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elements needed in the inverter. SiC devices can also operate at higher voltages and currents than standard silicon power semiconductors, increas- ing power density and reducing switch- ing losses even at high temperatures.
Racing power inverter
Formula E provides insight into how to maximize efficiency and extend battery life. Priscak pointed out that there are many design considerations in ensur- ing how to transfer energy in the pow- ertrain as efficiently as possible.
Race cars need technology that can withstand violent shocks, strong vibra- tions, and extreme temperatures. There’s also a design emphasis on effi- ciency because the more efficient the semiconductor device, the less power is dissipated and heat wasted, resulting in an improved power-per-watt ratio. Engineers also aim to reduce the com- ponent total to save weight and space.
The semiconductor technology used in Formula E cars is almost exclusively SiC, as Priscak pointed out. The power stage from the battery to the engine is pretty straightforward. Motor drive is a very complex mathematical algorithm, but the power transition is not much different from current EVs. “The prob- lem is that in Formula E, you have to be on the track for about 45 minutes, with a lot of acceleration and braking,” Priscak said. “So the biggest challenge is to recover as much energy as possi- ble. And that’s very difficult because you have big, short bursts of power, and the batteries can’t absorb all that.”
One of the biggest powertrain chal- lenges is whether it can capture suffi- cient energy during braking to charge the battery. Competition rules allow only one battery per race, so the goal of those working on the technology is not only to recover as much of the energy as possible but to use it as efficiently as possible.
Formula E is pushing the limits of power electronics technology and leading to a range of new silicon carbide solutions.
Electric energy storage technology is both an enabler and limiting factor for EV performance. There are vari- ous technologies for storing electrical energy, such as ultracapacitors, chemi- cal batteries, and solid-state batteries, among others. Lithium-ion chemical batteries currently offer the most prac- tical balance between performance and commercial viability.
Creating the next generation of gate drivers is another focus for onsemi as it works to maximize the conduction area of a SiC MOSFET. “The difference is that the race, as I said before, only has to last 45 minutes, whereas a [pro- duction] car has to last 10 years,” said Priscak. “So by pushing the limits of silicon carbide’s performance, we are learning how to maximize its lifetime. In Formula E, we are focused on the whole powertrain, from the digital processor to the motor; so not just the silicon car- bide, but also the gate design, the driver design, the isolation barrier, all the ele- ments that determine the efficiency of the powertrain.”
Monitoring is crucial in Formula E. It’s important to measure every amp that’s being circulated through the car. Every time you accelerate or brake or make a turn, you need to understand not only how much energy is lost but also how much could be recovered. If the driver is too aggressive, the battery will never make it to the end, Priscak pointed out. So the driving profile, acceleration, and braking in particular is monitored, and every aspect of the driveline must be analyzed.
“In all this, temperature [monitoring] is a big thing — both from the bat- tery point of view, to make sure the
terminals don’t get too hot in accelera- tion, and in all the power stages,” said Priscak. “There are lots of sensors not only for current and voltage but also for temperature.”
SiC is a very fast, high-voltage switch, which presents challenges for driving the motor, said Priscak. “The motor is a big inductor that hates fast switches,” he said. “If you have a fast switch going into a motor, the motor wants a sine wave. Silicon carbide is switching much faster than the inductive load can take. So there needs to be continuous innova- tion in the way we drive motors.”
Formula E is pushing the limits of power electronics technology and lead- ing to a range of new SiC solutions. EVs will benefit from the new SiC power solutions by having simpler cooling sys- tems, longer range, and better perfor- mance. SiC will also extend the battery life of the EV, and battery charging will be much faster with improved on-board chargers and DC/DC converters.
The numerous partnerships among chip companies and Formula E are expected to bring forth various engineering solu- tions, creating opportunities for both SiC and GaN chip manufacturers.
This article was originally published on EE Times on April 27, 2021.
Maurizio Di Paolo Emilio is editor-in-chief of Power Electronics News and EEWeb.
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ASPENCORE GUIDE TO SILICON CARBIDE