PWM controller repair reference

SG3525A Welder PWM Controller: Pin Checks and No-PWM Diagnosis

Use the checks below to separate controller, timing, shutdown, output-supply and downstream driver faults.

Fast diagnostic sequence

Order	Measure	Expected evidence	If missing
1	Pin 15 VCC to pin 12 GND	Stable controller supply within the board design and device operating range.	Repair the auxiliary supply, ground path or local decoupling first.
2	Pin 16 VREF	Approximately 5.1 V.	Check VCC, VREF loading and the IC.
3	Pin 5 CT and pin 4 oscillator output	Timing ramp and clock activity.	Check RT, CT, discharge, sync and timing traces.
4	Pin 8 soft-start	Rises after startup; not permanently clamped low.	Check soft-start capacitor and external trip circuits.
5	Pin 10 shutdown	Not held at an active shutdown level.	Trace overcurrent, overtemperature and protection logic.
6	Pin 9 compensation and pins 1/2	PWM command is not forced to minimum duty.	Check feedback, command input and protection pull-downs.
7	Pin 13 VC	Output-stage collector supply present.	Repair the VC feed or decoupling path.
8	Pins 11 and 14	Alternating A/B pulses with comparable shape and amplitude.	Isolate output loading, IC output damage or downstream buffer faults.

Pin	Function	Repair check
1	Inverting input	Feedback and protection command.
2	Non-inverting input	Current/voltage setpoint.
3	Sync	External synchronization or noise injection.
4	Oscillator output	Clock activity.
5	CT	Timing ramp and capacitor condition.
6	RT	Timing resistor and VREF connection.
7	Discharge	Timing discharge and dead-time path.
8	Soft-start	Startup ramp; low clamp blocks duty.
9	Compensation	Error-amplifier output / PWM command.
10	Shutdown	Active trip blocks both outputs.
11	Output A	First alternating drive output.
12	Ground	Control-ground reference.
13	VC	Collector supply for output transistors.
14	Output B	Second alternating drive output.
15	VCC	Controller supply.
16	VREF	Nominal 5.1 V reference.

Evidence	Check
VREF present, no CT ramp	CT short/leakage, RT open, pin 7 discharge path, pin 3 sync loading, damaged timing traces.
Frequency unstable	CT leakage, RT drift, noisy VREF, poor ground, external sync interference.
Pulse width abnormal on both outputs	Dead-time network, soft-start, compensation and feedback command.

Device reference: VCC 8 V to 35 V; oscillator range 100 Hz to 400 kHz. The actual welder frequency is board-specific.

Node	Fault indication	Next trace
Pin 8 soft-start stays low	Outputs remain blocked or at minimum duty.	Soft-start capacitor, discharge transistor and protection pull-down.
Pin 10 shutdown active	Both outputs disabled.	Current transformer, shunt amplifier, thermal input and protection latch.
Pin 9 compensation forced low	PWM command suppressed.	Error amplifier, optocoupler, current/voltage command and external clamp.
Pins 1/2 pinned at a limit	Feedback or command fault.	Reference divider, potentiometer, current feedback and connector signals.

Observation	Likely area	Action
Both outputs missing; timing and control valid	VC supply or IC output stage.	Verify pin 13 VC, local capacitor and output loading.
Only A or B missing	Loaded output, damaged output stage, buffer transistor or open trace.	Compare pins 11/14 and isolate downstream load where safe.
A/B present at IC, absent at driver transformer	Buffer stage or coupling network.	Check transistors, resistors, coupling capacitors and transformer primary.
Driver-transformer secondary unequal	Transformer branch, rectifier/clamp or gate network.	Compare both branches before installing power devices.

Prove VCC and control ground.
Prove the 5.1 V reference.
Prove RT/CT timing activity.
Prove soft-start and shutdown are released.
Prove compensation and feedback permit duty.
Prove VC is present.
Compare output A and output B.
If both outputs are correct, move downstream to buffer, transformer, isolation and gate branches.

Do not install or energize IGBTs if output A/B waveforms are missing, unequal or overlapping.
Do not continue powered testing with a shorted gate branch, buffer transistor or driver-transformer winding.
Do not reference the oscilloscope ground clip to the wrong primary-side node.
After driver repair, use current-limited staged power-up before full bus voltage and load.

Follow the evidence path from symptom to measurement, circuit and repair case.