Automotive & Transportation

Higher EV voltages demand innovative inverter materials

28 June 2023
EV inverter system. Source: Adobe/photostock.am

Electric vehicle (EV) manufacturers still have several key challenges to overcome before EVs become the first-choice for drivers in the e-mobility market. This is true despite the significant advances EVs have made in closing the market-share gap to fossil-fuelled autos. Chief among those challenges are EV range and time to recharge.

Also significant is the cost of the EV. Some sources peg the average cost of a new EV at $65,000, which is about $20,000 more than a petrol vehicle. Additionally, with energy efficiency and environmentalism a key global focus, OEMs and suppliers also have to improve system efficiency, without sacrificing performance, reliability or increasing cost.

One solution is to increase the voltage of the vehicle powertrains significant. Hence, there is a burgeoning effort move to 800 V voltages, or even higher. This will hasten charge times; enable use of smaller battery system, which will also save vehicle weight; and use less precious metals.

Therein lies another engineering problem for e-mobility solutions. High efficiency inverters will require high switching frequencies and increased power densities. The power modules will have a higher operating temperature and will ned to have an expected service life of 15 years. In turn, the traditional semiconductor technology – silicon (Si)-based insulated gate bipolar transists – will be usurped by silicon carbide (SiC) metal-oxide semiconductor field-effect transistors.

Source: Adobe/Blue Planet StudioSource: Adobe/Blue Planet Studio

IGBTs vs MOSFETs

IGBT semiconductors comprised of four layers of silicon have dominated the power-electronics market, the rapid progress of SiC and gallium nitride (GaN) semiconductor technologies has seen the steady adoption of these technologies.

These compound-material semiconductors, also referred to as wide-bandgap (WBG) devices, switch faster and operate at higher voltages than silicon. Inverters equipped with SiC MOSFETs and GaN high electron mobility transistors (HEMTs), with lower on-resistance, will dissipate less power and reduce switching and conduction losses. These devices improve efficiency by as much as 50% when compared with silicon IGBTs.

The gains are even more impressive under light or moderate loads. A 210 kW inverter using 1,200V SiC MOSFETs and freewheeling diodes of the same voltage rating, running at 10kHz can achieve an efficiency approaching 99%.

Once equipped with SiC MOSFETS, an 800 V system architecture delivers 350 kW with a single three-phase power module; to achieve the same power output with IGBTs, at least two power modules in parallel would be needed. This would require more space, double the gate driver boards and demand a more complex DC link/busbar structure.

SiC inverter efficiency gains are furthered by its temperature tolerance. In 800 V applications, IGBTs will have a six-fold increase in heat losses during the Worldwide Harmonize Light Vehicle Test Procedure, which is a global driving cycle test that measures a vehicle’s emissions, fuel efficiency and range. Today’s EVs are typically 400 V systems, where Si IGBTs relative inefficiency is more tolerable; typically just two or three times more than SiC MOFSETs.

[Learn more about inverters on GlobalSpec.]

An 800 V system also leaves capacity for new technologies that might not be common in todays EVs. Regenerative braking adds another 20 to 30 V. For a flyback converter, an additional 150 to 200 V must be added. Applying the usual 20% safety factor could push the system requirements to at least 1.33 kV – well outside of the Si IGBT’s comfortable operating range.

Despite SiC’s clear performance advantages, the adoption of the technology has not been as prolific as some hoped or predicted. This is mostly due to the material cost. Even though SiC is currently more expensive to manufacture than Si, when compared over the technology’s lifespan, SiC is likely to create meaningful savings in energy usage that outweight the initial expense.

How MOFSETs will change the EV paradigm

The weighted-average battery capacity of EV cars is on the rise worldwide, which adds to vehicle cost and increases pressure on battery supply chains. In addition, the hours-long recharge times of an EV are inconvenient for drivers accustomed to a 5-minute gas refill. SiC MOFSETS could also help address all of these issues.

The use of SiC WBG devices to stimulate the growth of 800 V architectures is compelling. An EV underpinned by 800 V architecture, with a 50 kWh battery and range of 200 miles, can achieve a 10% improvement in efficiency by substituting Si IGBTs with SiC MOSFETs. This could reduce energy consumption to roughly 4.4 miles per kilowatt-hour, allowing for a potential battery capacity reduction of between 4 to 5 kWh for the same range. At today's average prices, this would save between $500 and $600 on the battery pack alone - roughly the total cost of Tesla's SiC inverter on the Model Y, according Munro & Associates.

Similarly, a Wolfspeed test to compare IGBT and SiC on a platform equipped with a 77 kWh battery shows that the use of SiC WBGs could reduce the battery capacity by 7% without compromising range, or, alternatively, increase range by 7% with the same size battery.

The gains in efficiency are modest, although important, on an individual vehicle. If scaled to 50 million EVs, SiC inverters could save approximately 225 GWh of battery manufacturing capacity. This is more than the total battery demand expected from off-road electric vehicle sectors (e.g., heavy equipment, marine, aerospace) over the next 20 years. It could create a significant manufacturing advantage, allowing the industry to focus more on personal EVs and help alleviate a manufacturing bottleneck.

Finally, SiC inverters will help drivers charge their cars more quickly and reduce consumer’s range anxiety. With a higher operating temperature and faster switching speed, SiC is a better semiconductor material for fast charging solutions, which have thermal management needs due to the high currents and fast switching of AC-to-DC charging systems.

Is GaN next?

GaN also shows promise as an EV semiconductor material – it’s switching speed of up to 200 kHz is suitable for DC-DC converters. GaN’s primary limitation to date has been manufacturability of GaN-on-Si devices, which today limits voltage to about 1 kV.

GaN is primed for EV chargers and converters, since these components run at lower power than inverters. However, if the manufacturability improves, it could challenge SiC for market share of inverter applications.

For now, the industry is gradually adopting SiC technologies for the eventual shift to 800 V EVs. Doing so will be key to encouraging driver adoption of e-mobility technologies, and meeting key environmental and transportation regulations around the world in the coming decades.



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