Switching power supply common faults and maintenance tips

Power failures in electrical troubleshooting account for the majority of electronic equipment failures. Therefore, familiar with the common faults and maintenance techniques of the switching power supply, it is beneficial to shorten the electronic equipment fault repair time and improve the personal equipment maintenance skills.

First, common faults

(1) The fuse is blown:

In general, a blown fuse indicates a problem with the internal wiring of the power supply. Due to the high voltage and high current of the power supply, the fluctuations and surges of the power grid will cause the current in the power supply to increase instantaneously and the fuse will be blown. The key points should be to check the rectifier diode of the power input terminal, high-voltage filter electrolytic capacitor, inverter power switch tube, etc., check whether the component has breakdown, open circuit, damage, etc. If the fuse is really blown, you should first check the components on the board to see if the appearance of these components is burnt, and if there is any electrolyte overflow. If the above situation is not found, use a multimeter to measure the switch tube for short circuit. . Special attention should be paid to: Do ​​not directly detect the damage of a component when it is damaged. It is very likely that the other high-voltage components will still be damaged and the replacement components will be damaged. It is necessary to comprehensively apply all the high-voltage components of the above circuit. After the measurement is checked, the fuse blowout can be completely eliminated.

(2) No DC voltage output or unstable voltage output

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, short circuit phenomenon in the power supply, overvoltage, overcurrent protection circuit failure, auxiliary power failure, oscillating circuit not working, power supply overload, rectifier diode in high frequency rectification and filtering circuit It is broken down, the filter capacitor is leaking, and so on. After measuring the secondary component with a multimeter and eliminating the breakdown of the high-frequency rectifier diode and the short-circuit of the load, if the output is zero at this time, it is sure that the control circuit of the power supply has failed. If some of the voltage output indicates that the pre-stage circuit is working properly, the fault is in the high-frequency rectification and filtering 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 causes the circuit to have no voltage output, and the leakage of the filter capacitor may cause an unstable output voltage. Statically measure the corresponding component with a multimeter to check the damaged component.

(3) Poor power supply capacity

Poor power load capacity is a common fault, which usually occurs in old-fashioned or long-duration power supplies. The main reason is that the components are aging, the switching tube is unstable, and there is no heat dissipation in time. It is important to check whether the Zener diode is hot or leaky, the rectifier diode is damaged, and the high voltage filter capacitor is damaged.

Second, maintenance skills

The maintenance of the switching power supply can be divided into two steps:

(1) "Look, smell, ask, measure" in case of power failure

1 Look: Open the outer casing of the power supply, check whether the fuse is blown, and then observe the internal condition of the power supply. If it is found that there is burnt or broken component on the PCB of the power supply, the components and related circuit components should be checked.

2 Smell: Smell the inside of the power supply for a smell, check for charred components.

3 Q: Ask if the power supply has been damaged and whether the power supply is operated in violation.

4 Quantity: Before powering on, use a multimeter to measure the voltage across the high voltage capacitor first. If the switching power supply does not vibrate or the open circuit of the switch is faulty, in most cases, the voltage across the high-voltage filter capacitor is not bleed. This voltage is more than 300 volts, so be careful. Use a multimeter to measure the forward and reverse resistance and capacitor charging at both ends of the AC power line. 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. Disengage the load and measure the ground resistance of each group's output. When normal, the needle should have capacitor charging and discharging swing. The last indication should be the resistance of the bleeder resistor of the circuit.

(2) Power-on detection

After power-on, observe whether the power supply has burn-in insurance and smoke from individual components. If there is any need to cut off the power supply for maintenance.

Measure whether there is a 300 volt output at both ends of the high voltage filter capacitor. If there is no need to check the rectifier diode, filter capacitor, etc.

Measure whether there is output of the secondary coil of the high-frequency transformer. If there is no key to check whether the switch is damaged, whether it is oscillating, whether the protection circuit is working, etc. If there is, it should focus on checking the rectifier diode, filter capacitor and three-way stability of each output side. Pressure tube and so on.

If the power supply is stopped, 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, the power supply is in the protection state, and the cause of the protection should be checked.