How silicon-carbide technology can address high-voltage EV challenges

Sales of new electric vehicles (EVs) in the US exceeded five percent of the total market for the first time in 2022, joining 18 other countries worldwide that have surpassed the same milestone. Although EVs are achieving market acceptance that bodes well for future success, adoption must continue and accelerate. Changing to tailpipe-free vehicles is a pillar of governments’ plans to meet climate pledges under the Paris Accord and improve air quality in major cities.

Standing in the way of progress, the usability of EVs is a key issue for many would-be buyers. Charging an EV at home can be impractical for those living in properties without off-street parking or allocated spaces. Moreover, while today’s EVs offer a decent driving range to handle average daily usage, longer trips still require stops to recharge.

There’s also concern about the location and availability of suitable charging stations and the stop duration. Whereas a liquid-fuel tank can be refilled in a couple of minutes, a typical discharged EV battery requires about 25 minutes to recharge to 80% of its full capacity. Driving range and charging times still need better solutions to strengthen the case for EVs and drive faster market adoption to satisfy targets on emissions and sustainability.

Higher voltage, greater efficiency
One way automakers are advancing electric-vehicle architecture is by increasing battery voltages. Some prestige and high-performance EV models have voltages as high as 800 V. Increasing the voltage provides greater power transfer, enabling faster battery charging.

Additionally, a higher operating voltage means thinner and lighter cables can handle the power delivered into the charging and traction systems and lower “I2R” losses, contributing to a longer driving range.

While greater passenger EVs use 400 or 800 V, a new 1,250 V charging specification promises to cut the charging times of long-haul trucks, including the high-capacity Class 7 and 8 vehicles. Productivity is a major concern in commercial haulage. The Megawatt Charging System (MCS) has been proposed to ensure the fastest possible charging times — standardized as SAE J3271.

Currently, SAE J3271 is under development and specifies charging from 440 kW at 350 A and 1,250 VDC without cooling and up to 3.75 MW at 3000 A and 1,250 VDC with active cooling.

As a guide, charging at 1.6 MW for 30 minutes would deliver 400 miles of driving with a standard Class 8 tractor. The SAE J3271 specification standardizes aspects, such as the plug design, communications protocols, and safety requirements, providing interoperability between vehicles, charging stations, charging networks, and the electric grid.

With higher battery voltages, the driving range could be extended by about five to 10%.

Technology for the transition
Raising EV battery voltages can lead to faster charging and greater driving range, but it also puts extra demands on the vehicle’s essential power-handling functions. These include the traction inverter, onboard charger (OBC), and HVAC systems.

As battery voltages are increased to 800 and 1,250 V, silicon-carbide (SiC) technology is inherently better suited to these applications. SiC MOSFETs offer advantages, including smaller feature sizes, lower parasitic capacitances, and lower on-resistance in relation to their breakdown voltage rating (V_BR) compared to silicon IGBTs or MOSFETs, the typical alternative.

The lower capacitances permit faster switching with lower losses, resulting in greater energy efficiency. The lower on-resistance allows for fewer transistors in parallel, resulting in smaller module dimensions, lower weight, and a reduced bill of materials. SiC can also handle higher operating temperatures, and the devices have lower thermal impedance, which can help simplify the vehicle’s cooling systems while preserving reliability.

SiC in roadside chargers
Roadside EV fast chargers must be upgraded to meet increased EV battery voltages, especially to ensure faster charge times. Accordingly, SiC’s advantages in terms of device size and efficiency make this technology an ideal fit for charging applications where high speed, reliability, flexibility, and space savings are critical.

Many chargers are becoming more intelligent to better manage smart infrastructures powered by renewable energy sources. In a world where grid-connected storage is essential to maintain stability, the value of a nation’s fleet of EVs connected to the grid is inestimable as a storage solution.

An EV with a 40-kW battery could satisfy the energy demand of an entire household. The vehicle could be recharged when necessary and ready for the owner’s next trip. Vehicle-to-grid (V2G) communication is a specification that enables smart charging systems to use an EV as a resource to help balance energy flow through the grid and keep pace with continuously changing supply and demand.

