Case · protection/PWM
Case: UC3846 Shutdown Active With No PWM
VCC, VREF and oscillator were present, but both PWM outputs remained inhibited. The shutdown input stayed active because of a fault in the protection/current-sense path.
Case summary
VCC, VREF and oscillator were present, but both PWM outputs remained inhibited. The shutdown input stayed active because of a fault in the protection/current-sense path.
This is a reusable diagnostic pattern; exact values and reference designators remain board-specific.
Initial symptom
- Control power was present but both PWM outputs were missing.
- 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 |
|---|---|---|
| VCC / VREF | Controller ground | Present and stable |
| Oscillator | Controller ground | Present |
| Shutdown / current sense | Controller ground | Abnormally active |
| Outputs A/B | Controller ground | Inhibited until the path was isolated |
Diagnosis
The sequence separated a working controller from an active protection condition. Isolating the shutdown/current-sense path in stages restored PWM without replacing the controller.
Repair action
The failed protection-path component was repaired and current-sense polarity was verified. Protection was not permanently bypassed.
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.