DC to DC Power Distribution in MilAero Applications

Power system design is seeing impact from steady growth in the number, type, and complexity of electrical systems onboard aircraft. Advancements in cockpit avionics and more electrically actuated systems have designers paying attention to a broader spectrum of upgraded power needs and their connection to SWAP-C goals.

At the same time, innovators worldwide are investing in technologies supporting smart energy storage, viable for new transportation applications. Fuel cells and batteries are quickly advancing urban air mobility for tangible commercial advantage – and the U.S. Air Force’s Agility Prime initiative has come on board to amplify the impact.

Supporting the ideal of procuring commercial technology with military value, the Agility Prime program leverages various assets of the U.S. Air Force – such as safety certifications, test ranges, and assigned missions enabling steady logging of flight hours – with the goal of shaping, driving, and accelerating the urban air mobility market. Its stated role is intended to reduce risk and build confidence in the technology, create and incentivise investment opportunities, and ultimately accelerate the development of aircraft.

Cockpit avionics, which typically rely on 28-volt systems, may only require power conditioning from a local electrical bus or battery.

Aiming to field a new class of aircraft as soon as 2023, Agility Prime targets three different areas for military value: a people mover or air taxi type of craft, capable of transporting three to five individuals; a smaller variant designed for one to two people; and a cargo-only version. Resupply missions are top of mind, but broad value is anticipated from electrically-powered craft that can reduce or eliminate heavily manned missions into dangerous environments.

DC Power and Mitigating SWAP-C

While most aircraft power distribution has historically been AC (like a typical home), higher voltage DC distribution may be more efficient. It’s also more compatible with battery storage and an increasing portion of aircraft power load requirements. At the same time, the gas turbines present in traditional aircraft typically fuel AC auxiliary power. Its requisite AC-DC conversion can multiply power electronics and ultimately increase cost, complexity, and weight. The DC power advances demonstrated by urban air mobility are providing design insight, storing DC power in batteries and eliminating AC-DC power conversion at the point of load. For designers of power systems onboard more traditional aircraft, it’s an approach driving greater consideration of DC-DC power distribution and conversion strategies in aircraft design.

Power electronics are designed with attention to diverse factors such as magnetics, discrete components, expected performance, and thermal efficiency, helping developers define and address design problems earlier. This strategy also supports the increasing need for design scalability – reducing costs and accelerating timelines in an environment anticipating greater power needs across a spectrum of flight-qualified applications. These factors point to a more holistic approach raising in importance as well, recognising the enabling characteristics of DC-DC power conversion and distribution technologies.

Voltage conversion and power conditioning are required when power is distributed as DC, however these factors vary with specific power applications onboard the aircraft. One design may deliver a chosen voltage and then convert it up or down at the device; another may adjust a power level before it is distributed to its intended application. In choosing a path, designers must not only consider the aircraft and its role in flight operations, but also factors such as lifecycle, required reliability, scalability, system weight trade-off, and more. Because of these complexities, as well as continued rapid evolution of power demands for aerospace applications, a building block strategy proves useful. Developers can accelerate development with proven, modular designs that can be sourced for both performance and scalability.

The Royal Air Force’s venerable Typhoon fighter aircraft will be replaced by the Tempest, a 6th generation combat aircraft expected to enter service in the mid-2030s. (Image: BAE Systems)

DC Power Distribution Designs

Individual areas of an aircraft may only require power conditioning from a local electrical bus or battery. Cockpit avionics provide an example, typically relying on 28-volt systems. These systems, contained within a specific area of the aircraft, provide an example of power requirements that may only require power conditioning. Alternatively, designs may opt to centrally convert and distribute 28 VDC to specific systems and areas onboard. Because of the cable weight associated with wide distribution of power, it is the smaller, simpler architectures that are ideal candidates for this approach. Redundancy and manageable degradation of performance can be built-in. For military airframes, these values are critical to improving safety and survivability.

As a design tactic, increased voltage offers a functional trade-off against cable weight. This can be illustrated using a primary, 540-volt distribution supported by a localized secondary distribution of 28 volts. Because the secondary 28-volt network can be designed to support power needs of several devices via a single conversion, system developers can avoid designing voltage step down into each specific load. This adds critical value for power electronics servicing relatively isolated or difficult to access areas of the aircraft. It’s a strategy that also reduces cable distances and, therefore, weight as higher voltage power is distributed and converted locally at the device. Designers will, however, find complexity increases when secondary loads must deliver a range of voltages, powering systems, and subsystems requiring three, five, or 28 volts.

