Switching power supply maintenance problems
First of all, observe whether the power fuse is damaged. If the fuse is damaged, it cannot be replaced in a hurry. It is necessary to check whether there is a short circuit in the power supply. Method: Use the electric barrier of the multimeter to test the AC end (test point 1) behind the power supply fuse. The normal resistance should be above tens of kiloohms. If the resistance is zero, it means that there is an AC short circuit in the power supply. In addition, we should also focus on checking whether the AC filter capacitor of the power supply is damaged; at the same time, if there is a varistor, this resistance should also be checked.
If the above test results are normal, we should then test the four rectifier diodes of the power supply (test point 2). Under normal conditions, the forward resistance of the diode is several k (tested with a multimeter × 1k block), and the reverse resistance is close to infinity. If the test result is found to be abnormal, it needs to be replaced. Following the investigation, the next thing to do is to test the DC resistance of the power supply (test point 3), and its normal resistance is also several k. If the resistance is zero, it means that there is a DC short circuit. There are many reasons for the DC short circuit, such as short-circuit damage to the filter capacitor, damage to the power oscillating tube, damage to the oscillating integrated block and peripheral circuits, which may cause a short circuit. Here, it needs to be explained that before replacing the power oscillating tube, it must be confirmed that the oscillating integrated block and peripheral circuits are normal, otherwise the power tube will be damaged again.
After eliminating the above problems of switching power supply, it shows that most of the faults exist in the oscillation control circuit, sampling feedback circuit or load. At this time, we should first check whether the power supply circuit of the oscillating integrated block is normal (test point 4), and its normal voltage should be around 10V (special reminder: because the test voltage should be carried out under the condition of power on, and there is a high voltage on the power board. Mains power, so pay special attention to personal safety, do not try to touch any part of the power supply directly).
If there is no voltage at this point or the voltage is very low, first check whether the dropping resistor is damaged; secondly, check whether the oscillation integrated block and its external power supply circuit are normal. If no fault is found in the peripheral circuit, it is recommended to replace the oscillation integrated block. Of course, sometimes the power supply cannot output voltage normally, which may also be caused by the short circuit of the load and the protection of the power supply. For this power supply, we only need to unplug the output line and check whether the output voltage is normal (test point 5) to determine the fault location. After the previous troubleshooting, the feedback circuit part should be checked at last. Generally speaking, the faults of this part are mainly concentrated on the photocoupler and its amplifying circuit, so special attention should be paid.
