The 10-microsecond window
A standard IGBT in short-circuit mode (SCM) heats up rapidly through resistive losses. The IGBT manufacturer's data sheet specifies a Short Circuit Withstand Time (SCWT) of typically 10 µs at rated voltage. Beyond that, the device fails.
Within that 10 µs window, the entire fault detection and shut-off chain must execute:
| Step | Typical time budget |
|---|---|
| 1. Current rises to detection threshold | 1-2 µs |
| 2. Sensor detects + reports | < 1 µs |
| 3. Comparator triggers | 200 ns |
| 4. Gate driver desat circuit reacts | 1-2 µs |
| 5. Soft turn-off begins | 1 µs |
| 6. IGBT current drops to safe level | 2-3 µs |
| Total | ≈ 7-10 µs |
There is zero margin. The current sensor's response time is one of the largest budget items.
What "response time" actually means
A current sensor's response time has three components:
- Magnetic flux propagation through the core — typically 100-500 ns
- Hall element settling — 100-500 ns
- Output amplifier slew — 100-500 ns
A sensor specified at "<1 µs response time (10-90 %)" means the output reaches 90 % of final value within 1 µs of a step input.
Watch for: Some datasheets specify response time as the time to reach 50 % of final value. That's not enough for desat — desat detectors typically trigger at 70-90 % of trip threshold.
SiC is even less forgiving
SiC MOSFETs have much shorter SCWT — typically 2-3 µs at full voltage. The same fault detection chain has to fit into less than half the IGBT budget.
For SiC, your sensor needs:
- Response time < 500 ns to 90 %
- Bandwidth ≥ 200 kHz to avoid phase lag
- Low-noise output to allow tight comparator threshold
Sensor placement matters
Even with a fast sensor, parasitic inductance in the bus bar and cabling can slow current rise as seen by the sensor. Two best practices:
- Place the sensor in the bus bar carrying short-circuit current — not in the inverter output cable, where current is bandwidth-limited by load inductance.
- Minimize stray inductance between sensor and gate driver — locate the comparator on the gate driver board, not the control board.
A sensor with a 500 ns response delay can become a 2 µs effective delay if it sits 30 cm away from the gate driver.
Three failure modes I see in field
Failure 1: Sensor too slow
SCWT exceeded before turn-off completes. IGBT fails open-circuit, often taking adjacent devices with it.
Failure 2: False trip on switching transients
Sensor bandwidth is too high for the noise environment — every commutation triggers a trip. Customer disables desat. Next real fault destroys the module.
Failure 3: Compensation winding ringing
Closed-loop sensor's secondary winding rings on fast di/dt events. Output overshoots, comparator trips. Ironically caused by too sensitive a sensor.
The fix in all three cases: pick a sensor with specified response time AND bandwidth AND noise, not just one of them.
Specification template for desat-protected drives
| Parameter | IGBT spec | SiC spec |
|---|---|---|
| Response time (10-90 %) | ≤ 1 µs | ≤ 500 ns |
| Bandwidth (-3 dB) | ≥ 100 kHz | ≥ 200 kHz |
| Output noise (RMS) | ≤ 1 % of nominal | ≤ 0.5 % |
| dV/dt immunity | ≥ 25 kV/µs | ≥ 50 kV/µs |
| Overload (3 µs pulse) | ≥ 10× nominal | ≥ 10× nominal |
Quick checklist
- ✅ Datasheet specifies response time to 90 % (not 50 %)
- ✅ Bandwidth and response time both specified
- ✅ Sensor placed close to the gate driver
- ✅ dV/dt rating exceeds your bus + commutation rate
- ✅ Lab-tested with actual short-circuit at full bus voltage