Uncrewed Systems Technology 051 l Primoco One 150 l Power management l Ocius Bluebottle USV l Steel E-Motive robotaxi l UAVs insight l Xponential 2023 p Issue 51 Aug/Sept 2023 art 2 l Aant Farm TPR72 l Servos l Tampa Deep Sea Barracuda AUV

39 flight control surfaces, DC bus voltage regeneration and soft starter controls to limit the inrush current. This requires more IPMs with more intelligence. The latest hybrid power drive modules combine four power switches, a controller and a thermistor into a single package weighing 150 g and measuring 108 x 67 x 27 mm. The modules support voltages from 5 to 15 kV as standard using a silicon carbide (SiC) MOSFET transistor and a diode or a hybrid approach with a silicon IGBT power transistor and an SiC diode. SiC has better behaviour for a given power density than silicon IGBT devices. This reduces the overall junction temperature and gives a 1-2 percentage point efficiency boost. It also boosts battery life significantly. The modules use a silicon nitride (SiN) substrate for better stability and reliability than aluminium nitride. SiN provides enhanced bending strength, fracture toughness and thermal conductivity. The modules can be attached to the substrate with high-temperature solder or pressure silver sintering. The sintering process starts with treating the die with a silver layer a few tens of microns thick. A pressure of 1-10 MPa and a temperature of 250 oC is then applied to attach the die. This provides a better thermal interface than soldering. The modules can be supplied with or without a baseplate. The heat generated in the modules can be hundreds of watts so they need to be mounted on a heat sink or to a metal part of the airframe. Power architecture Another power architecture splits the functionality of the DC-DC converter into the isolation and step-down function, and the regulation of the voltage. With this architecture, a Pre Regulation Module (PRM) supports a wide input range to a voltage that is close to the optimal point that gives the best conversion efficiency. Compared to a hard-switched, multi-phase topology, the zero-voltage switching (ZVS) and zero-current switching (ZCS) topology, running at the highest practical frequency, is more space-efficient and dissipates less power. A ZVS and ZCS topology does not have the high-frequency, harmonicseries noise profile character that occurs with a hard switching topology. With an operating frequency above 1 MHz, the architecture avoids the 100-500 kHz frequency content that interferes with radio systems and other components. This low harmonic content and high fundamental conversion frequency means the noise filter is smaller, and engineers can create low common and differential-mode (CM and DM) noise designs, particularly when component arrangements and device interconnects are properly considered. As always, input and output filters are required and must be designed and placed properly, but the inherent nature of the architecture make this task easier. As an example, a power delivery network (PDN) for a converter with a 28 V input at 300 W is converted using an isolated, regulated DC converter. The low profile of the thermally efficient module enables a cooling solution using an available cold wall and leads to a 818 W/cu in power density that reduces the PDN’s footprint. Solar power Gallium arsenide solar modules are popular for UAV operation, as they provide high efficiency conversion with low weight, but they need sophisticated power management to provide the highest levels of efficiency. Power management | Focus SiC devices have a better behaviour for a given power density than IGBT, and give a 1-2 percentage point efficiency boost and a longer battery life Uncrewed Systems Technology | August/September 2023 A distributed architecture for UAVs (Courtesy of Vicor)