Repair of Common Faults in Switching Power Supplies
1. Fuse or fuse tube burns out
Mainly inspect the diodes, large filtering capacitors, and switch tubes of the rectifier bridge. Problems with the anti-interference circuit can also cause fuses or fuse tubes to burn out or turn black. It is worth noting that the burning of fuses or fuses caused by the breakdown of switch tubes is often accompanied by damage to overcurrent detection resistors and power control chips, and negative temperature coefficient thermistors are also prone to burning out together with fuses or fuses.
2. No output, but the fuse or fuse tube is normal
This phenomenon indicates that the switch power supply is not working or has entered a protective state after working. Firstly, measure whether there is a starting voltage on the starting pin of the power control chip. If there is no starting voltage or the starting voltage is too low, check whether there is leakage in the starting resistor and external components of the starting pin. If the power control chip is normal at this time, the fault can be quickly detected through the above inspection. If there is a startup voltage, measure whether there is a high or low level jump in the driving output pin of the control chip (thick film circuit does not have a driving output pin) at the moment of startup. If there is no jump, it indicates that the control chip is damaged, there are problems with the peripheral oscillation circuit components or protection circuit. You can replace the control chip first and then check the peripheral components. If there is a jump, it is usually due to poor or damaged switch tubes.
3. There is an output voltage, but the output voltage is too high
This type of fault often comes from the voltage regulation sampling and voltage regulation control circuit. We know that the DC output, sampling resistor, error sampling amplifier (such as TL431), optocoupler, and power control chip together form a closed control loop, and any problem in this link will cause the output voltage to rise.
For power supplies with overvoltage protection circuits, if the output voltage is too high, the overvoltage protection circuit will first activate. At this time, the overvoltage protection circuit can be disconnected to disable it and measure the main voltage of the power supply at the moment of startup. If the measured value is higher than the normal value, it indicates that the output voltage is too high. In actual maintenance, it is common for sampling resistance changes, error amplifiers, or optocouplers to malfunction.
