Silicon carbide inverters improve the performance of a car while enabling higher torque and acceleration. ON Semiconductor is taking part in the Formula E World Championship in collaboration with the Mercedes-EQ Formula E team to develop next-generation electric drives with SiC inverters.
The technical collaboration between ON Semiconductor and the High Power Performance (HPP) division of Mercedes AMG has brought technological advances for racing cars. In an interview with the EE Times, Dave Priscak, Vice President, Global Systems Engineering at ON Semiconductor, highlighted how Formula E design and testing go hand in hand, enabling the continuous improvement of inverter performance levels.
"When we started in Formula E, we came as a sponsor and only worked with the Mercedes team. We looked for ways to show what we have done and are doing for the electronic vehicle (EV) market," said Priscak. “Working with the development team, however, we quickly realized that we can learn a lot from each other from a technical point of view. And so it became more of a partnership than sponsorship. Our designers are working with the Formula E powertrain engineers to develop the next generation of traction inverters and power transmission 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 would find in a regular car. "
The heart of a Formula E vehicle is the engine, the drive system, which consists of three elements: battery, converter and motor. The inverter is the brain of the system. It is responsible for converting the direct current drawn from the battery into a high density alternating current that is sent to the motor. During the deceleration, however, the regenerative engine brake is activated and the current path is reversed. Formula E is the only motorsport event where the latest technology for next-generation electric vehicles is tested.
As a semiconductor with a large band gap, silicon carbide has a higher band gap energy than silicon (3.2 eV versus 1.1 eV). Since 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-source on-resistance (RDS (ON)), which is up to 300-400 times lower than that of silicon devices with the same breakdown voltage.
The benefits of using SiC technology in inverters include smaller circuitry and weight, improved weight distribution, and a reduction in overall power consumption. This is because SiC MOSFETs can operate at a much higher switching frequency, reducing the size of many of the circuit elements required in the inverter. SiC components can also work at higher voltages and currents than standard silicon power semiconductors, which increases the power density and reduces switching losses even at high temperatures.
Formula E car. Click to enlarge the image above (Source: ON Semi)
Racing power inverter
Formula E offers insights into maximizing efficiency and extending battery life. Priscak pointed out that there are many design considerations about how to transfer power in the drivetrain as efficiently as possible.
Racing cars require technology that can withstand violent shocks, strong vibrations and extreme temperatures. The more efficient the semiconductor device, the less energy is dissipated and heat is wasted, resulting in an improved power-per-watt ratio. At the same time, the engineers want to reduce the number of components in their cars in order to save weight and space.
In the Formula E area, it is almost exclusively silicon carbide, as Priscak emphasized. The power level from the battery to the motor is pretty straightforward. However, the motor drive is a very complex mathematical algorithm, but the power transition is not much different from current electric vehicles. “The problem is that in Formula E you have to be on the track for around 45 minutes with a lot of acceleration and braking. So the biggest challenge is to recover as much energy as possible. And that is very difficult. Because you have big, short power surges and the batteries can't hold all of that, ”said Priscak.
Currently, one of the biggest challenges in the powertrain is being able to capture all of the energy when braking and actually recharge the battery. The competition rules only allow one battery per race. The goal is therefore to work on the technology not only to restore as much of it as possible, but also to use it as efficiently as possible.
The performance of electrical energy storage technology (often viewed as the greatest enabler and limiting factor in an electric vehicle) is what holds and powers an electric vehicle engine. There are various technologies for storing electrical energy, such as: B. Ultracapacitors, chemical batteries and solid state batteries. Lithium-ion (Li) chemical batteries present the most practical balance between performance and economic viability.
Creating the next generation of gate drivers is another area ON Semiconductor is focusing on to maximize the conduction area of a silicon carbide MOSFET. “The difference is that, as I said, the race only has to last 45 minutes while a car has to last 10 years. By pushing the performance limits of silicon carbide, we learn how to maximize its service life. In Formula E, we concentrate on the entire drivetrain, from the digital processor to the engine. So not just the silicon carbide, but also the gate design, the driver design, the isolation barrier and all the elements that determine the efficiency of the powertrain, ”said Priscak.
Monitoring is vital in Formula E. It is important to measure every ampere of current that is circulating through the car. Every time you accelerate, brake or take a turn, you need to understand not only how much energy is lost, but also how much can be recovered. If the driver is too aggressive, the battery will never make it to the end, stressed Priscak. So there is the monitoring of the driving profile, especially the acceleration and braking, whereby every aspect of the drive train is analyzed.
“With all of this, temperature is a big deal, both from a battery perspective to ensure the terminals don't get too hot when accelerating, and at all power levels where temperature is monitored. There are many 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 that Priscak pointed out is how to drive the motor. “The motor is a big inductor that hates quick switches. If you have a quick switch that goes into a motor, the motor wants a sine wave. Silicon carbide switches much faster than the inductive load can absorb. So the way we drive engines needs to be continuously developed, ”said Priscak.
Formula E reaches the limits of power electronics and leads to a number of new SiC solutions. Electric vehicles will benefit from the new SiC power solutions through simpler cooling systems, greater range and better performance. They also extend the battery life of the electric vehicle and the charging of the batteries is significantly accelerated by improved on-board chargers and DC-DC converters.
The numerous partnerships between chip manufacturers and Formula E are expected to produce various technical solutions that offer opportunities for both SiC and GaN chip manufacturers.