SiC MOSFETs Bring Next-Gen EV Efficiency Gains

In their efforts to maximize energy efficiency in electric and hybrid vehicles, designers are focusing on devices in compact packaging and on assembly of high-thermal–reliability, low-switching–loss power electronics modules. Design parameters take into account the power level, conversion efficiency, and operating temperature of the vehicle powertrain system; thermal energy dissipation capacity; and the system package.

SiC versus GaN

The bandgap of a semiconductor material refers to the energy needed to knock an electron of the material from the valence band to the conduction band. Silicon carbide (SiC) and gallium nitride (GaN) are wide-bandgap (WBG) semiconductors, meaning this energy level is higher for those materials than it is for silicon (Si): Si’s bandgap is 1.1 eV, compared with 3.3 eV for SiC and 3.4 eV for GaN. Despite the similar bandgaps of SiC and GaN, components based on those materials are not interchangeable, and a designer’s choice of material will differ according to the parameters of use for the systems in which the devices will operate. SiC-enriched power electronics for electric vehicles (EVs) meet the design parameters required in high-power applications and thus contribute significantly to system performance and long-term reliability. “The adoption rate in the electric-vehicle industry has been tremendous,” said Jay Cameron, vice president and general manager of Power at Cree. “Global OEMs have announced more than US$300 billion in EV investments, and we expect a projected 20% of all model types manufactured to be electrified within the next 10 years.” GaN performance and reliability are related to temperature and the Joule heating effect on the channel. Substrates such as SiC and diamond integrated into GaN can improve heat management, making it possible to lower the operating temperature of the device. GaN-on-SiC works very well for very high-frequency (RF) applications and thus is expected to serve future devices that will work with 5G. GaN-on-Si, meanwhile, maintains its place in low-voltage (<200-V) products such as compact USB Power Delivery (PD) chargers. In the range between 600 V and 650 V, both technologies work very well in sub-2-kW applications. Because of its excellent performance in minimizing switching losses, GaN can be the right solution when switching frequencies in the megahertz range are required. But at increased switching frequency and current values, GaN’s weak resistance (2× to 3× greater than SiC’s) can limit GaN’s application at high temperatures in terms of both device and system cost. “Due to the massive adoption of silicon carbide in the automotive industry, we see a path for silicon carbide to become the most cost-effective solution in an even wider variety of power supply designs,” said Cameron. “Silicon carbide is a key solution for on-board chargers [OBCs]. The power-density and -efficiency improvements over silicon implementations enable a strong reduction in terms of volume and weight [Figure 1].” Cree’s products “enable bidirectional OBC designs that support the V2x [vehicle-to-vehicle or vehicle-to-grid] trend,” Cameron added. “With our portfolio of devices at 1,200 V and 650 V, we are able to meet the requirements for both 800-V and 400-V electric vehicle architectures.”

Cree’s solutions

With the release of its Wolfspeed 650-V SiC MOSFETs, Cree is targeting a broader range of industrial applications. Target markets include EVs, data centers, and renewable energy. “There is an array of applications that can benefit from Cree’s new 650-V MOSFETs,” Cameron said. “Electric vehicles and data centers are two areas that can see huge benefits from the technology that primarily stem from its high-efficiency, high-power-density traits. Compared with silicon alternatives, you can see half the conduction losses, 75% lower switching losses, and 70% greater power density [Figure 2]. Those benefits are a natural fit for EVs and data centers, as well as telecom power, UPS [uninterruptible power supplies], solar inverters, and others.” The 15- and 60-mΩ, 650-V, AEC-Q101–qualified devices, using third-generation Cree SiC C3M MOSFET technology, offer lower switching losses and lower on-state resistance than previous solutions. The SiC MOSFETs provide 75% lower switching losses and 50% lower conduction losses than silicon equivalents, resulting in a potentially 300% increase in power density. Increased efficiency and faster switching speeds allow customers to design smaller solutions with higher performance. “Power supply designers can achieve maximum efficiency in their products when they use silicon carbide, enabling them to get either more power out of the same form factor or the same power out of a smaller form factor, or [they can] maximize power density to reduce size, weight, or cost,” said Cameron. “The properties of the silicon carbide substrate are absolutely critical. For example, as a cousin to diamond — the best heat conductor in nature — silicon carbide has vastly superior thermal performance over silicon. The easier it is to get the heat out, the cooler the device runs, which multiplies the effect of the low on-resistance change over temperature [Figure 3].” Full SiC modules are becoming more widely available, both in standard footprints and new module designs optimized around the WBG material. “Our release of the XM3 family of 1,200-V silicon carbide half bridges shows the improvements possible when packaging is designed with silicon carbide in mind,” said Cameron. “We also actively support module manufacturers in the development of modules based on our silicon carbide die, and they have been able to achieve excellent performance with their innovative designs.” The strong adoption of SiC solutions has been driven by ever-increasing demands for performance in a range of industrial applications. According to market research, the most profitable markets for new power devices will be electric mobility and self-driving vehicles, in which WBG semiconductors will be used in inverters, OBCs, and LiDAR anti-collision systems. This is no surprise, given that the thermal characteristics and efficiency of the new devices meet demands to optimize the performance of the accumulators. This article was first published on EE Times Europe