Maintenance of common faults of switching power supply
1. The fuse or fuse is blown.
Mainly check the rectifier bridge diodes, large filter capacitors and switch tubes, etc. If there is a problem with the anti-interference circuit, the fuse or fuse will be blown and blackened. It is worth noting that the fuse or fuse blown due to the breakdown of the switch tube is often accompanied by the damage of the overcurrent detection resistor and the power control chip, and the negative temperature coefficient is also easy to burn out together with the fuse or fuse.
2. No output, but the fuse or fuse tube is normal.
This phenomenon shows that the switching power supply is not working, or it has entered a protective state after working. First, measure whether the starting pin of the power control chip has a starting voltage. If there is no starting voltage or the starting voltage is too low, check whether the starting resistor and components connected to the starting pin have leakage. At this time, if the power control chip is normal, the fault can be quickly found through the above inspection. If there is a starting voltage, measure whether the driving output pin of the control chip (the thick film circuit does not drive the output pin) has a high-low level jump at the moment of starting. If there is no jump, it means that the control chip is damaged and there is something wrong with the components of the peripheral oscillation circuit or the protection circuit. You can replace the control chip first, and then check the peripheral components. If there is a jump, it is generally a bad or damaged switch tube.
3. There is output voltage, but the output voltage is too high.
This kind of fault often comes from the voltage stabilizing sampling and voltage stabilizing control circuit. As we know, DC output, sampling resistor, error sampling amplifier (such as TL431), photoelectric coupler and power control chip together form a closed control loop. In this link, any problem will lead to an increase in output voltage.
For the power supply with overvoltage protection circuit, the overvoltage protection circuit will act at first if the output voltage is too high. At this time, the overvoltage protection circuit can be disconnected to make it ineffective, and the main voltage of the power supply at the instant of startup can be measured. If the measured value is higher than the normal value, the output voltage is too high. In actual maintenance, sampling resistance variation, error amplifier or photoelectric coupler defect are common.
4. The output voltage is too low
According to the maintenance experience, in addition to the low output voltage caused by the voltage stabilizing control circuit, there are other reasons that will cause the output voltage to be too low. There are mainly the following points.
① There is a short circuit fault in the switching power supply load. At this time, all loads of the switching power supply circuit should be disconnected to distinguish whether the switching power supply circuit is defective or the load circuit is faulty. If the voltage output of the disconnected load circuit is normal, it means that the load is too heavy; if it is still abnormal, it means that the switching power supply circuit is faulty.
② The failure of rectifier diode and filter capacitor at the output voltage end can be judged by substitution method.
(3) the performance of the switch tube declines, which will inevitably lead to the failure of the switch tube to conduct normally, which will increase the internal resistance of the power supply and decrease the load capacity.
(4) Bad switching transformer not only causes the output voltage to drop, but also causes insufficient excitation of the switch tube, thus repeatedly damaging the switch tube.
⑤ The large filter capacitor (i.e. 300V filter capacitor) is not good, resulting in poor load capacity of the power supply, and the output voltage drops as soon as the load is connected.
