Repair Case
SG3525 No PWM Output
Use this page when an SG3525-based welder control board has power but the PWM outputs are missing, locked, or too weak to drive the next stage.
What must be true before replacing the chip
An SG3525 can be healthy and still produce no useful drive if its shutdown, feedback, oscillator, soft-start or output path is being held in an abnormal state. Do not judge only by chip VCC.
Measurement sequence
| Node | Expected check | Fault meaning |
|---|---|---|
| VCC and ground | Stable chip supply at the correct pins | Missing or pulsing VCC points to auxiliary supply or local regulator fault |
| Reference output | Reference voltage present and not collapsed | Shorted external load or damaged IC |
| RT/CT oscillator | Timing ramp or oscillator activity | Open timing resistor/capacitor or failed oscillator section |
| Shutdown / soft-start | Not clamped in shutdown state | Protection circuit, optocoupler or transistor is blocking PWM |
| Output pins | Alternating PWM drive present | If inputs are normal but outputs are dead, suspect IC or output load |
Diagnostic flow
Confirm auxiliary rails
↓
Confirm SG3525 VCC and reference
↓
Check oscillator pins
↓
Check shutdown / soft-start clamp
↓
Check output pins and driver load
↓
Only then replace SG3525 if needed
Common causes
- Missing auxiliary supply or bad local regulator.
- Shorted timing capacitor or cracked timing resistor.
- Protection transistor holding shutdown active.
- Optocoupler feedback path clamping the control loop.
- Shorted driver transistor loading the SG3525 output.
- Cold solder joints around the IC socket or driver transformer path.