Aerospace, Military & Defence
Optimizing DC-DC converter design to maximize efficiency in more electric aircraft applications
DC-DC converters are critical to the ongoing transition toward More Electric Aircraft (MEA), where electric solutions replace hydraulic and pneumatic systems. Henri Huillet, chief executive officer at GAIA Converter, looks at how a Size, Weight, and Power (SWaP)-optimized LLC converter design can maximize efficiency and power density while meeting the stringent environmental, voltage, and power requirements imposed by MEA applications
I n aerospace, the term MEA1 describes the
current trend toward replacing pneumatic, hydraulic and mechanical energy sources in conventional aircraft with electrical equivalents. This transformation aims to reduce the size and weight of components, paving the way for fully electric propulsion in the future. In electrical power conversion equipment, this reduction is enabled by better-operating efficiency at the ever- higher power levels demanded. Fewer losses also mean lower temperature rise in power electronics, benefitting lifetime and reliability. In addition, less energy is wasted from the aircraft's primary power sources, ultimately reading across to more miles per gallon and fewer emissions.
The acronym ‘SWaP’ encapsulates the key factors – Size, Weight, and Power - that need to be optimized. This article examines a recent research project that demonstrates how an optimal power conversion design can be achieved. The design was developed by the company GAIA Converter in collaboration with researchers at the Centro de Electrónica Industrial of the Universidad Politécnica de Madrid (Spain). Let’s start by examining the typical power conversion specifications and requirements of MEA applications.
Power conversion specifications in MEA applications
Electrical power rails in aircraft have been traditionally 115VAC at 400Hz and 28VDC, derived from three-phase generators (driven by an engine or APU) followed by AC-DC converters, with batteries backing up the DC rail. In MEA applications, the higher electrical loads mandate higher voltages to keep currents lower and cabling lighter, so 270VDC nominal is an accepted standard. The specified variation is 235V to 285VDC, but the connected equipment may need to operate under the abnormal conditions of 220V to 320VDC. Therefore, a common requirement is to convert the nominal 270VDC
20 July/August 2022
bus voltage down to 28VDC, with isolation, and then to lower to end-voltages such as 5V, 3.3V, or even sub-1V using downstream Point- of-Load converters.
Achieving optimal power conversion design
We will now dive into the project carried out by GAIA Converter and Universidad Politécnica de Madrid. The main objective was to develop a SWaP-optimized converter with the following specifications:
Input:
Output: Load:
Efficiency: Losses: Size:
235VDC – 285VDC normal, 220V – 320VDC abnormal 28VDC
250W minimum, 1.0kW nominal, 1.5kW maximum
96% minimum across normal input range at 1kW load 49.5W maximum
Temperature: To operate fixed to a baseplate at 90°C
58mm x 61mm x 13mm maximum
Additionally, any switching frequency variation from nominal had to be limited to +/-15% maximum while aviation EMI and shock/vibration standards also applied. An initial decision was to select the appropriate conversion topology. A ‘soft- switched’ circuit was chosen to ensure high efficiency and low EMI. This approach also enables high-frequency operation, allowing smaller magnetics, lower EMI, and higher power density.
There are two main categories of soft-switched converters: fixed-frequency, non-resonant, or variable-frequency, fully- resonant. The former includes the Phase Shift Full Bridge (PSFB), Dual active Bridge (DAB), Asymmetric half-bridge (AHB), and others. Although current stresses tend to be less-than-fully-resonant types, they are not optimal for wide load ranges, as they
Components in Electronics
Figure 1: LLC topology
do not guarantee soft-switching transients at light loads. They all need an additional inductor, which can be of high value, depending on the desired controllability, and not easily integrated.
Fully resonant converters have their downsides as well, but the ‘LLC’ type was chosen as it can maintain soft switching transitions even at no-load conditions and consequently, for its ability to regulate down to light loads with high efficiency across the operating range. The transformer winding inductance Lm is one ‘L’ in ‘LLC’ while leakage inductance Lr can be the other ‘L’ (figure 1). This is often an external discrete component, as its value
is small and avoids compromise in the transformer design, reducing efficiency. The power switches gate signals are simply 50 per cent duty cycle PWMs pulses under variable frequency control. For best efficiency, the secondary side rectification is often performed by synchronous switches, typically MOSFETs.
Operation and detailed design decisions The two inductors Lr and T1 primary (Lm), along with capacitor C1 and circuit parasitics in the LLC circuit, form a ‘tank’ with two resonances, which can be represented as a gain plot (see Figure 2).
Figure 2: The gain of the tank circuit formed by the LLC components
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