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38 waste power and potentially damage the ESC. A faster diode, with a lower reverse recovery time, will better capture the current spikes, helping greatly to reduce overshooting. MOSFETs with lower resistance tend to have higher input capacitances. This and similar trade-offs entail a difficult balancing act when designing ESC hardware architectures. Fortunately, advances in microelectronics have spurred a wide availability of high-quality MOSFETs, which are both compact in form factor and able to achieve very high slew rates, making them especially suitable for UAV ESCs. It is above 200-250 V that matters become less settled, however. Silicon MOSFETs operating at these voltages begin to suffer excessively high electrical resistance and input and output capacitances, as well as slower-reacting body diodes, all of which contribute to much slower switching. Therefore, at around 250 V it becomes beneficial to switch to motor controllers based on IGBTs or SiC transistors. IGBTs do not have ohm resistances as such, but are instead characterised by their collector-emitter voltage (Vce). Vce does not grow exponentially with switching frequencies in the way that copper losses do in MOSFETs, making IGBTs ideal for sustaining high currents. However, they are not optimal for high switching speeds, as they suffer from ‘tail currents’ – current leakages across the emitter-collector junction, even when the IGBT is turned off – during operation. This is spurring interest in SiC transistors. In the past couple of years, SiC transistors have emerged which have resistances low enough to sustain the currents required for high-performance UAV motors. Also, their diodes have the lowest reverse recovery times of most transistor technologies, giving them consistent and efficient switching patterns at very high voltages and slew rates. For example, some SiC-based transistors have been able to achieve switching from 0 to 450 V with 50 A currents within 50 ns, and with just 5-10 V of overshoot. Attempting the same with a silicon MOSFET results in overshoots of around 100-150 V. However, SiC supply chains are still prone to intermittent shortages and can have lead times exceeding six months, given how new the technology still is. Lead times are dropping though, particularly as suppliers make use of larger substrate wafers during initial construction. Capacitors Capacitors serve as power regulators in ESC architectures, smoothing out voltage and current, aiding in cooling, and protecting the motor from current spikes during braking or hard acceleration for example. Different types of capacitor can be used on ESC boards, and their selection should generally be on a case-by-case basis. Ceramic capacitors for example offer the best performance in terms of high-frequency response and low impedance against currents. The latter quality is especially important, as an ESC being driven at 500 A might still suffer ripple currents of more than 1000 A. Standard electrolytic capacitors can withstand only so many such ripples before they, and by extension the ESC, are destroyed. In very high vibration environments, however, ceramic capacitors can become cracked and suffer other forms of damage. They are also more expensive than other types, as a result of their lead times and the high quantities in which they are being taken up by the EV industry. Ceramic capacitors are able to filter high-frequency noise (primarily from voltage ripple) and low-frequency noise though. However, where low- frequency noise is a more pressing concern, electrolytic capacitors can be a safe choice. Made from aluminium or tantalum, they also cost less and are easier to obtain. Most motor controllers will contain two or three electrolytic capacitors, the exact number being tied to the level of interference the power bus can tolerate and to the type of electric motor being driven. Lower inductance motors for example will produce higher peak currents during switching, leading to higher levels of ripple and therefore noise, which will stress the capacitors and reduce the likely lifespan of the ESC. Also, if the input lines from the battery to the motor controllers are excessively long, the resulting greater input impedance means more capacitors will be needed to stabilise the voltage at the ESCs’ terminals. Many manufacturers therefore include notes in their documentation to add a certain amount of capacitance (for a certain amount of load) if any input leads exceed 1 m – and some ESCs are being designed with dozens of capacitors in low- impedance arrangements, to share the current safely between them. Controlling ESCs Pulse width modulation (PWM) is a well-established and widely understood format for communicating control inputs from flight controllers to motor controllers. It is generally accepted as the simplest December/January 2021 | Unmanned Systems Technology Although UAS motor controllers generally operate using silicon MOSFETs, IGBTs will become important for heavy-lift UAVs and urban air taxis (Courtesy of Embention)