Silicon carbide (SiC) devices for the next-generation of electric vehicles

Before electric cars become a familiar sight on Europe’s roads, the technology behind them needs to evolve to improve the size and efficiency of their drivetrains, the speed that they can be charged and the range that they can drive. The silicon carbide inverter could be crucial to helping to overcome these issues

18 Mar 2021
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Before electric cars become a familiar sight on Europe’s roads, the technology behind them needs to evolve to improve the size and efficiency of their drivetrains, the speed that they can be charged and the range that they can drive. The silicon carbide inverter could be crucial to helping to overcome these issues

Sales of battery-electric vehicles (BEVs) are increasing as consumers and automakers alike benefit from European government subsidies for electric vehicles. Momentum is gathering pace as automakers including Ford, Jaguar Land Rover and Volvo have all announced plans to phase out internal combustion engines in their vehicles.

Despite the small steps being taken so far by the technology, the move towards an electrified fleet in Europe is unstoppable and the days of petrol and diesel engines are already numbered. It is a question of when, not if, we will all be driving electric cars in the future. 

But before we reach this point, the technology behind BEVs needs to evolve to overcome a number of challenges, which include the size and efficiency of their drivetrains, the speed at which they can be charged and the range that they can cover.

A crucial element in all this is the inverter. This component controls the input of electricity from the battery or batteries to the motor. Aside from converting incoming direct current to alternating current, the inverter controls the variability of power supplied to the motor when the driver demands it. As motors develop and migrate from 400-volt electrical systems to the far more versatile 800-volt units, the inverter is playing a crucial role. 

Increasingly, the industry is moving away from silicon to the use of silicon carbide inverters (SiCs), which have a number of significant advantages when twinned with high-voltage systems.

Thomas Steffen, Senior Lecturer in Control Engineering at Loughborough University in the UK, explains: “A conventional inverter is about 97% to 98% efficient in moving energy from the battery to the motor, and a SiC-based inverter can push this to an amazing 99%. Although this is only a modest increase of 1% to 2%, the benefits for the whole vehicle can be more significant.”

The two materials are very different in that they have varied physical parameters. Silicon carbide has the advantage that it is well suited to higher thermal conductivity, which is important in high-revving, high-voltage motors.

“Compared with other silicon conductors, silicon carbide has a higher maximum operation temperature, which means you can squeeze more power out of it,” says Roland Bittner, a senior engineer at Semikron Elektronik, which is a partner of Drivemode, an EU research project. Its goal is to produce small adaptable electric modules consisting of power electronics including an SiC-inverter, a gearbox, and the motor itself. These modules are capable of being scaled up from one unit to four according to the power requirements of different vehicle segments.

SiC inverters are ideal in such a modular approach because they allow a reduction in the size of all the crucial components. Bittner says: “If you go to higher battery voltage [such as 800V] and require less current, you also need smaller cables and smaller cables means less cost, less weight in the vehicle. It is also easier to assemble [the modules] in the vehicle.” 

Watch the video interview on SiC inverters in electric cars with Roland Bittner, engineer at Semikron, Germany 

Steffen adds: “Through reduced weight and increased regenerative braking, the range and the efficiency of the vehicle can increase by about 3%. For a typical vehicle, this translates into about 10 miles (16 km) more range.”

Driver anxiety in relation to the distance a BEV can drive between charges is one of the biggest hurdles in their acceptance among consumers and key to next-generation models. A small 800-volt enabled module, as used by Drivemode, frees up additional space that not only benefits internal passenger and storage possibilities, but also enables the placement of additional batteries in order to extend range in BEVs. 

Hyundai Motor Group is among the first automakers to commercialise 800-volt systems with their new Electric-Global Modular Platform (E-GMP). The company unveiled its IONIQ 5 on the platform last month, and the automaker says the crossover model can reach 80% of its full power within 18 minutes on a fast charger. This is roughly a fifth of the time a conventional 400-volt car needs to reach the same charge, and the added efficiency of the SiC inverter means more of this power survives to drive the vehicle.

“We want to offer best efficiency and this starts [with less lost power] from charging, up to driving and recuperation. With 800-volt charging, the current can go directly to the battery without any transformation (possible losses),” a spokesman said. The automaker is expecting to launch 23 new Hyundai and Kia BEV models by 2025 in a bid to hit 1 million cars by mid-decade. 

Like the Drivemode project, the company has packaged its electric powertrain system into a compact module that includes a SiC inverter, something that Thomas Steffen expects to see more and more automakers adopting in the move to next-generation BEVs.

“Now with SiC inverters, it makes sense to connect the inverter and motor into one unit with a common case and cooling circuit,” he says. “What is interesting about the Drivemode project is that they aim for a drive unit that is both integrated and modular. So the inverter is integrated into a common case with the motor,” he adds.

Such technology is leading industry observers to forecast that BEVs will likely reach cost parity with internal combustion engine vehicles by mid-decade, which means they could soon be a more common sight on Europe’s roads.  

By David Jolley

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