Common faults and maintenance skills of switching power supply

Maintenance of switching power supply can be divided into two steps:

I. In the case of power failure, "see, smell, ask, measure"

1. Look:

Open the shell of the power supply, check whether the fuse is blown, and then observe the internal situation of the power supply. If the PCB board of the power supply is found to be burnt or the components are broken, the components and related circuit components should be checked. Asset management

2. Smell:

Smell the inside of the power supply for mush and check for burnt components.

3. Question:

Ask the power supply damage process, whether illegal operation of the power supply.

4. Quantity:

Before the power is off, use a multimeter to measure the voltage at both ends of the high-voltage capacitor. If the fault is caused by the non-vibration of the switching power supply or the open circuit of the switching tube, in most cases, the voltage at both ends of the high-voltage filter capacitor is not discharged, and the voltage is more than 300 volts. Caution should be taken. Use a multimeter to measure the positive and negative resistance and capacitor charging at both ends of the AC power cable. The resistance value should not be too low; otherwise, there may be short circuit inside the power supply. Capacitors should be able to charge and discharge. Remove the load, respectively measure the output end of each group to the ground resistance, normal, the dial should be capacitor charge and discharge swing, the last indication should be the path of the drain resistance value.

5, power detection

Observe whether the power supply has burning insurance and individual components smoke phenomenon, if there is to cut off the power supply in time for maintenance.

Measure whether there is 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 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 so, focus on checking the rectifier diode, filter capacitor and three-way voltage regulator on each output side.

If the power supply starts to stop, then the power supply is in a protected state, can directly measure the voltage of the PWM chip protection input foot, if the voltage exceeds the specified value, it indicates that the power supply is in a protected state, should focus on checking the cause of protection.

two Common fault

1. The fuse is blown

Normally, a blown fuse indicates a problem with the internal wiring of the power supply. Because the power supply works in the state of high voltage and high current, the voltage fluctuation and surge of the grid will cause the current in the power supply to increase instantaneously and make the fuse break. Focus on checking the power input rectifier diode, high voltage filter electrolytic capacitor, inverter power switch tube, etc., check whether this component has breakdown, open circuit, damage, etc. If it is indeed a blown fuse, should first look at the various components on the circuit board, see whether the appearance of these components is burned, there is no electrolyte overflow, if not found the above situation, then use a multimeter to measure the switch tube breakdown short circuit. Need to pay special attention to is: must not be found in the damage of a component, after the replacement of direct boot, so it is likely to be due to the failure of other high voltage components and replacement components damaged, must be on the circuit of all high voltage components after a comprehensive inspection measurement, in order to completely eliminate the fault of the fuse blown.

2. No DC voltage output or unstable voltage output

If the fuse is intact, all levels of DC voltage have no output under load. This situation is mainly caused by the following reasons: the phenomenon of open circuit and short circuit in the power supply, the failure of the overvoltage and overcurrent protection circuit, the failure of the auxiliary power supply, the oscillation circuit does not work, the overload of the power supply, the breakdown of the rectifier diode in the high-frequency rectifier filter circuit, and the leakage of the filter capacitor. In the multimeter measurement of secondary components, eliminate the high-frequency rectifier diode breakdown, load short circuit, if the output is zero, you can be sure that the control circuit of the power supply out of the fault. If part of the voltage output indicates that the former circuit works normally, the fault is in the high-frequency rectifier filter circuit. High frequency filter circuit is mainly composed of rectifier diode and low voltage filter capacitor DC voltage output, rectifier diode breakdown will make the circuit no voltage output, filter capacitor leakage will cause the output voltage instability and other faults. The damaged component can be detected by static measurement of the corresponding component with a multimeter. For example: After a 24 volt DC motor power supply is energized, there is no DC 24 volt output. Open the power supply housing, observe that the fuse is not burned and the circuit board has no obvious burnt or broken components. In the case of no power, the resistance value of AC input and DC output is normal, and the resistance value of the switch tube, rectifier bridge, rectifier tube and other important components are normal. Estimate protection circuit action. It is checked that the switching power supply adopts U3842 PWM control chip. After searching relevant information, it is known that when the voltage of the three terminals of the U3842 chip is higher than 1 volt, the internal current sensitive comparator will output high level, and the PWM latch will be reset to turn off the output. After checking relevant circuits, it is found that voltage regulator D2 is broken down, as shown in FIG. 3. Therefore, PC1 is switched on, resulting in high level at 3 ends of U3842, no output at 6 ends, no switch tube working, and no DC output at DC side. Replace the pressure regulator D2 of the same model, and the fault is rectified.

3, the power load capacity is poor

The poor load capacity of the power supply is a common fault, generally appearing in the old or long working time of the power supply, the main reason is the aging of the components, the unstable work of the switching tube, no timely heat dissipation. Should focus on checking whether the regulator diode heat leakage, rectifier diode damage, high voltage filter capacitor damage. Example: Our laser spectrometer (VECTOR 22) in the near red area cannot complete self-test and alarm after starting up, and the indicator light of the main board keeps flashing. After inspection, for the spectrometer motherboard DC 5V power supply only about 2.3 volts, off the 5V DC power load, power again measured 5V DC power supply, then there are 5V, preliminary judgment of the 5V DC power supply with load capacity is poor, open the power shell for maintenance, because there is no load, power with DC 5V output, Therefore, it is important to check the output rectifier circuit at the side of the secondary coil, and connect the 5-volt power supply to the dummy load for measurement. It is found that the voltage between the first and second pins of the three-way voltage regulator 7805 is 5.2 volts, while the voltage between the second and third pins is 2.3 volts, as shown in Figure 4. Therefore, it is judged that the performance of the three-way voltage regulator 7805 is deteriorated, and the fault is solved by replacing the three-way voltage regulator 7805.