Complete Guide to Switching Power Supply Repair: From Fault Diagnosis to Safe Restoration

Date：2026-01-15 03:13:03 Visit：7

Common Fault Phenomena and Cause Analysis

1. SMPS No output, fuse is normal

Possible cause: The power supply is not working or has entered protection mode.
Diagnostic steps:
Measure the voltage at the start-up pin of the power supply control chip.
If there is no start-up voltage or the voltage is too low, check whether the start-up resistor and peripheral components are leaking current.
If there is start-up voltage, check whether there is a voltage transition at the chip's output when powering on.
If there is no transition, the chip may be damaged, or there may be a fault in the oscillation circuit or protection circuit.
If there is a transition, it usually indicates that the switching transistor is faulty or damaged.

2. Fuse Burnout or Explosion

Key Inspection Points:
- 300V large filter capacitor
- Rectifier bridge diodes
- Switching transistor
- Anti-interference circuit
Precautions:
- A breakdown of the switching transistor often also damages the current-sensing resistor and the power control chip.
- Negative temperature coefficient thermistors are also prone to be damaged along with the fuse.

3. SMPS Output Voltage Too High

Fault Range: Voltage regulation sampling and voltage regulation control circuit.
Check the closed-loop control circuit:
DC output sampling resistor
Error sampling amplifier (such as TL431)
Optocoupler

Power control chip

4. SMPS Output voltage too low

Possible causes:
- Abnormality in the voltage regulation control circuit
- Failure of rectifier diodes or filter capacitors at the output (can be determined by substitution method)
- Decreased performance of the switching transistor, causing poor conduction, increased internal resistance of the power supply, and reduced load capacity
- Systematic fault troubleshooting process

Handling Special Phenomena SMPS

1.Output appears briefly at power-on but then drops: This could be due to the protection circuit activating or a load short circuit; the load circuit should be checked.
2.Unstable output voltage: Check the input voltage, overload protection, switching transistors, and rectifier diode condition.
3.High-frequency whistling: This may originate from transformer issues or poor feedback control.
4.Intermittent operation: Inspect solder joints and the condition of electrolytic capacitors.
5.Overheating: Ensure the diodes are functioning properly and that there is adequate heat dissipation.

SMPS Maintenance Safety and Techniques

Safety Operating Guidelines

Always disconnect the power before performing maintenance.
When testing live circuits, it is recommended to use an isolation transformer.
If you are not familiar with high-voltage circuits, seek professional assistance.