Silicon carbide: coming of age in electric vehicle powertrains
Using silicon carbide (SiC) for power switches enables higher power densities and switching efficiencies in electric vehicle (EV) powertrains.
The automotive industry is going through an unprecedented transformation from internal combustion engine (ICE) vehicles to electric vehicles (EVs). Driven by regulations across the globe to curb carbon dioxide emissions, EVs are expected to reach 45% of the total new vehicles sold by 2030 [1,2]. Against this evolving backdrop of mandatory regulations, consumer acceptance of EVs is likewise increasing.
This article addresses the benefits of the rapid adoption of silicon carbide (SiC),awide-bandgap semiconductor switch in power electronics systems in EVs, and the value of wafer-level substrate manufacturing. Power electronics based on SiC enable EVs to achieve longer driving ranges, faster charging, and lower total system-level cost of ownership. These benefits are achieved by leveraging the highly differentiated set of material properties of SiC to design powertrain systems that are more efficient, robust, and compact.
Increasing power density for performance improvement
While significant strides are being made to lower the battery cost by increasing battery capacity, also known as energy density, EV powertrains are also increasing in power density, which is defined as the ratio of power efficiency to overall size, and decreasing in overall size, weight, and cost. This is being achieved by leveraging SiC power switches, particularly in onboard chargers (OBCs) and traction inverters in powertrain systems [3].
The following are the key benefits of power electronics based on SiC:
- Ability to operate at higher temperatures: SiC power devices can operate at much higher temperatures than conventional silicon-based devices, eliminating the need for cooling components and bulky heat-sink materials. As power levels rise — for example, in traction inverters that drive motors in EVs — thermal management of silicon power devices such as insulated gate bipolar transistors (IGBTs) becomes challenging due to maximum operating temperature limitations and allowable junction temperatures. This challenge requires the incorporation of cooling components in powertrain systems, such as large copper blocks with water jackets, especially in a traction inverter where power levels can rise higher than 100 kW. These cooling components add to vehicle size, weight, and cost. Conversely, SiC has a much higher allowable junction temperature of 175°C and above. Additionally, the thermal conductivity of SiC is two to three times higher than that of silicon.
- Higher current carrying capability: SiC power devices can carry current densities of up to five times higher than silicon power devices. This allows a higher power density per chip, leading to smaller devices and more compact packages.
- Higher switching frequencies: SiC-based power devices are also capable of 10 times faster switching frequencies, to at least 20 kHz in traction inverters and hundreds of kilohertz in OBCs. At these higher frequencies, the size of passive components such as capacitors and inductors can be greatly reduced, enabling significantly smaller systems overall.
- High withstand voltage: SiC also enables higher withstand voltage, power, and switching efficiencies, allowing for the design of high-power traction inverters with significantly reduced losses.
For a given power level and battery capacity, SiC power devices can be smaller in size, and this translates into assemblies of EV subsystems with integrated powertrain systems. For example, in some designs, the motor drive and traction inverter are integrated into a one-box solution that further reduces size, weight, and cost. The cost can also be lowered at the system level by eliminating or minimizing the mechanical blocks for cooling and the amount of material for passive elements and casing. Figure 1 shows a summary of the benefits of power electronics using SiC power switches.
Figure 1. Benefits of SiC power electronics.
800 V architecture: reducing driving-range anxiety, cost, and charging time
Taking advantage of the higher voltage sustainability and current carrying capacity of SiC devices, OEMs are increasingly moving toward an 800 V architecture [4]. This architecture benefits consumers, as it alleviates driving-range anxiety and enables reduced charging times.
For a given battery capacity, SiC power switches offer a 10% improvement in efficiency over IGBTs in the traction inverter system. The reduced power loss of SiC devices can be leveraged to reduce the cost and size of the battery. For the same power level, the cabling weight and cost are significantly reduced due to the lower current requirements at 800 V vs. 400 V. Furthermore, higher voltages reduce the need for large amounts of copper in the motor windings. This permits smaller motor designs. All of these component size and weight reductions help trim the cost of electric vehicles, significantly contributing to EVs achieving cost parity (or better) with traditional ICE vehicles. For luxury vehicles that can accommodate larger power levels at 800 V with higher currents, their OBC systems enable faster charging times.
Wafer-level substrate manufacturing
The largest market for SiC in the next five years is in the EV market for power electronic switches. To keep pace with the growth trajectory of the EV market, the SiC market is expected to grow two times faster than the EV market [2]. One of the most significant improvements over the last few decades in the SiC manufacturing process has been producing defect-free wafer-level substrates at a low cost.
It is well known that increasing wafer size allows for significant cost reduction of the devices. However, increasing the wafer size creates challenges to eliminate defects. Primary defects occurring during SiC substrate manufacturing are stacking faults, micropipes, pits, scratches, stains, and surface particles. All of these defects adversely affect the performance of SiC devices. Furthermore, higher levels of defectivity occur more frequently on 150 mm wafers, which are today the most prevalent wafer size for SiC manufacturing. Only a handful of suppliers have mastered the art of producing high-quality, defect-free 150 mm wafers, after decades of research and development. This has enabled the supply chain to build power devices in high volumes, and take advantage of the superior properties of SiC that have been known for a while, using defect-free wafers today. Producing such high-quality wafers with high yield alone separates the few SiC wafer-level substrate suppliers from the rest in the power SiC supply chain. Moving forward, these substrate suppliers have already set their sights on scaling up to 200 mm in the coming years.
Summary: SiC power electronics are making a global impact
According to the Center for Climate and Energy Solutions, 31% of U.S. greenhouse gas emissions in 2017 was from electricity and heat and 15% from transportation. Therefore, leveraging renewable sources of energy to power our zero-emissions cars, our factories, and our lives has the potential to reduce U.S. greenhouse gas emissions by half. It explains why governments around the world, who are eager to contain global warming, are generally moving to ban ICEs by the middle of the century and are planning now to invest in infrastructure such as electric energy storage at scale and charging stations for electric vehicles. SiC, referred to as a third-generation wide-bandgap semiconductor, has established itself over the last three to five years as a technology with the potential to have a global impact in the transportation space. SiC is also finding new applications in renewable energy generation, transmission systems, and factory automation, which combined represent a rapidly growing market.
Dr. Nagarajan Sridhar, Senior Director, Automotive Vertical Marketing Coherent Corporation
References:
- “Global EV Outlook,” International Energy Agency, 2022.
- “Defining the Opportunities for Success Across Electric Vehicle Ecosystem,” Strategy Analytics, 2022.
- Maurizio Di Paolo Emilio, “Silicon Carbide for the Success of Electric Vehicles,” Power Electronics News, August 3, 2020.
- Peter Sigal, “EV Industry Seen Shifting to 800-Volt Architectures,” Automotive News, April 16, 2022.
