TSL 120kVA: burned IGBT module
What happened
A 120 kVA UPS failed with a destroyed IGBT module (identified in the case as SEMiX405TMLI12E4B); on teardown, the damage turned out to reach beyond the module itself.
What was checked
- Fault analysis of the failed IGBT module: one transistor blown through.
- Check of the gate driver board and its components: supply rails, gate-network parts and the driver optocoupler.
- Bench testing of the drivers outside the UPS: as reported in the case, the engineers fed the 400 Hz driver transformers with low voltage, no more than 12 V, to prove the driver outputs before fitting a new module.
Cause
The engineers found the probable root cause was a failed -5 V linear regulator on the driver board. With that rail down, one transistor in the IGBT module, a fast diode on the -5 V line and the protective zener in the gate circuit were destroyed — the drivers went along with the module, which is typical when an IGBT fails.
The cause is probable and was not fully confirmed.
What was done
Replace the burned components on the driver board (the regulator, the fast diode, the gate zener and the optocoupler listed in the case) and fit a new IGBT module — the engineers were still sourcing the SEMiX405TMLI12E4B at the time of the case. Prove the driver outputs on the bench before reconnecting the module, otherwise the replacement is likely to fail the same way.
Safety
Before touching the power stage, isolate the UPS from mains and battery and verify with a meter that the DC bus capacitors are fully discharged — a 120 kVA UPS stores lethal energy long after shutdown.
Lesson: An IGBT failure is usually a symptom, not the cause: always check the gate driver board and its supply rails, and prove the drivers on a bench, before fitting a new module.
Covered in the course: Power and Cooling for Medical Equipment — Level 2: Fault-Finding →
Written from an anonymised case shared by service engineers. It is not a manufacturer instruction and grants no authorisation. Independent note; names are trademarks of their owners.