By: Jonathan Liao, Sr. Product Line Manager, Automotive Traction Solutions, onsemi
The adoption of electric vehicles (EVs) is on the rise, driven by growing consumer demands, environmental concerns/regulations, and available options. According to a recent Goldman Sachs research, EV sales accounted for 10% of global car sales in 2023.By 2030, this forecast is expected to grow to 30%; by 2035, EV sales could account for half of global car sales. However, “range anxiety,” the fear of not being able to travel a desired distance between battery charges, is one of the main barriers to EV adoption.And the key to overcoming it will be extending vehicle range without significantly increasing costs. This article illustrates how using silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs) in the traction inverter can extend the range of an EV by up to 5%. It also discusses why some original equipment manufacturers (OEMs) have been reluctant to transition from silicon-based insulated gate bipolar transistors (IGBTs) to SiC devices and onsemi’s efforts to mitigate these concerns and inspire confidence in this maturing wide bandgap semiconductor technology.
Automotive traction trends
The traction (main) inverter in an EV transforms the DC battery voltage into the AC voltage required by the electric traction motor, which is responsible for vehicle propulsion. Recent trends in traction inverter design include:
- Increasing power: the greater the power output of the inverter, the faster a vehicle can accelerate and responds to the driver.
- Maximizing efficiency: the amount of power consumed by the inverter must be minimized to increase the availablepower for propulsion.
- Higher voltage: Until recently, 400V batteries have been the most common, but the automotive industry is moving towards 800V to reduce current,cable thickness,and weight. The traction inverter in an EV must be able to handle this higher voltage level and use the right components.
- Reduced weight and size: SiChas higher power density (kW/kg) compared to silicon-based IGBTs. Higher power density can reduce the system size (kW/Liter), which helps to reduce the traction inverter’s weight with less load for the electric motor. Lower vehicle weight helps to extend the vehicle’s range with the same battery, while making the drivetrain smaller, increasing the space available for occupants and trunk/frunk payload.
Figure 1: Recent trends in EV traction inverter designSiC advantages over silicon
Silicon carbide has several material advantages over silicon, making it a better choice for traction inverter designs. The first is its physical hardness – 9.5 Mohs compared to 6.5 Mohs for silicon – making it better for high pressure sintering and giving it greater mechanical integrity. Its thermal conductivity (4.9W/cm.K)is more than four times that of silicon (1.15 W/cm.K), meaning it can operate reliably at higher temperatures by transferring heat more efficiently. Finally, SiC has an 8x higher breakdown voltage (2500kV/cm versus 300kV/cm) and its wide bandgap nature allows it to turnon and off faster, making it a better choice for EVs’ increasingly higher voltage (800V) architectures, while its wider bandgap voltage means it exhibits lower losses than silicon.
Addressing legacy concerns about adopting SiC
Despite the apparent advantages of SiC, some automotive OEMs have been slow to switch away from using more traditional silicon-based switching devices like IGBTs for traction inverters. Reasons for the reluctance to adopt SiC include perceptions that it is:
- not a mature technology
- difficult to implement
- not available in packaging suitable for traction
- supply is not as readily available as silicon-based devices
- more expensive than IGBTs
The following section presents a multi-faceted approach showing why these perceptions are unfounded and why OEMs should confidently use SiC in an EV traction inverter.
Proving SiC increases traction inverter efficiency
The first step in inspiring confidence is demonstrating the clear performance advantage achievable by using SiC in traction inverter designs. onsemi’s NVXR17S90M2SPB (1.7mΩ Rdson) and NVXR22S90M2SPB (2.2mΩ Rdson), EliteSiC Power 900 V six-pack power modules were simulated using circuit design software and their performance compared to that of the 820A VE-Trac Direct IGBT (also from onsemi). Simulations on a traction inverter design demonstrated:
- For a 450V DC bus voltage with 550Arms power delivery at 10KHz switching frequency, the Tvj (junction temperature) of the SiC modules (111°C) was 21% lower than that of the IGBT (142°C) for the same cooling conditions.
- Compared to the IGBT, the averaging switching losses in the NVXR17S90M2SPB were 34.5% lower, while the NVXR22S90M2SPB’s losses were 16.3% lower.
- Overall losses were over 40% lower for a full traction inverter design implemented using the NVXR17S90M2SPB, while there was a reduction of power losses up to 25% using the NVXR22S90M2SPB compared to an IGBT-based design.
While these enhancements were specific to a traction inverter, they translate into a 5% efficiency gain in the overall EV performance, enabling a 5% range extension. For example, an EV with a 100kW battery offering a range of 500km could travel up to 525km using a traction inverter designed using onsemi’sEliteSiC power modules. Significantly, the cost of using SiC in such a traction inverter would also be 5% lower than silicon IGBTs.
SiC has higher power delivery than IGBTs in a similar footprint
For OEMs considering moving away from IGBT, onsemi provides SiC modules in a similar mechanical footprint for easier integration, which also eases the implementation without any changes in the manufacturing process. Furthermore, they give the added advantage of higher power delivery at the same junction temperature. For example, the NVXR17S90M2SPB can deliver 760Arms, compared to only 590Arms for an IGBT (Tvj =150ºC), representing a 29% power increase. Furthermore, onsemi sinters the SiC diceon a direct bond copper plate, enabling up to 20% lower thermal resistance between the device junction and the coolant (Rth junction to fluid = 0.08ºC/W).
Transfer-molded packaging using advanced interconnect technology further contributes to these modules’ high-power density and also offers low stray inductance (significant for high-speed switching efficiency), higher switching frequency can reduce the size and weight of some passive components in the system. In addition, this packaging type, with options up to 200ºC operating temperature, reduces OEMs cooling requirements and potentially employs smaller pumps for thermal management.
Switching to SiC makes sense beyond the traction inverter
As EV battery voltages increase, electrical currents can be lowered to achieve the same power output. On the system level, cables in the automobile get thinner.The switch to SiC will become increasingly logical since SiC devices produce less heat than silicon, enabling even higher power density levels, not alone in the traction inverters but across the broader EV architecture.
onsemi addressing the supply concerns of OEMs
onsemi has invested heavily in creating a fully integrated and mature SiC supply chain and ecosystem, including wafering epitaxy and 150mm fabrication (200mm planned) of discrete products, integrated circuit devices, modules and reference application designs. Over a decade in the making, onsemi’sexpertise can provide the ultimate reassurance required to dispel any concerns automotive OEMs may have about switching to SiC.