Power-to-Weight Ratio: The Key Metric for Tethered UAVs

April 3, 2018

UAVs (“drones”) are being used for many different applications. Maximizing flying time is one of the most important things designers of these systems must consider, and so they are often tethered to the ground with a cable, allowing indefinite flying time.

Weight is a key factor that determines the flying time of battery-powered UAVs. Using a tether will enable a heavier payload, which allows the UAV to carry more equipment to perform more functions.

tethered UAVOne customer was designing a UAV, powered from a multi-kilowatt vehicle-based power system via a tether that delivered 24V to the on-board electronics. The tether, however, still contributed a significant amount of weight, and so the engineers looked for ways to reduce the impact of the tether on the maximum payload.

With power losses in the tether proportional to the square of the current, increasing the voltage can significantly reduce the size and weight of the tether. In the instance a customer chose to use a 400V DC supply. This reduced the current by almost 20x, allowing for a smaller cross-section conductor to be used, cutting the weight of the tether.

To power the on-board electronics, the 400V supply had to be stepped down to 24V with a power system that could deliver 75A, while adding minimal weight to the system. Vicor DCMs in a ChiP package are ideal solutions as they can be easily paralleled without additional components to deliver the power required and weigh only 29.2g each. A further benefit of the DCMs was their small size (47.91 mm x 22.8 mm x 7.26 mm), which left ample space on the UAV for additional payload.

The final solution uses four DCMs with a nominal 300V input. The wide input range of DCMs means that these power components can accept inputs from 200V to 410V, and even with a heatsink the complete 1.8kW solution had a footprint of just 43.69cm3. The high power density of the solution, and the significant savings in tether weight meant that the customer could offer UAVs that could be deployed with a significantly larger payload than was possible when the 24V tether was used.

The DCM isn’t the only power component that enables lighter tethers. BCMs are bi-directional components that can step-up the voltage before transmission along the tether cable, as well as step-down the voltage on the drone. Our High Voltage BCMs support up to 410V transmission and will supply up to 125A at the load. Recently, we announced 700V Ultra-High Voltage BCMs, which further reduce the current, enabling even smaller conductors and therefore lighter tethers.

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