ON Semi SiC Powers Formula E Inverters

Article By : Maurizio Di Paolo Emilio

The technical collaboration between ON Semiconductor and Mercedes AMG's High Power Performance (HPP) division has provided technological advances for the race car.

Power inverters made using silicon carbide improve a car’s performance while also allowing increased torque and acceleration. ON Semiconductor is participating in the Formula E World Championship with a partnership with the Mercedes-EQ Formula E team on the development of next-generation electric powertrains equipped with SiC inverters.

The technical collaboration between ON Semiconductor and Mercedes AMG’s High Power Performance (HPP) division has provided technological advances for race cars. In an interview with EE Times, Dave Priscak, vice president, global systems engineering at ON Semiconductor, highlighted how design and testing go hand in hand in Formula E, enabling the continuous improvement of the inverter power stages.

“When we started in Formula E, we came in as a sponsor, working only with the Mercedes team, and we were looking for some ways to show what we did and are doing for the electronic vehicles (EV) market,” said Priscak. “But very quickly, working with their development team, we realized that there is a lot we can learn from each other from an engineering perspective. And so it became more of a partnership than a sponsorship. Our design engineers are working with the powertrain engineers at Formula E to develop the next generation of traction inverters and power transfer systems for Formula E. Most of what we learn can be applied to commercial solutions but the torque ratios are very different from what you find in a normal car.”

The heart of a Formula E vehicle is the power unit, the propulsion system, which consists of three elements: battery, inverter, and motor. The inverter is the brain of the system. It is responsible for converting the direct current taken from the battery into a high-density alternating current to be sent to the engine. During deceleration, however, the regenerative motor brake is activated, and the current’s path reverses. Formula E is the only motorsport event to test the latest technology in next-generation electric vehicles.

SiC Technology
As a wide bandgap semiconductor, silicon carbide exhibits larger bandgap energy than silicon (3.2eV versus 1.1eV). Because more energy is required to excite a valence electron in the conductive band of the semiconductor, higher breakdown voltages, higher efficiency, and better thermal stability at high temperatures can be achieved. The main advantage of a SiC MOSFET is the low drain-to-source ON-resistance (RDS(ON)), up to 300-400 times lower than that of silicon devices at the same breakdown voltage.

The benefits of using SiC technology in inverters include smaller circuits and less weight, improving weight distribution and reducing overall power consumption. This is because SiC MOSFETs can be operated at a much higher switching frequency reducing the size of many of the circuit elements needed in the inverter. SiC devices can also operate at higher voltages and currents than standard silicon power semiconductors, increasing power density and reducing switching losses even at high temperatures.

SiC formula E
Formula E car. Click to enlarge the image above (Source: ON Semi)

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 as to how to transfer energy in the powertrain as efficiently as possible.

Racing cars need technology that can withstand violent shocks, strong vibrations, and extreme temperatures. In addition, the more efficient the semiconductor device, the less power is dissipated and heat wasted, resulting in an improved power-per-watt ratio. At the same time, engineers also aim to reduce the components of their cars to save weight and space.

In the Formula E space, it is almost exclusively silicon carbide, as Priscak pointed out. The power stage from the battery to the engine is pretty straightforward. However, motor drive is a very complex mathematical algorithm, but the power transition is not much different from current EVs. “The problem is that in Formula E you have to be on track for about 45 minutes with a lot of acceleration and braking. So the biggest challenge is to recover as much energy as possible. And that’s very difficult. Because you have big, short bursts of power, and the batteries can’t absorb all that,” said Priscak.

Right now, some of the biggest challenges in the powertrain are if it can capture all the energy during braking and actually charge the battery. Competition rules only allow one battery per race, so the goal is to work on the technology to not only recover as much of it as possible but to use it as efficiently as possible.

Electric energy storage technology (often regarded as the biggest enabler and limiting factor for an EV) performance is what holds and supplies electricity to an EV motor. There are various technologies for storing electrical energy, such as ultracapacitors, chemical batteries, solid-state batteries, among others. Lithium-ion (Li) chemical batteries currently offer the most practical balance between performance and commercial viability.

Creating the next generation of gate drivers is another area ON Semiconductor is focused on, in order to maximize the conduction area of a silicon carbide MOSFET. “The difference is that the race, as I said before, only has to last 45 minutes, whereas a car has to last 10 years. So by pushing the limits of performance of Silicon Carbide, we are learning how to maximize the lifetime. In Formula E, we are focused on the whole powertrain, from the digital processor to the motor.  So not just the silicon carbide, but also the gate design, the driver design, the isolation barrier, all the elements that determine the efficiency of the powertrain,” said Priscak.

Monitoring is crucial in Formula E. It’s important to measure every amp of current 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 there is the monitoring of the driving profile, acceleration and braking in particular, analyzing every aspect of the driveline.

“In all this, the temperature is a big thing, both from the battery point of view to make sure the terminals don’t get too hot in acceleration and in all the power stages monitoring the temperature. There are lots of sensors not only for current and voltage but also for temperature,” said Priscak.

Silicon carbide is a very fast, high-voltage switch, and the biggest challenge Priscak pointed out is driving the motor. “The motor is a big inductor that hates fast switches. 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 innovation in the way we drive motors,” said Priscak.

Formula E is pushing the limits of power electronics technology and leading to a range of new SiC solutions. Electric vehicles will benefit from the new SiC power solutions by having simpler cooling systems, longer range, and better performance. They will also extend the battery life of the electric vehicle, and battery charging will be much faster with improved onboard chargers and dc-dc converters.

The numerous partnerships among chip companies and Formula E are expected to bring forth various engineering solutions, creating opportunities for both SiC and GaN chip manufacturers.

This article was originally published on EE Times.

Maurizio Di Paolo Emilio holds a Ph.D. in Physics and is a telecommunication engineer and journalist. He has worked on various international projects in the field of gravitational wave research. He collaborates with research institutions to design data acquisition and control systems for space applications. He is the author of several books published by Springer, as well as numerous scientific and technical publications on electronics design.

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