No VREF
Supply, reference loading or controller fault.
Fault · shutdown / no PWM
Welder Control Board Shutdown or No PWM
Do not replace the controller until you know whether the signal is lost in supply, oscillator, protection, output loading or the driver stage.
Check these before replacing the PWM controller
- Controller VCC and VREF.
- RT/CT oscillator or sync input.
- Soft-start and enable state.
- Shutdown, protection and current-sense inputs.
- Output A/B with the driver load isolated.
- Driver input, isolated supply and gate output.
Define the fault boundary
VREF present, no oscillator
RT/CT, synchronization loading or controller fault.
Oscillator present, outputs inhibited
Shutdown, soft-start, feedback or current limiting.
Outputs present, no gate drive
Buffer, driver transformer, isolated driver or supply fault.
Only one branch missing
Local winding, resistor, clamp, connector or solder fault.
Reference-ground boundaries
Controller ground, driver-primary return, isolated secondary return and IGBT emitter may be different nodes. Label the reference before every live measurement.
Controller and protection checks
| Evidence | What it proves | Next step |
|---|---|---|
| Stable VREF | Controller bias is at least partly established | Check oscillator and control pins. |
| Oscillator present | Timing section is running | Check shutdown, soft-start and feedback. |
| Shutdown active | Valid or false inhibit condition | Isolate current sense, protection and leakage paths. |
| A/B return when driver load is removed | External loading suppresses the outputs | Check buffer or driver input. |
From PWM output to gate driver
Compare both channels and every isolated supply. For an M57962AL, verify input, supply, fault output and gate output using the exact module suffix and board return.
Verification before the power stage
- Controller outputs are both present and coherent.
- Shutdown/current-sense conditions are normal.
- Every driver branch is present.
- No false pulse or slow turn-off remains.
- Protection responds in a controlled low-energy test.
Stop conditions
- Any power-stage or output short remains.
- The live-measurement reference is uncertain.
- Shutdown remains active without explanation.
- One gate branch is absent or distorted.
- An isolated driver supply is unstable.
Technical sources
- Texas Instruments UC3846 — official current-mode control, current limiting, shutdown, UVLO and dual-output functions.
- onsemi SG3525A datasheet — oscillator, soft-start, shutdown and alternating-output functions.
- Reviewed M57962AL reference set — used for driver supply, input, fault-output and isolation checks; confirm the exact module suffix before using a pin map.
- WelderData ZX7-250 field notes — used for measurement order and branch comparison; raw courseware and board-specific values are not published as universal specifications.
Related repair map
Follow the evidence path from symptom to measurement, circuit and repair case.