Practical Methods for Fault Detection in Switching Power Supplies
Firstly, observe whether the power fuse is damaged. If the fuse is damaged, it cannot be replaced urgently. It is necessary to first check whether there is a short circuit in the power supply. Method: Use the resistance range of a multimeter to test the AC terminal behind the power fuse (test point one). The normal resistance should be several tens of kiloohms or more. If the resistance is zero, it indicates that there is an AC short circuit in the power supply. In addition, we should also focus on checking whether the AC filtering capacitor of the power supply is damaged; If there is a varistor, it should also be checked.
If the above test results are normal, we should proceed to test the four rectifier diodes of the power supply (test point two). Under normal conditions, the forward resistance of the diode is several k (tested with a multimeter in 1k mode), and the reverse resistance is close to infinity. If abnormal test results are found, replacement is required. Following the investigation, the next step is to test the DC resistance of the power supply (test point three). Its normal resistance is also in the range of k. If the resistance is zero, it indicates the presence of a DC short circuit. There are many reasons that can cause DC short circuits, such as short-circuit damage to filtering capacitors, damage to power oscillation tubes, and damage to oscillation integrated blocks and peripheral circuit parts, all of which may cause short circuits. It should be noted that before replacing the power oscillation tube, it is necessary to ensure that the oscillation integrated block and peripheral circuits are normal, otherwise it may cause further damage to the power tube.
After eliminating the above-mentioned issues with the switching power supply, it indicates that most of the faults exist in the oscillation control
circuit, sampling feedback circuit, or load. At this point, we should first check whether the power supply circuit of the oscillation integrated block is normal (test point four), and its normal voltage should be around 10V (special reminder: as the test voltage should be carried out under power on, and there is high voltage mains on the power board, special attention should be paid to personal safety, and no attempt should be made to directly touch any part of the power supply).
If there is no voltage or the voltage is very low at this point, the first thing to check is whether the voltage reducing resistor is damaged; Next, check whether the oscillation integrated block and its external power supply circuit are normal. If no faults are 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 load short circuit and power protection. For this power supply, we only need to unplug the output line and check if the output voltage is normal (test point five) to determine the fault location. After the previous troubleshooting, the feedback circuit should also be checked. Generally speaking, this part of the fault is mainly concentrated in the optocoupler and its amplification circuit, and special attention should be paid.
