Power failure accounts for the majority of electronic equipment failures in electrical troubleshooting. Therefore, being familiar with the common faults and maintenance skills of switching power supply will help shorten the maintenance time of electronic equipment faults and improve the maintenance skills of personal equipment.

1、 Common faults

(1) Fuse blown:

In general, a blown fuse indicates that there is a problem with the internal circuit of the power supply. Because the power supply works at high voltage and high current, the fluctuation and surge of the grid voltage will cause the current in the power supply to increase instantaneously and cause the fuse to blow. It is important to check the rectifier diode, high-voltage filter electrolytic capacitor, inverter power switch tube, etc. at the input end of the power supply, and check whether these components have breakdown, open circuit, damage, etc. If the fuse is really blown, you should first check the components on the circuit board to see if the appearance of these components is burnt and if there is electrolyte overflow. If the above conditions are not found, use a multimeter to measure whether the switch tube has a breakdown short circuit. Special attention should be paid to the fact that when a certain element is found to be damaged, it is not allowed to start the machine directly after replacement. It is likely that the replaced element will be damaged due to the failure of other high-voltage elements. It is necessary to thoroughly check and measure all high-voltage elements of the above circuits before thoroughly eliminating the fuse blowing fault.

(2) No DC voltage output or voltage output is unstable

If the fuse is intact, there is no output of DC voltage at all levels under load. This situation is mainly caused by the following reasons: open circuit and short circuit in the power supply, overvoltage and overcurrent protection circuit failure, auxiliary power failure, oscillation circuit not working, power overload, breakdown of rectifier diode in high-frequency rectifier filter circuit, leakage of filter capacitor, etc. After measuring the secondary components with a multimeter and eliminating the breakdown of high-frequency rectifier diode and load short circuit, if the output is zero at this time, it can be sure that the control circuit of the power supply has failed. If there is partial voltage output, it indicates that the front circuit works normally, and the fault is in the high-frequency rectifier filter circuit. The high-frequency filter circuit is mainly composed of rectifier diode and low-voltage filter capacitor to form the DC voltage output. The breakdown of rectifier diode will make the circuit have no voltage output, and the leakage of filter capacitor will cause the output voltage instability and other faults. Use a multimeter to measure the corresponding components statically to check the damaged components.

(3) Poor power load capacity

The poor load capacity of the power supply is a common fault, which generally occurs in the old power supply or the power supply with a long working time. The main reason is that the components are aging, the switch tube is unstable, and the heat dissipation is not carried out in time. It is important to check whether the voltage regulator diode has heat and leakage, the rectifier diode is damaged, and the high-voltage filter capacitor is damaged.

2、 Maintenance skills

The maintenance of switching power supply can be carried out in two steps:

(1) In case of power failure, "see, smell, ask and measure"

① Look: open the shell of the power supply, check whether the fuse is blown, and then observe the internal condition of the power supply. If there is scorch or component fracture on the PCB board of the power supply, focus on checking the components and relevant circuit components.

② Smell: Smell whether there is a smell of paste inside the power supply and check whether there are charred components.

③ Q: Ask about the process of power supply damage and whether there is any illegal operation on the power supply.

④ Measurement: Before powering on, measure the voltage at both ends of the high-voltage capacitor with a multimeter. If the fault is caused by the failure of the switching power supply or the open circuit of the switch tube, in most cases, the voltage at both ends of the high voltage filter capacitor is not discharged. This voltage is more than 300 volts. Be careful. Use a multimeter to measure the positive and negative resistance at both ends of the AC power line and the charging condition of the capacitor. The resistance value should not be too low, otherwise there may be a short circuit inside the power supply. The capacitor shall be able to charge and discharge. Disconnect the load and measure the resistance to the ground of each group of output terminals. Under normal conditions, the meter needle should have the capacitor charge and discharge swing, and the last indication should be the resistance value of the discharge resistance of the circuit.

(2) Power-on detection

After power-on, observe whether the power supply has the phenomenon of burning fuse and smoking of individual components. If yes, cut off the power supply in time for maintenance.

Measure whether there is 300V output at both ends of the high voltage filter capacitor. If not, check the rectifier diode, filter capacitor, etc.

Measure whether the secondary coil of the high-frequency transformer has output. If not, focus on checking whether the switch tube is damaged, whether it vibrates, whether the protection circuit acts, etc. If yes, focus on checking the rectifier diode, filter capacitor, three-way regulator tube, etc. at each output side.

If the power supply is stopped once it is started, the power supply is in the protection state, and the voltage of the PWM chip protection input pin can be directly measured. If the voltage exceeds the specified value, it means that the power supply is in the protection state, and the cause of the protection should be focused on.