Power management | Focus case, larger conductors are needed, which adds copper weight and cost along the length of the tether. The maximum permitted AC line impedance presented to a UAV’s onboard power supply is more aggressive at lower voltages for a given power level. The copper is lighter, and the impedance stabilisation is more straightforward for a UAS that uses an LV source. The higher voltage, however, brings with it higher safety requirements to control shock hazards, which tends to drive the need for thicker insulation on the tether power line. The longer the cable is, the greater the expected voltage drop. This drawback can be solved by increasing the cable’s cross-section, but the weight of the cable directly offsets payload weight capacity. For example, a 10 m copper cable pair with 1 mm diameter weighs 140 g without insulation, and a diameter of 2 mm will add 560 g for every 10 m. Heavier cables have higher material costs and are less flexible. In military applications, using a thick power line in a system that should remain undetected might be a disadvantage, as a UAV capable of lofting it could generate more acoustic noise or be more easily spotted. This is another situation where the distributed regulation and DC-DC conversion architecture can be used. This architecture supports both ELV and LV input options, with most UAS designers using higher voltages. An isolated, regulated DC-DC converter using the ZVS topology operates from the unregulated, wide-range input to generate an isolated, regulated output. The modules allow the design of an extremely flexible system with a wide input voltage range. One such module has an input voltage range of 200-420 VDC with a regulated 24 V output at up to 600 W. The architecture offers the ability to adapt to changing requirements. If the cable needs to be longer, the input range can accommodate the additional voltage drop per unit of tether length. Modules suitable for LV applications are available in 4623 through-hole packages that measure about 48 x 23 x 7 mm with an output of up to 600 W from a typical package weighing 29 g. Saving several grams on each DC motor enables more payload capacity. Driverless cars The same distributed architecture is increasingly being adopted in driverless cars. It can support not only the highpower AI chips but also the sensor network with their multiple cameras, radar and powerful Lidar sensors. The AI processors need local power of hundreds of amps from the 48 V bus, with local regulation where it is needed. The devices are also scalable. The architecture is based around current sharing, so they can be implemented in arrays. The DC-DC converters do not need to communicate as they balance the current flow, so that if one device is impaired the other automatically provides more current. That is key for the safety design to ensure the key functions in the vehicle continue operating. These discrete, distributed converters are 50% smaller in volume and 50% lighter than traditional versions, which is critical in the overall vehicle architecture. Their smaller size makes it easier to install them for the best routing in the vehicle and to use two smaller systems to create redundancy for safety while adding the diagnostic signals for ASIL-B safety qualification. The controller chips have now been AECQ-qualified for automotive use. There are two competing safety states for the power architecture in driverless cars. The most obvious state is to never fail to stop delivering power on demand, such as in a collision when the airbag is deployed. The other failure mode, where one converter fails, is more challenging for the power management. The scalability of the distributed ZVS architecture allows for an array of 2 kW chips, so if one chip fails the others continue to function. All the converters have their own silicon controllers and protect themselves with safety rules for over- and undercurrent and temperature. If a fault is detected, the devices go into a safety state until the condition is resolved, signalling a higher level controller via a PMbus link. That doesn’t have an impact on the power conversion, as the devices use current sharing with sine amplitude conversion, so if one goes down the others will pick up the power, up to the current and voltage limit. Conclusion New power management architectures are providing more efficiency for 45 Uncrewed Systems Technology | August/September 2023 A tethered UAV with ground-based power management systems (Courtesy of Vicor)
RkJQdWJsaXNoZXIy MjI2Mzk4