Introduction
In the era dominated by IGBTs, power devices were generally expected to withstand short-circuit events without immediate failure. However, there is a lingering perception that SiC MOSFETs are less "robust," and their reliability under fault conditions remains a point of concern for many engineers.
What is Short-Circuit Withstand Capability?
In both consumer and industrial sectors, inverters are ubiquitous—from EV traction inverters to general-purpose Variable Frequency Drives (VFDs). During operation, system short-circuits can be triggered by mechanical failures, insulation breakdown, or human error during maintenance. Consequently, power semiconductors must be rugged enough to survive these transient fault conditions.
Traditional IGBTs are renowned for their robust Short-Circuit Withstand Time (SCWT). It is common to see a rated SCWT of 5–10μs in datasheets for both IGBT modules and IGBT discrete components. For manufacturers, this rating often includes a significant safety margin. This window provides ample time for gate driver designers to implement desaturation (Desat) protection. For a device rated at 10μs, a system with a 3μs response time and a 3μs soft turn-off slope still leaves plenty of headroom, fostering high confidence in IGBT reliability.
How is Short-Circuit Capability Defined in Datasheets?
Short-circuit endurance is not infinite; it subjects the die to extreme thermal and mechanical stress. Since it is a fault state, it cannot occur indefinitely. Manufacturers like SHYSEMI specify the maximum allowable number of short-circuit events in their datasheets.
In practice, the actual capability of an IGBT often exceeds these conservative ratings. Interestingly, many SHYSEMI datasheets specify short-circuit current values rather than time, whereas brands like onsemi tend to provide more direct time-based ratings.
How is the short-circuit resistance tested?
SHYSEMI previously shared an experiment involving four 1200V/3600A IGBT3-based modules. Under a 10μs pulse width at a 1/3Hz frequency, all four modules survived over 10,000 short-circuit cycles.
During my previous tenure, we also used a 10,000-cycle benchmark to validate system-level reliability.
SiC MOSFET vs. IGBT: A Technical Comparison
SiC MOSFETs exhibit significantly weaker short-circuit ruggedness compared to IGBTs. Failure is essentially a thermal issue—the accumulation of energy over time. Improving this requires lower energy density, larger die area, or higher temperature tolerance.
- Power Density: A 1200V/100A SiC MOSFET typically has a die area of roughly 20mm², while an equivalent IGBT is closer to 100mm². The IGBT volume is approximately five times larger.
- Saturation Current: SiC MOSFETs have a much higher short-circuit saturation current, often reaching 10x the rated current ($I_{nom}$), whereas IGBTs typically limit this to about 4x.
Given these factors, under identical fault conditions, a SiC MOSFET handles over twice the power in one-fifth the volume. Consequently, the power density per unit volume in a SiC MOSFET is over 10 times that of an IGBT.
Furthermore, the heat distribution in SiC is more concentrated due to its thinner drift region. While an IGBT fails near the intrinsic temperature of 400°C, SiC material can technically withstand up to 600°C.
However, the bottleneck is not the SiC itself, but the interconnects and wire bonds. When surface temperatures exceed 175°C, the aluminum metallization undergoes "reconstruction," leading to degradation or bond-wire liftoff. This is why achieving 10,000 safe short-circuit cycles is nearly impossible for current SiC modules, especially when considering current-sharing imbalances in multi-chip parallel designs.
Additional Challenges: Gate Oxide & Bipolar Degradation
Beyond thermal stress, SiC MOSFETs face Gate Oxide Degradation. High-field electron tunneling during short-circuits can shift the threshold voltage ($V_{th}$).
Another critical but under-researched issue is Bipolar Degradation. In a half-bridge phase-to-phase short-circuit, even if one SiC MOSFET shuts down safely, a massive current (up to 10x $I_{nom}$) may flow through the body diode of the complementary switch during freewheeling. This prolonged, high-current stress on the body diode is a severe catalyst for bipolar degradation.
Summary
Currently, SiC MOSFETs cannot match the short-circuit robustness of IGBTs. The root cause is the excessively high saturation current, which triggers thermal runaway, gate oxide wear-out, and potential body diode issues. Reducing the desaturation current is key to solving these challenges.
If you are facing design hurdles regarding short-circuit protection, contact SHYSEMI. Our expert engineers are ready to provide optimized solutions for your high-power applications.