New silicon carbide (SiC) designs are emerging to meet the growing high-power requirements for electric vehicles.
Power electronics solutions continue to be largely based on standard silicon devices. While three-level and other silicon circuit topologies are emerging to improve efficiency, new silicon carbide designs are emerging to meet growing high-power requirements for electric vehicles.
In interviews, power devices managers at Mitsubishi Electric US highlighted the promise of SiC when compared with standard silicon implementations.
They said efficiency improvements can be achieved with hybrid technologies that combine silicon with SiC. For example, Si-based insulated-gate bipolar transistors (IGBTs) with SiC Schottky-barrier diodes achieve efficiency improvements with relatively minor cost increases. For many applications, this represents a compromise between cost and performance.
Without changing topologies, SiC is one of very few ways to increase efficiency significantly, the Mitsubishi engineers asserted.
SiC remains considerably more expensive than silicon. Hence, it’s important to identify applications in which the economics keep pace with energy savings or some other technical advantage to justify the cost. Mitsubishi Electric has focused on SiC for high-power devices primarily because they are vertical components that operate at higher voltages. “Gallium nitride is a material that we have some experience with within our RF group,” said Adam Falcsik, product manager for power devices at Mitsubishi Electric. “And we think it certainly has very useful applications in lower-power applications.
“So far, our power device development has focused on silicon carbide, primarily because it’s better-suited for higher-power applications,” he added. “And so we have device modules [in production] rated up to 1,200 A, and we have voltage ratings up to 3.3 kV.”
SiC technology is viewed as unproven and therefore risky by power engineers, who tend to be conservative. Many would prefer to wait for evidence of reliable performance before taking the plunge — a preference and a practice that can slow the pace of SiC adoption.
Indeed, Mitsubishi engineers noted that customers remain in wait-and-see mode. “If early adopters are successful with this technology, delivering the desired benefits, there’s going to be significantly more adoption, and I think we’re gradually working our way through that phase,” said Mike Rogers, a power devices application engineer at Mitsubishi Electric.
Design changes are required to make the best use of SiC, resulting in substantial reworking of PCBs. The resulting designs must be capable of handling much higher operating frequencies, the Mitsubishi engineers added.
EV, storage apps
Automotive applications stand to benefit significantly from SiC technology, especially for EV drivetrains along with battery recharging, either on-board or at charging stations.
“There is a strong desire to reduce the size and weight of electronics” for EVs, said Tony Sibik, Mitsubishi power devices manager. “Silicon carbide helps in that effort both by shrinking inverter size [and] by increasing efficiency, thus reducing the size of battery needed for a given range,” he added.
Energy-storage applications on the scale of electrical utilities are another potential driver of SiC adoption. The sector is benefiting from the shift to renewable sources such as solar and wind to provide power when the sun doesn’t shine and the wind doesn’t blow.
Providing power during periods of peak demand requires sufficient capacity to store energy and, therefore, more converters and inverters. SiC is a promising candidate for those power-conversion steps.
As more alternative energy sources come online, power flow requires special attention, including active filtering and harmonic correction. All require power semiconductors. Meanwhile, wide-bandgap SiC technology promises to boost renewable energy storage.
One reason is that SiC delivers dielectric strength 10× that of Si, thereby offering a framework for building devices operating at higher voltages while meeting field requirements for remote charging infrastructure and smart-grid applications. Moreover, higher switching frequency allows designers to reduce the physical size of magnets, inductors, and other filter components, including transformers.
Mitsubishi Electric engineers note that Si IGBTs, in general, have relatively slow switching, which slows further as the blocking voltage increases. IGBTs in the high-voltage range, such as 3.3 kV, are quite slow and exhibit high switching losses, limiting them to low switching frequencies.
“Silicon carbide offers its advantage for 3.3-kV and, shortly, 6.5-kV devices,” said Eric Motto, Mitsubishi’s chief engineer for power devices. “More importantly, they can switch at considerably higher frequencies than a silicon device ever could.”
Said Falcsik: “We’re seeing this today in … subway applications; we are mass-producing 3.3-kV silicon carbide devices for that application. They’re still pretty expensive devices, but the efficiency improvements they get not only in the inverter but in other components of the powertrain make them applicable.”
Low harmonics due to higher switching frequency allow for significant improvements in motor efficiency, enabling wider adoption of SiC technology in high-voltage power applications. Mitsubishi Electric believes high-voltage DC transmission is pushing the limits of Si devices, making SiC a more attractive option for those applications.
Higher device costs could therefore be offset by energy savings ranging as high as tens of thousands of watts. SiC devices, especially at high voltage, provide faster and more efficient switching. Considering conduction losses, the best Si IGBT is limited to about a 1.2-V drop, even if operated well below its rated current. SiC exhibits almost no voltage drop at low currents, depending on how much chip area is used.
Mitsubishi Electric’s development roadmap implements optimization and new structures to improve SiC performance.
“On the other hand, silicon IGBT technology doesn’t have much left to improve, where we’ve optimized that technology so much that it’s up against the physical limits of silicon,” Rogers said. “There are still some incremental improvements, especially in terms of optimization, that are possible, but nothing as dramatic as we can achieve with silicon carbide.”
Mitsubishi Electric expects SiC to remain more expensive than silicon for some time. Hence, early applications must justify the cost via improved efficiency.
The strategy targets applications “that benefit the most, recognizing that any application that uses silicon IGBTs today could be more efficient using silicon carbide MOSFETs,” said Sibik. “At some point in the future, silicon IGBTs will be completely obsolete, but how far into the future is still quite unclear.”
Schottky diodes also offer benefits. Mitsubishi Electric produces SiC Schottky diodes from 600 V to 3.3 kV for high-volume applications such as traction inverters requiring high current. DC/DC converter applications also require a diode, meaning SiC could provide power-factor correction. Long term, the goal is providing next-generation SiC devices that optimize the cost/performance ratio. Citing volume and competitiveness with IGBTs, Mitsubishi Electric acknowledges that cost optimization will be critical for fine-tuning wafer process phases to support ever-growing production volumes.
Among the technical hurdles is the quality of the substrates made from SiC wafers. Wafer defects continue to hinder yields.
Those defects translate into higher SiC device costs, which ultimately hinder adoption.
This article was originally published on EE Times.
Maurizio Di Paolo Emilio holds a Ph.D. in Physics and is a telecommunication engineer and journalist. He has worked on various international projects in the field of gravitational wave research. He collaborates with research institutions to design data acquisition and control systems for space applications. He is the author of several books published by Springer, as well as numerous scientific and technical publications on electronics design.