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Silicon Carbide: The Foundation of a Successful BEV Powertrain

Jun 23, 2020
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The electric vehicle (EV) market is growing rapidly by all accounts. The International Energy Agency’s 2019 EV Outlook reports that the global electric car fleet exceeded 5.1 million in 2018, up 2 million from the previous year. There were also 460,000 electric buses, 250,000 light commercial vehicles (LCVs), and 260 million electric two-wheelers by the end of 2018. Together, EVs consumed about 58 TWh of electricity that year, saving 36 million tons of carbon dioxide equivalent compared to the same number of internal combustion engine (ICE) vehicles.1

That green trend is expected to continue and analysts like Research and Markets estimate that the global EV market will grow by 19.8% over 2020–2030 to reach nearly $1.3 trillion.2

The EV powertrain

Our drive toward all-electric transport is made possible by powerful, lightweight, and efficient powertrains of battery-powered EVs (BEVs). The powertrains (Figure 1) comprise:

  1. An on-board charger with an AC/DC converter that converts the grid power to DC intermediate voltages and a DC/DC block that converts DC to the voltage required for fast-charging the battery. The charger also directs energy harvested by regenerative braking to the battery. Ideally, it should be bidirectional to allow it to take advantage of future smart grids and deliver energy back to the grid or home as needed.
  2. A battery pack.
  3. An inverter, which is part of the drivetrain supplying power to the motor, usually either a permanent magnet synchronous motor (PMSM) or an induction motor.

Figure 1: The BEV powertrain comprises an on-board charger (left of the battery), the battery itself, and a drivetrain that includes an inverter driving a traction motor (right of the battery).

Success in the BEV market hinges on automakers addressing “range anxiety” among customers. To increase a BEV’s range, or distance travelled between charges, automakers can increase battery size, increase system efficiency, decrease weight, or implement a combination of all those options.

For instance, market researcher Adamas Intelligence claims that Tesla dominates in deployed battery capacity because its BEVs use batteries ranging from 60 to 100 kWh against the global sales weighted average of 19.7 kWh for all EVs (BEVs and all other types) sold in 2019 H13. That trend is catching up with the new Volvo XC40 crossover housing a 78-kWh battery pack, 2020 Mercedes EQC (80 kWh), 2019 Audi E-Tron (95 kWh), and the 2019 Nissan Leaf Plus (62 kWh).

Additional battery capacity, however, adds to the BEV’s weight as well as cost. Therefore, reducing the powertrain system weight and increasing its efficiency can further increase the BEV range or keep costs in check.

The key enabling technology for this is silicon carbide (SiC). Compared with the traditional silicon (Si) technology, SiC devices offer:

  • A 2× to 3× lower on-state voltage drop than Si
  • Lower leakage currents than Si at a given temperature in the off state
  • Higher edge rates during switching by virtue of being majority-carrier devices
  • 10× higher breakdown field than Si, allowing SiC devices to withstand higher voltage in the same package
  • Higher thermal conductivity enables safe operation at higher chip temperatures

Shedding weight

The cooling system needed to keep components of the powertrain within a safe operating temperature range adds significantly to the weight that a BEV must drive around at all times. Silicon carbide’s high thermal conductivity helps carry away the heat more quickly, and a SiC device’s ability to operate at higher temperatures eases the pressure on thermal solutions.

Silicon carbide devices can operate at 100× to 1,000× higher switching frequencies than Si devices. This reduces the size of the magnetics required in converters, which further reduces the system size and weight.

For example, a BEV’s Si-based drivetrain could weigh more than 15 kg. High-performance SiC-based inverters have been known to save 6 kg in weight and 43% in volume.4

Increasing efficiency

Silicon carbide’s higher switching frequencies and lower intrinsic losses increase system efficiency. In a bidirectional on-board charger, SiC enables the use of a totem-pole PFC topology that is highly efficient and cost-effective compared with implementations possible with Si-based devices. The DC/DC side takes switching frequencies into the 150-kHz to 300-kHz range that is up to 3× faster than Si-based circuits.

Such a reference design for a 6.6-kW charger uses 16 SiC MOSFETs, such as Wolfspeed’s latest silicon carbide series, to achieve peak efficiencies greater than 96%.5

Revolutionizing inverter design

The drivetrain is a challenging application in that it deals with powers ranging from 90 kW to more than 350 kW and there is no industry standard. However, SiC’s advantages of high efficiency, smaller size, and lower weight can be extended to the inverter in the BEV drivetrain as has been well established through tests.

Silicon carbide technology enables up to 30% smaller system size, up to 80% lower losses, and a lower system cost based on a benchmark comparison of SiC- and IGBT-based 200-kW inverters by Delphi.6 Because BEVs are not driven at the optimum full-load conditions most of the time on the road, a decrease in losses further gains in significance to increasing range.

Another comparison at an even higher power of 300 kW has shown that a CRD300DA12E-XM3 SiC-based inverter weighs just 6.2 kg against the 14.06-kg Si-based design and delivers a volumetric power density of 32.25 kW/L against Si’s 3.2 kW/L.7

However, the revolution goes beyond power density. Silicon carbide’s faster switching speed, lower dead time, and smaller voltage drop and output capacitance result in a lower phase voltage distortion and a higher bandwidth. Besides the inverter-level benefits, the SiC MOSFET-based designs affect features of the dynamic performance of a motor drive system, such as delivering faster response, and higher relative stability and robustness compared with Si-based implementations.8

Enabling the BEV market

It comes as no surprise that the power SiC market continues to grow and is expected to surpass the $3 billion mark by 2025, posting a year-to-year growth rate higher than 13%, according to Yole Développement, and driven mainly by the EV market.9

From the fast-charging off-board infrastructure to the BEV powertrain, SiC technology is fast proving itself to be not just revolutionary but foundational as it helps automakers finally address consumers' range anxiety.

Wolfspeed’s SiC components, full-SiC power modules, and SiC power die and services are providing the pillars for that foundation.

References

  1. IEA, Global EV Outlook 2019 (https://www.iea.org/reports/global-ev-outlook-2019)
  2. Research And Markets, Global Electric Vehicle (EV) Market 2020-2030 (https://www.researchandmarkets.com/reports/4990283/global-electric-vehicle-ev-market-2020-2030-by)
  3. Adamas Intelligence, State of Charge: EVs, Batteries and Battery Materials
  4. Can't take the heat? Silicon Carbide Holds the Answers to Your Applications (https://www.arrow.com/en/research-and-events/articles/silicon-carbide-holds-the-answers-to-your-applications)
  5. Guy Moxey, Wolfspeed, SiC Enabling EV Applications
  6. Guy Moxey, Wolfspeed, Silicon Carbide: Transforming the Future of Power (https://www.wolfspeed.com/knowledge-center/article/silicon-carbide-transforming-the-future-of-power)
  7. Daniel Martin, Wolfspeed, Revolutionizing Inverter Power Density using SiC MOSFET Modules (https://www.wolfspeed.com/knowledge-center/article/revolutionizing-inverter-power-density-using-sic-mosfet-modules)
  8. Xiaofeng Ding, et al., Impact of Silicon Carbide Devices on the Dynamic Performance of Permanent Magnet Synchronous Motor Drive Systems for Electric Vehicles (https://pdfs.semanticscholar.org/5606/ddb55b325a376fa510a52880d4111c3ccd7e.pdf)
  9. Yole Développement, Compound semi. Quarterly Market Monitor, March 17, 2020 (http://www.yole.fr/Compound_Semiconductor_Monitor_Q1.aspx)
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