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today in locomotives, such as subway applications. We are mass-producing 3.3-kV silicon carbide devices for that application. They’re still pretty expen- sive devices, but the efficiency improve- ments they get not only in the inverter but in other components of the pow- ertrain make them applicable.”
The low harmonics that result from higher switching frequencies allow for significant improvements in motor efficiency. This is enabling the adoption of SiC in high-voltage power applica- tions. Mitsubishi Electric said that high- voltage DC transmission is testing the limits of high-voltage silicon devices. Such applications are potentially much more efficient using SiC, the engineers said, and the resultant energy savings could measure in the thousands or even tens of thousands of watts. The energy savings thus offsets the cost premium for SiC solutions.
The SiC device, especially at high volt- age, is an important technical key: You have much faster and more efficient switching. Considering conduction losses, the best silicon IGBT is limited to
The SiC device is an important technical key: You have much faster and more efficient switching.
about a 1.2-V drop, even if you operate well below its rated current. However, SiC can have almost no voltage drop at low currents, depending on how much chip area you use.
Mitsubishi Electric said that there are generations of development on the roadmap that implement optimization and new structures to improve SiC per- formance. “On the other hand, silicon IGBT technology doesn’t have much left to improve; we’ve optimized that tech- nology so much that it’s up against the physical limits of silicon,” according to the company. “There are still some incremental improvements, especially in terms of optimization, that are pos- sible, but nothing as dramatic as we can achieve with silicon carbide.”
Mitsubishi Electric nonetheless expects SiC to remain more expensive than silicon for some time. Therefore, “our strategy today is to target the
applications that benefit the most, recognizing that any application that uses silicon IGBTs today could be more efficient using silicon carbide MOSFETs,” according to Mitsubishi Electric. “It’s quite credible that at some point in the future, silicon IGBTs will be completely obsolete, but how far into the future is still quite unclear.”
SiC also offers benefits for Schottky diodes. Mitsubishi Electric produces SiC Schottky diodes from 600 V to 3.3 kV in mass-production applica- tions that require a lot of current, such as traction inverters. In DC/DC converter applications that require a diode, SiC is advantageous for power- factor correction.
The company is working to optimize the cost/performance ratio for next- generation SiC devices. To get there, it will be essential to optimize the wafer process phases to support ever- growing production volumes. Two of the biggest hurdles are still timing and quality of the raw SiC wafers, which still often have imperfections in the crys- tal structure, thus negatively impact- ing yield and driving up the cost of the devices.
And in the final analysis, cost is always a key driver for adoption of any new technology.
This article was originally published on Power Electronics News on March 25, 2021.
Maurizio Di Paolo Emilio is editor-in-chief of Power Electronics News and EEWeb.
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