Inverter control-loop diagnostics
Inverter Welder PWM Feedback and Protection Diagnosis
Separate auxiliary power, command signal, oscillator, shutdown protection, driver permission and power-stage response before replacing boards.
Database summary
Use this workflow when an inverter welder has auxiliary power, fan or display activity but no real output, weak loaded output, no gate drive, no PWM, protection lockout or current control that does not behave under load.
The purpose is to separate command signal, oscillator, PWM output, shutdown permission, driver permission and power-stage response.
Evidence stack
| Layer | What to prove | Typical mistake |
|---|---|---|
| Auxiliary supply | 15V, -15V, 5V, 24V or driver rails are present and stable. | Assuming display power means PWM power is healthy. |
| Command signal | Panel potentiometer, remote command or set-current voltage changes normally. | Replacing the PWM IC before checking the command path. |
| Oscillator / timing | SG3525, UC3846 or similar controller has timing activity and reference voltage. | Reading VCC only and declaring the controller good. |
| Shutdown / protection | Overcurrent, undervoltage, thermal and feedback pins are not forcing lockout. | Bypassing protection without finding the power-stage cause. |
| Driver permission | Gate-drive transformer, optocoupler or driver module receives a valid PWM signal. | Replacing IGBTs when the gate signal is missing or asymmetric. |
| Power-stage response | Switching, output rectification and real current agree with the command. | Treating OCV as proof of loaded welding output. |
Diagnostic sequence
- Confirm DC bus and auxiliary rails before probing PWM outputs.
- Check reference voltage, oscillator/timing activity and command input.
- Check protection pins and shutdown conditions before condemning the controller IC.
- Verify driver stage supply, isolation and gate-drive symmetry.
- Use a limiter or staged power-up after any power-stage short.
- Validate with loaded current evidence, not only no-load output voltage.
Diagnostic path
| Symptom | Fan/display works but no output, no PWM, no gate drive, current cannot be adjusted or protection light stays on. |
|---|---|
| Evidence | Auxiliary rails, command voltage, oscillator activity, shutdown pins, driver signal and output response. |
| Circuit section | PWM controller, feedback network, protection inputs, driver board and power stage. |
| Next pages | Control board shutdown / no PWM · SG3525 PWM controller · ZX7 current not adjustable |