An SMPS that does not start can fail before the controller receives power, during controller startup, in the switching stage or because a protection condition immediately shuts the supply down.
Check the input path
Inspect the fuse, input protection, rectifier and high-voltage DC bus. If the bus is absent, troubleshooting should remain in the input and rectification section rather than moving to the secondary side.
Check controller startup supply
Many PWM controllers require a startup voltage on their VCC pin before switching begins. A failed startup resistor, auxiliary supply fault, leaky capacitor or excessive load can prevent this voltage from reaching the required threshold.
Look for switching activity
If the controller supply is valid, determine whether gate-drive or switching activity is present using correctly rated and safely referenced equipment. No drive and drive that starts then stops point to different diagnostic paths.
Consider protection and secondary faults
A shorted secondary rectifier, overloaded output or feedback problem can cause repeated startup attempts. Check whether the supply is truly inactive or cycling rapidly under protection.
Follow the fault path instead of changing parts blindly.
BoardIQ helps structure measurements from startup supply through switching and output checks.
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