Case · gate branch
Case: IGBT Failed Again Because One Gate Branch Was Weak
After IGBT replacement, the same position failed again. Unloaded branch comparison found a weak gate path caused by an open resistor/connection.
Case summary
After IGBT replacement, the same position failed again. Unloaded branch comparison found a weak gate path caused by an open resistor/connection.
This is a reusable diagnostic pattern; exact values and reference designators remain board-specific.
Initial symptom
- The same IGBT position failed again after replacement.
- Full bus power was not applied during diagnosis.
- Equivalent channels were compared under the same condition.
Power-off evidence
- DC bus discharged and no residual short remained.
- Gate resistors, clamps and connectors were compared.
- The controller was not condemned only because PWM was missing.
Controlled live evidence
| Point | Reference | Observed pattern |
|---|---|---|
| PWM outputs | Controller ground | Both channels coherent |
| Driver primary | Primary driver return | Inputs coherent |
| Affected secondary | Matching isolated return | Weaker than equivalent branches |
| Gate-emitter | That IGBT emitter | Slow turn-off / reduced amplitude |
Diagnosis
Controller output and driver-primary pulses were correct. One isolated secondary and its gate-emitter waveform had lower amplitude and slow turn-off. The fault was local to that branch, not the controller.
Repair action
The open gate resistor/connection and parts damaged by the earlier short were repaired. Every equivalent branch was verified before the power devices were installed.
Verification
- Controller and driver verified with the power stage disconnected.
- Equivalent branches confirmed.
- First bus power-up current-limited and unloaded.
- Short load test completed without abnormal protection or heating.
Stop conditions
- A short or unequal branch remains.
- The measurement reference is not confirmed.
- Protection would have to be bypassed to continue.
- Driver supply is unstable.
Technical sources
- Texas Instruments UC3846 — official current-mode control, current limiting, shutdown, UVLO and dual-output functions.
- onsemi SG3525A datasheet — oscillator, soft-start, shutdown and alternating-output functions.
- Reviewed M57962AL reference set — used for driver supply, input, fault-output and isolation checks; confirm the exact module suffix before using a pin map.
- WelderData ZX7-250 field notes — used for measurement order and branch comparison; raw courseware and board-specific values are not published as universal specifications.
Related repair map
Follow the evidence path from symptom to measurement, circuit and repair case.