely different challenge. Many of the power conversion systems for voltages required don’t yet exist, as they’ve never been needed. Although the automotive industry may provide a model that has solved many of these issues for vehicle power requirements, flight-qualified systems are inherently different.
They must be engineered to meet SWAP-C concerns and demonstrate an enhanced level of reliability suited to the nature of air transport and travel. In today’s civil aviation market, 270 Volts has emerged as the current DC power distribution system standard. That said, future platforms are looking to extend this to 540 Volts, given that +/-270 Volts can be achieved effectively on an aircraft even at altitude – an environmental factor with unique impact on power system design. Performance characteristics such as breakdown or arcing behave differently under the effects of altitude, adding a new layer of design challenge to the already complex world of high voltage power distribution. However, electric aircraft, such as urban air taxis, generally do not operate at altitudes comparable to civil or military aircraft.
Absent this limitation – and given their need for greater electrical requirements – these crafts can more readily operate at higher voltages and may mimic the 800 Volt standard currently established in automotive markets.