Repair Case
UC3846 Shutdown Pin Active: No PWM Output
Use this case when a UC3846/KA3846 control board has supply voltage but the PWM drive is blocked because the shutdown or protection path is active.
Why the shutdown pin matters
On many inverter welder control boards, the UC3846 shutdown path is tied to overcurrent, undervoltage, overtemperature, feedback and soft-start circuits. A machine can look like it has a bad PWM chip even when an external protection branch is intentionally stopping it.
Checks to perform
| Check | Expected result | If abnormal |
|---|---|---|
| UC3846 VCC | Stable control supply | Auxiliary supply or regulator fault |
| Reference output | Reference present | Shorted reference load or damaged IC |
| Oscillator | Timing waveform present | Timing component or IC oscillator issue |
| Shutdown pin | Not forced into shutdown during idle test | Protection transistor, optocoupler or comparator path active |
| Output pins | PWM appears when shutdown is released | Driver load, IC output or downstream gate-drive issue |
Isolation logic
Verify VCC and reference
↓
Measure shutdown pin state
↓
Trace protection transistor / comparator path
↓
Check optocoupler and current-feedback branch
↓
Release false clamp and retest PWM
Likely failed parts
- Leakage or short in a small signal transistor connected to shutdown.
- Faulty optocoupler in feedback or protection path.
- Comparator output stuck low/high depending on board design.
- Open current-sense resistor path causing false protection.
- Bad solder joint around the UC3846 socket or driver connector.