Chip
SG3525
PWM control chip used in inverter welder control modules, soft-start, compensation and no-PWM diagnosis.
Welder-specific role
SG3525 appears in many ZX7-style and older inverter welder control modules. It normally generates push-pull PWM for a driver transformer stage, while external amplifiers and comparators handle current regulation, thermal protection, overcurrent and shutdown.
Application flow
Current setting pot
↓
Current feedback amplifier
↓
SG3525 error amp / compensation
↓
Oscillator and soft start
↓
PWM outputs
↓
Driver transistor pair
↓
Gate-drive transformer
↓
MOSFET / IGBT power stage
Measurement table
| Check | Normal / Expected | Abnormal Meaning |
|---|---|---|
| VCC | Stable supply | Auxiliary supply or connector fault |
| Reference voltage | Stable | IC or load problem |
| Soft-start pin | Ramps up | Soft-start clamp or capacitor fault |
| Shutdown pin | Inactive | Protection path active |
| Output pins | Complementary PWM | SG3525 disabled or damaged |
| Driver transistor outputs | Drive waveform | Z24/9Z24/S8050/S8550 stage fault |
Common faults
- SG3525 has power but no output because shutdown is active.
- Soft-start capacitor or clamp path prevents startup.
- CA3140/LM324 current feedback pulls the control loop abnormal.
- Output pins are normal but driver transistors or transformer input are abnormal.
Repair-case pattern
Many SG3525 cases are not solved by replacing the SG3525. The failure may be in the feedback amplifier, soft-start path, driver transistor pair or protection input.