Trench-assisted planar-gate technology
Although the performance advantages of SiC devices have been well documented, designing SiC MOSFETs for the real world involves compromises between performance, reliability, and manufacturability. Typically, the choices are between a planar or trench architecture.

Trench architectures can offer lower on-resistance per die area and faster switching performance. However, the manufacturing yield can be low, and the gate oxide thickness is difficult to control, often leading to device failures in the field.

Planar devices benefit from superior gate ruggedness and short-circuit capability, as well as simpler manufacturing processes. There’s also scope for future generations of the technology to deliver additional improvements in die size and cell performance.

The GeneSiC MOSFET platforms stretch from 650 to 6,500 V, addressing a range of high-voltage, fast-charging systems. Patented trench-assisted planar-gate designs combine the established strengths of planar technology with fast switching capability, extended operating lifetime, and high manufacturing yield. Table 1 compares the relative merits of the three architectures.

Table 1. Planar, trench, and trench-assisted planar comparison.

With low RDS(ON) at high temperatures and low energy losses at high speeds, trench-assisted planar devices outperform alternatives, including trench-gate structures. Their extremely low RDS(ON) temperature coefficient is significant.

In datasheets, RDS(ON) is typically stated at 25° C, but conventional devices can suffer from a significant increase in resistance at elevated temperatures. GeneSiC MOSFETs have been shown to operate with up to 15% lower RDS(ON) over the rated temperature range (Figure 1).

Figure 1. Lower RDS(ON) temperature coefficient of GeneSiC MOSFETs.

This reduces energy losses, leading to increased system efficiency. Also, reducing device self-heating can lower the case temperature by as much as 25° C compared to equivalent alternative SiC devices operated with the same gate drive and ambient conditions (Figure 2). The 25° C cooler operation translates into a device lifetime that’s three times longer.

Figure 2. Operating temperature reduced by 25° C.

Future trends
The future must contain more fast-charging locations if EV adoption is to grow. However, installing more fast chargers can demand a significant financial commitment to upgrade the supplying utility infrastructure and could slow the rate of progress.

A fast and relatively low-cost approach involves upgrading ordinary Level 2 charging points by integrating an energy storage system to boost the charger output. The storage is charged continuously from the same low-voltage infrastructure used to supply the Level 2 charger, and discharged quickly into the EV battery when connected.

The electrification of heavy-goods vehicles is also significant to the transition to climate-neutral mobility. The US has committed to ensuring all new trucks sold will be zero-emission vehicles by 2040. It’s expected that replacing 100,000 40-ton trucks with electric equivalents on long-haul routes could save 10 million tons of CO2 each year.

To support these efforts, certain long-haul routes could be targeted to change first — prioritizing those that let vehicles cover long distances at a constant speed, with MCS chargers placed in specific locations along the route. This approach, which could establish a minimum viable infrastructure as a platform for further progress, would give fleet operators and vehicle manufacturers confidence to make investments in zero-emission haulage.

Off-highway applications, such as agricultural and construction vehicles, are expected to transition to electric powertrains more slowly. Arguably, such vehicles cover less distance than passenger cars and long-haul trucks. However, leading brands are answering the call to reduce emissions by developing diesel-electric drivetrains and electric pumps and generators.

Finally, hydrogen fuel-cell technology is expected to have a role in future e-mobility. It can leverage the energy efficiency, ruggedness, compactness, and reliability of SiC power semiconductors in hydrogen production by electrolysis and in the electric powertrains of hydrogen fuel-cell vehicles.

Conclusion
Automotive OEMs have demonstrated that passenger and commercial EVs can be practical, reliable, convenient, and cost-effective. However, further improvements in charging times are critical to reduce range anxiety and accelerate adoption. The latest 800 and 1,250 V systems require 1,700 V or 3,300 V SiC FETs and diodes, which must be efficient and reliable to meet critical needs for the migration to faster charging.

References

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