Chip
UC3846
Current-mode PWM chip used in inverter welder control boards, peak current feedback, slope compensation and shutdown diagnosis.
Welder-specific role
UC3846 is widely used in current-mode inverter welder control boards. In these machines it is not just a generic PWM chip: it sits between the current command, current feedback, peak-current sampling, shutdown chain and driver stage.
Application flow in a welder
Current setting / command
↓
Current feedback and slope compensation
↓
UC3846 PWM comparator
↓
Shutdown / protection gate
↓
PWM outputs
↓
Driver transformer or opto driver
↓
IGBT / MOSFET gate drive
Key diagnostic points
| Check | Normal / Expected | Abnormal Meaning |
|---|---|---|
| VCC | Stable operating voltage | Auxiliary supply fault if missing |
| Reference | Stable reference | IC or loading fault |
| Peak-current input | Low/normal during idle | False overcurrent if high |
| Shutdown input | Inactive during normal operation | Protection path blocking output |
| Output pins | PWM waveform | No drive if missing |
Common no-output causes
- Shutdown pin is active because an optocoupler or transistor path is pulling it.
- Peak-current feedback pin sees abnormal voltage due to sampling resistor or CT circuit fault.
- The chip has supply but no reference due to IC damage or excessive load.
- Outputs are present but driver transformer or gate stage is open.
Repair-case pattern
A typical UC3846 repair case records auxiliary power as normal, PWM output as missing, a shutdown/feedback node as abnormal, and a small transistor, optocoupler or current-sense part as the actual cause.