1. Blown fuse or fuse
Mainly check the diodes, large filter capacitors and switch tubes of the rectifier bridge. Problems in the anti-interference circuit will also cause the fuse or fuse tube to blow and turn black. It is worth noting that the fuse or fuse blowing caused by 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 thermistor is also easy to burn out together with the fuse or fuse. .
2. No output, but fuse or fuse is OK
This phenomenon indicates that the switching power supply is not working, or has entered a protection state after working. First, measure whether the startup pin of the power control chip has a startup voltage. If there is no startup voltage or the startup voltage is too low, check whether the startup resistor and the components connected to the startup pin have leakage current. At this time, if the power supply control chip is normal, the above Inspection can quickly find faults. If there is a start-up voltage, measure whether the drive output pin of the control chip (thick film circuit has no drive output pin) has a high-low level transition at the moment of power-on. If there is no jump, it means that the control chip is damaged, the peripheral oscillation circuit components or the protection circuit are faulty, you can replace the control chip first, and then check the peripheral components. If there is a jump, it is generally because the switch tube is bad or damaged.
3. There is output voltage, but the output voltage is too high
This kind of failure often comes from the voltage-stabilizing sampling and voltage-stabilizing control circuits. We know that circuits such as DC output, sampling resistor, error sampling amplifier (such as TL431), optocoupler and power control chip form a closed control loop. In this link, any problem will lead to output voltage rises.
For a power supply with an overvoltage protection circuit, if the output voltage is too high, the overvoltage protection circuit will first act. If the measured value is higher than the normal value, the output voltage is too high. In actual maintenance, it is common to change the value of sampling resistance, error amplifier or photoelectric coupler.
4. The output voltage is too low
According to maintenance experience, in addition to the voltage regulator control circuit will cause the output voltage to be too low, there are also some reasons that will cause the output voltage to be too low. There are mainly the following points.
①The load of the switching power supply has a short circuit fault. At this time, all loads of the switching power supply circuit should be disconnected to distinguish whether the switching power supply circuit is bad 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.
②The failure of the rectifier diode and filter capacitor at the output voltage end can be judged by the substitution method.
③ The performance of the switch tube will decrease, 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 reduce the load capacity.
④The poor switching transformer will not only cause the output voltage to drop, but also cause insufficient excitation of the switching tube, which will cause repeated damage to the switching tube.
⑤ The large filter capacitor (ie 300V filter capacitor) is not good, resulting in poor load capacity of the power supply, and the output voltage will drop as soon as the load is connected.
