First, the repair method
The maintenance of a switching power supply can be carried out in two stages
(1) When the power is cut off, it is "Look, Listen, Listen, Measure"
Look: Open the power box, check whether the wire is disconnected, look inside the power supply, and find that the PCB substrate of the power supply is scorched or the components are cracked, it is necessary to check the circuit components related to this component.
Smell: Check if there is a burning smell inside the power supply and if there are any burnt parts.
Q: Please listen to the history of the damage to the power supply and admit whether the power supply is violated.
Quantity: Before powering on, please use a multimeter to measure the voltage at both ends of the high-voltage capacitor. If it is caused by the failure of the switching power supply to oscillate or the disconnection of the switch tube, in most cases, the voltage at both ends of the high-voltage slippery capacitor does not leak, and the voltage is above 300, which needs to be paid attention to. Use a multimeter to measure the positive and negative resistance of both ends of the AC power cord and the charging status of the capacitor, if the resistance value is too low, the power supply may be short-circuited. The capacitor should be able to charge and discharge. Unload the load, measure the resistance to the ground at the output end of each group, the normal clock hand needs the charge and discharge swing of the capacitor, and the last indication is the resistance value of the leakage resistance of the same channel.
(2) Power-on detection
After the power is turned on, check whether the power supply is burned or smoked, and cut off the power supply as needed to check it.
It should not be important to check whether there is a 300 output at both ends of the high-voltage slippery capacitor, rectifier diode, slippery capacitor, etc.
To measure whether the secondary winding of the high-frequency transformer has an output, when there is no need to check whether the switch is damaged, oscillation, whether the protection circuit is working, etc., it is necessary to check the rectifier diode, slippery capacitor, three-way regulator tube, etc. on each output side.
If the power supply stops immediately after starting, the power supply is in a protection state, so the voltage of the protection input pin of the PWM chip can be directly measured, if the voltage exceeds the specified value, it means that the power supply is in a protection state, and the reason for the protection should be investigated.
2. Analysis of common problems
(1) Stable wire fuse
Generally speaking, a blown wire is an indication of a problem with the internal circuitry of the power supply. 1. Short circuit: short circuit problem occurs on the line side, and the safe wire is disconnected 2. Overload: the load current exceeds the rated current of the safe wire, and the safe wire is disconnected by long-term heating (generally 1.1 times the rated current is fused for about 1 hour) 3. Pulse: The circuit is started or the power supply is unstable and punctual, and the large current is disconnected in a moment
In addition, the screw is not tightened when the wire is installed, or the wire is damaged and the wire is blown off. A current short circuit and a current overload are different. The current short circuit is directly connected by the wire and the neutral wire, and without passing through the electrical equipment, it will soon produce a current loop with almost no blockage, and the extra-large current passes through the wire, and the resistance of the wire suddenly increases, and the heat energy generated is blown off in a moment
The so-called current overload refers to the excessive total power of the electrical appliances, that is, the current consumed is too large, and when the open electrical appliances are increased, the current passing through the wire increases, the resistance of the wire increases, and the heat energy is fused.
(2) There is no DC voltage output or the voltage output is not stable
The steady wire is intact, and when there is a load, the DC voltage at all levels is not output. This situation is mainly due to the phenomenon of open circuit and short circuit of the power supply, the fault of the overvoltage and overcurrent protection circuit, the defect of the auxiliary power supply, the failure of the oscillation circuit, the heavy load of the power supply, the damage of the rectifier diode in the high-frequency rectifier filter circuit, and the leakage of the filter capacitor.
The steady wire is intact, and when there is a load, the DC voltage at all levels is not output. This situation is mainly due to the phenomenon of open circuit and short circuit of the power supply, the fault of the overvoltage and overcurrent protection circuit, the defect of the auxiliary power supply, the failure of the oscillation circuit, the heavy load of the power supply, the damage of the rectifier diode in the high-frequency rectifier filter circuit, and the leakage of the filter capacitor. In addition to the condition that the high-frequency rectifier diode is damaged and the load is short-circuited, if the output is zero, the operation circuit of the power supply will definitely be faulty. When a part of the voltage output indicates the normal operation of the pre-circuit circuit, a problem occurs in the high-frequency rectifier filter circuit. The high-frequency filter circuit is mainly composed of a rectifier diode and a low-voltage filter capacitor to form a DC voltage output, in which the rectifier diode is damaged when the circuit has no voltage output, and the leakage of the filter capacitor will cause the output voltage to be unstable and other problems. The corresponding parts can be statically measured with a multimeter to detect damaged parts.
For example: after the power supply of a 24-lying DC motor is energized, there is no output of the DC 24-lying down, remove the power supply shell, and see that the stable wire is not disconnected, and the circuit board is not obviously scorched or cracked, and when it is not energized, the resistance value of the capacitor AC input terminal and the resistance value of the DC output terminal are normal, and the capacitor switch tube, rectifier bridge, rectifier tube and other important components The results show that the switching power supply uses the U3842 PWM operation chip, query the relevant information, and the voltage of the 3rd terminal of the U3842 chip exceeds 1 special time. The internal current sensor output is high, resets the PWM latch, and turns off the output. The 3 terminals of the power-on measurement U3842 are higher than 1 and the 6 terminals have no output, and the result of checking the relevant circuits is that the stable pressure pipe D2 is damaged. As shown in the figure below, PC1 is turned on, and the 3 terminals of U3842 become high, so that 6 terminals have no output, the switch tube does not operate, and there is no DC output on the DC side. Replace the stable trachea D2 of the same model, and the problem will be eliminated.
(3) Poor load capacity of the power supply
The difference in the load capacity of the power supply is a common problem, which is generally presented in the old power supply and the long-term power supply, mainly due to the aging of each component, the unstable operation of the switch tube, and the inability to dissipate heat in time. It is necessary to check whether the Zener diode is heating and leaking, whether the rectifier diode is damaged, and whether the high-voltage slippery capacitor is damaged.
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