Unmanned Systems Technology Dec/Jan 2020 | Phoenix UAS | Sonar focus | Construction insight | InterGeo 2019 | Supacat ATMP | Adelan fuel cell | Oregon tour | DSEI 2019 | Copperstone Helix | Power management focus

93 Power management | Focus VTOL rotary UAV uses a single connector per servo line to eight servos, with a connector for both the battery packs and the avionics. A developer of a smaller UAV wanted to reduce the number of connectors, so each connector had more pins, but that increases the risk of a failure. Kelvin leads OBCs are emerging as a key application for 650 V and 1200 V GaN and SiC devices, as both support higher temperature operation, allowing higher frequency switching. That allows smaller magnetic components to be used and so gives a higher power density. The higher frequencies often mean the new GaN and SiC devices often cannot work with the existing controllers, called gate drivers, that are designed for silicon IGBT or MOSFET devices. However, taking a different approach to power management allows SiC devices to use the existing gate drivers. That means the SiC devices can be dropped into existing designs. This will give higher efficiency, as they can run at higher temperatures and so reduce the amount of cooling needed – also leading to higher reliability – but will have limited effect on increasing the power density. Increasing the power density is achieved by co-packaging a silicon- junction JFET with a SiC cascode device that is controlled by the JFET. By altering the resistance of the JFET gate, the device can run at a higher frequency, but that creates problems with older packages that have three leads. These three-lead packages have a high inductance and give designers problems with ‘ringing’, which is where the voltage struggles to settle down when the device switches. This usually means the devices run at lower frequencies. Chip makers have added a fourth lead, called a Kelvin lead, to allow faster operation. It requires more work on the layout and EMI electrical interference issues, but it allows designers to use that fourth lead for a gate loop. That avoids false triggering by using the additional connection to the top source metal pad. In a three-lead device the 40-50 A that flows through the device has to flow through the source wires, and if all that is part of the gate-driving loop there can be large spikes in the voltage. The additional lead is used to bypass the source wires and make a direct connection to the source metal so that all the high current is flowing in the central lead. That means the Kelvin lead only carries a small amount of the gate current, so none of the big voltage drops are seen by the gate driver circuit, allowing the existing gate drivers to be used. The SiC component is also small, which cuts the capacitance, allowing the SiC devices to switch faster. It also means a small snubber ceramic capacitor can be used, with a value of 220 pF, which can switch 900 V in 15 ns. That allows higher frequencies for smaller OBC designs, at around 500 kHz, up to around 6 kW. Before this, the three- lead packages were used at 50-125 kHz. There is also some headroom for tweaking the devices to support 1 MHz switching for designs with higher Unmanned Systems Technology | December/January 2020 Researchers are looking at ways to use standard printed circuit boards to mount high- power wide-bandgap silicon carbide transistors (Courtesy of the Fraunhofer Institute) Gallium nitride devices can now be built on a silicon carbide substrate, which could boost the performance of power transistors by a factor of three (Courtesy of SweGaN)