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Additionally, SiC enables higher operating tempera- tures, thanks to its intrinsically high junction temperature and excellent thermal stability. Thus, it suits applications in which the ambient temperature is higher than 125˚C — for example, deep-well drilling for oil and gas. For the high- voltage– and high-temperature–supported gate driver, capacitive isolation supersedes traditional optocouplers with >125˚C recommended junction temperature, [enabling] a higher lifetime at high operating voltage.
Why is gate driver isolation important in most
high-power motor control applications?
Gangyao Wang, systems and applications engineer, Isolated Gate Drivers, Texas Instruments: For appli- cations higher than 600-V operation, galvanic isolation is a must-have feature to achieve high operating voltage, as the level-shift junction isolation can support bus voltage only up to 450 V. An isolated gate driver, which integrates both a galvanic isolator and a high-performance gate driver, will greatly reduce the design complexity, bill-of-materials cost, and printed-circuit–board area and volume, and will increase system mean time to failure.
In addition, SiC-based motor drives have significantly faster switching speeds, with over 5× faster rising and falling time during switch commutation. This is beneficial to switching loss but increases common-mode transient immunity [CMTI] requirements. A capacitive-isolation-technology–based iso- lated gate driver can achieve a minimum 150-V/ns CMTI, which is 3× to 5× higher than the state-of-the-art junction isolated gate driver and the traditional optical-based isolated gate driver. Capacitive isolation technology has the significant advantages of high working voltage, long lifetime based on the time-dependent dielectric breakdown [TDDB] test, and high CMTI capability.
Speer: Any application with high-side switches will need isolated gate drivers. If we ask why gate driver isolation is important to high-power motor control in general, we can say that this adds a level of protection and reliability by safeguarding against common-mode noise, which can create interference between the gate driver and power loops. The very fast switching speeds possible with SiC, of course, can exacerbate this effect, making gate driver isolation even more critical.
Is SiC gate oxide reliability still an issue for last-generation devices? If so, how can it be handled?
Speer: As a SiC supply community, we can happily report that oxide reliability is a concern of the past. What’s often lost when we talk about SiC is the history of silicon and the les- sons we learned. Indeed, we are following a similar evolution as the early silicon MOS structures did. It began with identi- fying the material and process problems. This was followed by R&D to find new oxide growth kinetics and post-oxidation surface-passivation methods, which have since been reduced to high-manufacturable practice. Finally — and just as is done in silicon — we use end-of-line screening to eliminate the few devices on the wafer that still have so-called extrin- sic, or process-related, defects. The end objective is to leave only intrinsic defects and related failure modes, which can then be used to predict oxide lifetimes. At Microchip, these oxide lifetimes, even at a junction temperature of 175˚C, will survive for more than 100 years. That sounds sufficient for a lot of use cases.
Wang: Reliability is a key concern for motor drive applica- tions due to lifetime and robustness expectations. The indus- try is working on improving SiC gate oxide reliability, though it may take a few more years of field application to prove its maturity.
However, a gate driver with gate-monitoring capability can
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ASPENCORE GUIDE TO SILICON CARBIDE