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96 power density. This will reduce losses by a factor of three or four. The devices around the power system also have to improve to keep up with the higher performance. For example, film capacitors have been developed with a new high-temperature dielectric material to make them suitable for designs using SiC semiconductors. To minimise the lead lengths, and thus the parasitic inductances, the capacitors are connected directly to the power modules by means of busbars. The problem here is that the chips are operated with high barrier termination temperatures, which can also be conducted via the busbars to the DC link capacitors. The temperature limit of conventional film capacitors with a dielectric of biaxially oriented polypropylene (BOPP), however, is only 105 C.  The dielectric is a combination of two basic materials. One is semi- crystalline polypropylene, which is ideal for processing into films; the other is amorphous cyclic olefin copolymer (COC), which can tolerate high temperatures. The resulting dielectric (COC-PP) can be used at temperatures in excess of 125 C with much lower derating, while retaining the self-healing properties of BOPP. In addition, it enables films as thin as 3 µm to be manufactured. The substrates and packaging for the power electronics are also key to power management. A project in Germany for example is developing a module for SiC transistors on a traditional printed circuit board design that is already established in industry and is easy to implement, rather than a ceramic or metal substrate. The difference is that the module will not use a wire bond connection but connections that are embedded directly into the circuit via a galvanically produced copper contact, meaning the cable can be shortened and the power routing optimised. New materials A hybrid GaN-on-SiC material is promising significant power savings in EVs and charging systems. The technology, called Transmorphic Heteroepitaxy, grows an atomic layer less than 1 nm thick on a silicon carbide substrate. This accommodates the mismatch between the crystal lattices of the GaN and SiC, which is the major cause of reliability problems. It also allows high-quality GaN-based structures to be built on top. A team in Sweden has used the technique to build a GaN transistor that has a total thickness of less than 300 nm on the SiC substrate. It has a critical breakdown field of 2 MV/cm and a vertical breakdown voltage of more than 3 kV – nearly three times higher than that of GaN devices built in the standard way on a mainstream silicon wafer. That means the ON-resistance of the device has the potential to be 10 times lower, leading to much lower losses and higher switching frequencies for devices at 650 V and higher. It also leads to smaller system designs. AlN is also becoming more popular as a power management technology. Researchers in Germany have built aluminium scandium nitride (AlScN) devices for the first time, using a technique called metal-organic chemical vapour deposition (MOVCD) rather than traditional sputtering. This is a faster, higher quality way to build an AlScN substrate. However, there was no source of scandium available for MOVCD, so the researchers developed a new material and technique that they are now patenting to grow the AlScN layers with a high crystal quality and the right amount of scandium to develop the next generation of power transistors. These first layers have a sheet resistance of about 200 Ω /sq, a mobility of about 600 cm 2 /Vs and a charge carrier density of about 4.0 x 10 13 /cm 2 . These layers will be used to build AlScN transistor for power applications. As new materials increase the power density and handle much higher levels of power, so the risk of a failure has ever more significant consequences. With driverless taxis operating for most hours of the day, and UAVs operating well beyond visual line of sight, making sure the designs are as safe and reliable as possible is essential. This is leading to a new generation of components for safety-critical designs in power management. Acknowledgements The author would like to thank Mike Kultgen at Analog Devices, Andrew Dunlop at Millswood Engineering, Benjamin Tschida at VisionAirtronics, Dr Anup Bhalla at UnitedSiC, Tai Chen at SweGaN and Niall Lyne at Renesas Electronics for their help with researching this article. December/January 2020 | Unmanned Systems Technology Focus | Power management A 100 mm hybrid wafer for building gallium nitride power devices on a silicon carbide substrate (Courtesy of SweGaN)