Switching power supply fault repair skills and frequent problem response

　　At present, switching power supply technology has been fully integrated into the design of various electronic products. However, with the expansion of its use, switching power supplies are beginning to face various problems, and damage is one of them. In this case, a technology is needed to repair the switching power supply, which is the switching power supply maintenance technology. Strictly speaking, the maintenance of switching power supplies belongs to the category of chip-level maintenance, and it plays a very important role in the field of industrial maintenance. This article will introduce some tips for switching power supply maintenance and answer some common faults.

　　Maintenance Tips

　　The maintenance of the switching power supply can be carried out in two ways, one is to observe it when the power is off, and the other is to test it when the power is on. When the power is off, the cause of the damage to the switching power supply can be determined by "looking, smelling, asking, and measuring".

　　Power failure detection

　　Look: Open the power supply casing, check whether the fuse is blown, and then observe the internal situation of the power supply. If burnt areas or broken components are found on the PCB board of the power supply, you should focus on checking these components and related circuit components.

　　Smell: Smell the inside of the power supply to see if there is a burnt smell and check if there are any burnt components.

　　Q: Please tell me how the power supply was damaged and whether there was any illegal operation on the power supply.

　　Measurement: Before powering on, use a multimeter to measure the voltage across the high-voltage capacitor. If the fault is caused by the switching power supply not vibrating or the switch tube being open, in most cases, the voltage across the high-voltage filter capacitor is not discharged. This voltage is more than 300 volts, so be careful. Use a multimeter to measure the forward and reverse resistance at both ends of the AC power line and the charging of the capacitor. The resistance value should not be too low, otherwise there may be a short circuit inside the power supply. The capacitor should be able to charge and discharge. Disconnect the load and measure the resistance to ground of each group of output terminals respectively. Under normal circumstances, the needle should swing with the capacitor charging and discharging, and the final indication should be the resistance value of the discharge resistor of this circuit.

　　Power-on detection

　　After power is turned on, you can observe the power supply to determine whether there is any damage, such as a burnt fuse or smoke from components. If there is, cut off the power supply in time for maintenance. Measure whether there is a 300-volt output at both ends of the high-voltage filter capacitor. If not, focus on checking the rectifier diode, filter capacitor, etc. Measure whether there is output at the secondary coil of the high-frequency transformer. If not, focus on checking whether the switch tube is damaged, whether it is oscillating, whether the protection circuit is working, etc. If so, focus on checking the rectifier diode, filter capacitor, three-way voltage regulator tube, etc. on each output side.

　　If the power supply starts and then stops, it is in protection state. You can directly measure the voltage of the PWM chip protection input pin. If the voltage exceeds the specified value, it means that the power supply is in protection state. You should focus on checking the cause of the protection.

　　Common malfunctions

　　Fuse blown

　　If the fuse blows, there is a high possibility that there is a problem with the circuit inside the power supply. Since the power supply works under high voltage and high current, the fluctuation and surge of the grid voltage will cause the current in the power supply to increase instantly and cause the fuse to blow. The focus should be on checking the rectifier diode, high-voltage filter electrolytic capacitor, inverter power switch tube, etc. at the input end of the power supply to check whether these components are broken down, open circuit, damaged, etc. If the fuse is indeed blown, you should first check the components on the circuit board to see if the surface of these components is burnt or if there is any electrolyte overflow.

　　If none of the above situations occur, you need to use a multimeter to measure the switch tube to check whether the switch tube is broken. It is important to note that when a component is damaged, you must not directly start the machine after replacing it. This is likely to damage the replaced component due to other high-voltage components still having faults. You must conduct a comprehensive inspection and measurement of all high-voltage components in the above circuit to completely eliminate the fault of the fuse.

　　No DC voltage output or unstable voltage output

　　Under load, if the fuse is intact and the DC voltage of each level is not output. This situation is mainly caused by the following reasons: open circuit and short circuit in the power supply, failure of overvoltage and overcurrent protection circuit, failure of auxiliary power supply, failure of oscillation circuit, overload of power supply, breakdown of rectifier diode in high-frequency rectifier filter circuit, leakage of filter capacitor, etc. After measuring the secondary components with a multimeter and excluding the breakdown of high-frequency rectifier diode and load short circuit, if the output is zero at this time, it can be confirmed that the control circuit of the power supply is faulty. If there is partial voltage output, it means that the front-stage circuit is working normally, and the fault is in the high-frequency rectifier filter circuit. The high-frequency filter circuit mainly consists of a rectifier diode and a low-voltage filter capacitor to form a DC voltage output. The breakdown of the rectifier diode will cause the circuit to have no voltage output, and the leakage of the filter capacitor will cause faults such as unstable output voltage. The damaged components can be checked by statically measuring the corresponding components with a multimeter.

　　Poor power load capacity

　　This phenomenon is quite common, as it often occurs in old-fashioned or long-time working power supplies. The main reasons are aging of components, unstable operation of the switch tube, and failure to dissipate heat in time. It is important to check whether the voltage regulator diode is heated and leaking, whether the rectifier diode is damaged, and whether the high-voltage filter capacitor is damaged. Example: Taking the infrared laser spectrometer as an example, after starting up, it cannot complete the self-test and alarms, and the mainboard indicator light keeps flashing. After inspection, it was found that the DC 5V power supply for the spectrometer mainboard had only about 2.3 volts left. The load of the 5V DC power supply was disconnected, and the 5V DC power supply was powered on and measured again. At this time, there was 5V. It was preliminarily determined that the load-bearing capacity of this 5V DC power supply was poor. The power supply casing was removed for inspection. Because there was a DC 5V output when there was no load, the output rectifier circuit on the secondary coil side was checked in detail. A dummy load was connected to the 5V power supply and it was powered on for measurement. It was found that the voltage between pins 1 and 2 of the three-way voltage regulator 7805 was 5.2 volts, but there was 2.3 volts between pins 2 and 3, as shown in the figure below. Therefore, it was determined that the performance of the three-way voltage regulator tube 7805 had deteriorated. The fault was solved by replacing the three-way voltage regulator tube 7805.

　　Modern electronic devices are becoming increasingly smaller and lighter, and most of them are inseparable from switching power supplies. If you want to fully master the maintenance skills of switching power supplies, you must be fully familiar with the working principles of switching power supplies and some common faults. Through this article, I hope everyone can understand the maintenance of switching power supplies and try to solve some problems in the circuit by themselves.