New Voltage Standards Taking Cues From Automotive Advances

Power system design for systems at altitude is entirely different from the design of power systems integrated into automotive and racing vehicles. That said, is it surprising that a motorsports technology provider is part of the project team for Tempest, the UK’s 6th generation combat aircraft?

Tempest replaces the Royal Air Force’s Typhoon and is expected to come into service in the mid-2030s. The fighter is an ambitious undertaking that will predictably need extended amounts of electric power as well as upgraded, higher power for distribution to various advanced systems across the aircraft. Tempest anticipates including augmented and virtual reality interactive cockpit displays, drone management capabilities, and advanced stealth and data fusion technologies.

Here the automotive industry has insight to share, long focused on energy storage and designing secondary loads powered by DC sources. Implementing battery-driven systems may be a more familiar challenge for automotive power system designers, however, the demand is still shifting toward higher voltage systems used for heavy end-loads and distribution across a vehicle. Incorporating 800 VDC systems as a standard, Porsche Taycan provides a luxury example – cable size and weight are minimized, supporting high-end vehicle performance and speedy charging.

In contrast, 270 volts has emerged as the DC power distribution system standard in the current civil aviation market. But future platforms are intent on extending this to 540 volts since +/-270 volts can be effectively attained on an aircraft even at typical altitudes – an environmental factor that has unique impact on power system design. For example, breakdown or arcing perform differently at altitude, adding a new design challenge in the already complicated world of high-voltage power distribution.

However, electric aircraft, including urban air taxis, do not normally operate at the altitudes required by civil or military aircraft. Without this limitation and given their need for more robust electrical requirements, these craft can easily operate at higher voltages and may mimic the automotive market’s currently established 800 volt standard. The altitude parameters for military applications, however, may drive to a different optimal distribution voltage.

Experience gained by the automotive industry in terms of energy storage and designing secondary loads powered by DC sources can someday be applied to the air mobility market. (Image: iStock)

Modular Power Designs

Using pre-defined parametric models featuring full-design models and analysis, developers can customize secondary power systems quickly. Ideally, these building block devices are engineered to meet a range of requirements, a “superset” of design parameters. By focusing on the device’s efficiency – and expecting trade-offs to maintain a simple, speedy design process – power system developers can significantly streamline the resources required for a full-custom design. Building blocks developed as a family of options are interchangeable, each operating within a window of performance and creating a clear path for designers to scale up or down as efficiency dictates. A key benefit here is flexibility, with the same building blocks applicable to systems ranging from a low-power, 100-Watt control unit to end-loads as high as 6000 Watts.

The ability of urban eVTOL aircraft to transport people and supplies can readily extend to meet the needs of the U.S. armed forces – keeping troops out of harm’s way, delivering equipment, conducting search and rescue missions, and more. (Image: iStock)

Designers can further capitalize on modularity within the end-unit itself. Power distribution design for a 4 kW or 6 kW application can tap into a 2 kW converter, designed for more than just one type of use. Such a system design could integrate five 2 kW converters – separate but coordinated system components – delivering a nominal 10 kW. Proven technology is applied to meet customised power levels, while automatic power routing simultaneously ensures system reliability, even in the unlikely event of failure of any single converter.

Isolated performance has been a common theme in power control system design used in aircraft applications. However, built-in power redundancy is a basic tenet of all-electric aircraft – lighter than “A Lane/B Lane” duplication strategies and proving to be an optimized design path for electric aircraft power.

Urban air mobility is imminent and already transforming aviation as we know it. Its ability to transport people and supplies can readily extend to meet the needs of the U.S. armed forces – keeping troops out of harm’s way, delivering equipment, conducting search and rescue missions, and more. DC power technologies unlock the value of electrified aircraft and represent an important shift in the design of power systems today – and tomorrow.

This article was written by Julian Thomas, Engineering Director, Aerospace & Defence, TT Electronics (Woking, UK). For more information, go here .