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The global transition to electric mobility has led to a transformation of the automotive sector, called upon by demand for automotive OEMs to create increasingly efficient and high-performance electric vehicles. Electronic technologies, such as wide bandgap semiconductors, play an essential role in enabling the achievement of these objectives.
Increasingly, power-conversion technologies that are both effective and affordable are sought after by electric and hybrid vehicles. Wide-bandgap (WBG) semiconductors outperform traditional silicon in terms of performance thanks to the advantages provided by silicon carbide (SiC), and will soon replace and outperform traditional silicon-based power devices—especially those used in electric vehicle (EV) design.
EV design challenges
The success of electric mobility is closely linked to the achievement of high efficiency. In an EV, efficiency reaches its maximum potential under full load conditions (typically, when load > 90%). In city driving, where the load can step down to 10%, systems like regenerative braking play an essential role in increasing vehicle efficiency by up to 30%.
For this to be possible, the power consumption of all electronic components and systems installed in the EV shall be minimized, while respecting the space and weight constraints imposed by the automotive sector.
Other challenging requirements automotive components must meet include high reliability (the number of defective parts per million has reached single digit) and excellent thermal management. Silicon carbide is a semiconductor material able to meet these requirements, replacing and outperforming traditional silicon-based power devices, such as MOSFETs and IGBTs.
STMicroelectronics (STMicro) is one company taking advantage of SiC for automotive design.
“STMicro is very active in proposing solutions to the automotive industry that can allow designers to improve efficiency in all the main electric vehicles systems, which include traction inverters, DC-DC converters, onboard chargers, and also climate control units,” Filippo Di Giovanni, strategic marketing, innovation, and key programs manager at STMicro, told EE Times. “We pioneered the first automotive-grade silicon carbide MOSFETs, paving the way to electric vehicles capable of running longer distances for a given battery pack, thus reducing the range anxiety.”
SiC Applications in EVs
With a critical electric field strength of 2.8 MV/cm—much higher than that of silicon (0.3 MV/cm)—SiC allows designers to apply thinner epitaxial layers to the semiconductor substrate, reducing the surface resistance of the component and the power losses. Furthermore, this property allows SiC to reach very high breakdown voltages, even of the order of a few kV.
As a result, these devices can switch effectively at frequencies that are far higher than those reached by traditional silicon. The size of passive components and magnetic devices like inductors also lowers because of the higher switching frequency. Consequently, the system’s overall size is significantly decreased, thereby increasing the power density. Further reducing system weight and volume, SiC’s wide bandgap and strong thermal conductivity enable high-temperature operation with straightforward cooling control.
New high-performance and long-range EVs will be based on SiC because conventional silicon-based power devices, such as IGBTs, are not able to further reduce their dissipation, weight and size, and these are all prerequisites for improvements in efficiency. Furthermore, the imminent transition of high-voltage batteries from 400 V to 800 V imposes even more stringent voltage requirements on the power devices used.
The main use case: main inverter
One of the main applications of SiC in EVs is the main inverter, which is the circuit that converts the high DC voltage coming from batteries into the AC voltage required to power the traction electric motor.
Compared to IGBT-based inverters with the same topology, SiC offers an efficiency improvement between 6-10%. For the main inverter, the lower conduction losses of SiC are a key advantage, especially under partial load conditions. This improvement in efficiency translates into longer range or smaller battery size, which saves space and costs.
“Based on our success story regarding silicon carbide, I have to say that SiC MOSFETs are, and will still be for quite a long time, the main choice for building traction inverters because of the large availability of 1200 V devices, their proven and well-established ruggedness, and a simpler gate drive design,” Di Giovanni said.
DC-DC converter and onboard charger
In battery-powered EVs (BEVs) and plug-in EVs (PHEVs), the electric motors are powered by a traction inverter that receives energy from large, high-voltage battery packs. The inverter can be either connected directly to the high-voltage battery, or through a DC-DC converter depending on the battery’s nominal voltage, 400 V or 800 V respectively, as shown in Figure 1.
Inside the vehicle, there are also different systems that operate at 12 VDC or 48 VDC. These voltages are supplied from the high-voltage battery pack via DC-DC converters.
Charging requires a DC supply. However, AC supply is more easily accessible in homes and commercial spaces, and therefore an onboard charger that converts AC to DC is required. The exception is fast DC charging, only available in some charging stations.
Both DC-DC converters and OBCs require power devices that provide high efficiency, high switching frequency, and excellent thermal management. Meeting these challenging requirements, SiC has become the first choice, providing a combination of high efficiency, low switching losses, high switching frequencies, lower heat dissipation, and smaller magnetic components.
SiC manufacturing
Once successfully adopted into the EV arena, SiC devices will further increase manufacturing. This will eventually scale down prices similar to silicon-based devices following mass manufacture. The decrease in cost is a crucial milestone because SiC devices are more expensive than silicon-based ones, with SiC material costing nearly 2× to 3× as much as silicon.
“After having created a vertically integrated and full supply chain, we’re now building a new integrated fab close to our main production site in Catania, Italy,” Di Giovanni said. “It will be the first of its kind in Europe and our target is to source by 2024 at least 40% of our SiC wafer needs using in-house internal substrates produced in this new plant.”
The new SiC manufacturing processes, with wafers from 200 to 300 mm, will make it possible to achieve better yields and reduce production costs, making them comparable to those of traditional silicon.
“We have already proven our ability to build 200-mm SiC wafer prototypes in Sweden,” Di Giovanni said. “It is also important to underline that our equipment and machinery can already handle 200 mm, and therefore we expect the transition to be quite smooth.”
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