Common Fault Repair of Power Adapter
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Update time : 2021-11-10 15:52:43
Power adapter provides great convenience for everyone, and many industries cannot do without it, which means that its development potential is still great. The power adapter technology is also developing rapidly, with higher efficiency and smaller size. , Less electromagnetic pollution, more reliable power adapters, new products appear almost every year.
If the ac dc power adapter fails, it is usually caused by the opening of the starting coil or the damage of the starting drive tube. You can try to repair it with a substitution method. The maintenance ideas are as follows:
1. Measure whether the rectifier diode is open or disconnected.
2. Measure whether there is an open circuit or open circuit at the three ends of the switch tube.
3. Measure whether there is any open circuit in the coil of the switch transformer at each end.
4. Fuse or fuse blown.
Mainly check the diodes of the rectifier bridge, large filter capacitors, and switch tubes. Problems in the anti-interference circuit can also cause the fuse or fuse to blow and become black. It is worth noting that the blowout of the fuse or fuse caused by the breakdown of the switch tube is often accompanied by the damage of the overcurrent detection resistor and the power control chip. The negative temperature coefficient thermistor is also easy to burn out together with the fuse or the fuse.
5. No output, but the fuse or fuse is normal
This phenomenon indicates that the power adapter is not working, or it has entered a protection state after working. First, measure whether the startup pin of the power control chip has a startup voltage. If there is no startup voltage or the startup voltage is too low, check whether the startup resistor and the external components of the startup pin have leakage. At this time, if the power control chip is normal, the above Checking can quickly find the fault. If there is a starting voltage, measure whether the drive output pin of the control chip (the thick film circuit has no drive output pin) has a high and low level jump at the moment of power-on. If there is no jump, it means that the control chip is damaged, the peripheral oscillator circuit components or the protection circuit are defective, you can replace the control chip first, and then check the peripheral components. If there is a jump, it is generally a bad or damaged switch tube.
6. There is output voltage, but the output voltage is too high
Excessive output voltage generally comes from voltage stabilization sampling and voltage stabilization control circuits. In the closed control loop composed of DC output, sampling resistor, error sampling amplifier such as TL431, optocoupler, power control chip and other circuits, any one of the components will cause the output voltage to rise.
7. The output voltage is too low
The following are the main causes of low output voltage:
① Power adapter load short-circuit failure (especially DC/DC converter short-circuit or poor performance, etc.) , at this time, first disconnect all loads of the switching power supply circuit, and check whether the switching power supply circuit is faulty or the load circuit is faulty. If the load circuit is disconnected and the voltage output is normal, it means that the load is too heavy; or it is still abnormal, it means that the switching power supply circuit is faulty.
②The failure of the rectifier diode and filter capacitor at the output voltage end can be judged by the substitution method.
③The performance of the switch tube will decrease, which will inevitably lead to the failure of the switch tube to conduct normally, which will increase the internal resistance of the power supply and reduce the load capacity.
④ Poor switching transformer not only causes the output voltage to drop, but also causes insufficient excitation of the switching tube and thus repeated damage to the switching tube.
⑤ The 300V filter capacitor is poor, resulting in poor load carrying capacity of the power supply, and the output voltage will drop as soon as the load is connected.
8. Maintenance of repeated failures of switch tubes
Repeated damage power switch tube (or thick film circuit, thick film circuit contains a switch tube) is the key and difficult point of the power adapter circuit maintenance, the following is a system analysis. The switch tube is the core component of the power adapter. It works in a high-current and high-voltage environment, and its damage ratio is relatively high. Once damaged, it is often not possible to remove the fault by replacing it with a new tube, or even damage the new tube. , It is more troublesome to troubleshoot the failure of this kind of repeated damage switch tube, which often makes it impossible for beginners to start.
If the ac dc power adapter fails, it is usually caused by the opening of the starting coil or the damage of the starting drive tube. You can try to repair it with a substitution method. The maintenance ideas are as follows:
1. Measure whether the rectifier diode is open or disconnected.
2. Measure whether there is an open circuit or open circuit at the three ends of the switch tube.
3. Measure whether there is any open circuit in the coil of the switch transformer at each end.
4. Fuse or fuse blown.
Mainly check the diodes of the rectifier bridge, large filter capacitors, and switch tubes. Problems in the anti-interference circuit can also cause the fuse or fuse to blow and become black. It is worth noting that the blowout of the fuse or fuse caused by the breakdown of the switch tube is often accompanied by the damage of the overcurrent detection resistor and the power control chip. The negative temperature coefficient thermistor is also easy to burn out together with the fuse or the fuse.
5. No output, but the fuse or fuse is normal
This phenomenon indicates that the power adapter is not working, or it has entered a protection state after working. First, measure whether the startup pin of the power control chip has a startup voltage. If there is no startup voltage or the startup voltage is too low, check whether the startup resistor and the external components of the startup pin have leakage. At this time, if the power control chip is normal, the above Checking can quickly find the fault. If there is a starting voltage, measure whether the drive output pin of the control chip (the thick film circuit has no drive output pin) has a high and low level jump at the moment of power-on. If there is no jump, it means that the control chip is damaged, the peripheral oscillator circuit components or the protection circuit are defective, you can replace the control chip first, and then check the peripheral components. If there is a jump, it is generally a bad or damaged switch tube.
6. There is output voltage, but the output voltage is too high
Excessive output voltage generally comes from voltage stabilization sampling and voltage stabilization control circuits. In the closed control loop composed of DC output, sampling resistor, error sampling amplifier such as TL431, optocoupler, power control chip and other circuits, any one of the components will cause the output voltage to rise.
7. The output voltage is too low
The following are the main causes of low output voltage:
① Power adapter load short-circuit failure (especially DC/DC converter short-circuit or poor performance, etc.) , at this time, first disconnect all loads of the switching power supply circuit, and check whether the switching power supply circuit is faulty or the load circuit is faulty. If the load circuit is disconnected and the voltage output is normal, it means that the load is too heavy; or it is still abnormal, it means that the switching power supply circuit is faulty.
②The failure of the rectifier diode and filter capacitor at the output voltage end can be judged by the substitution method.
③The performance of the switch tube will decrease, which will inevitably lead to the failure of the switch tube to conduct normally, which will increase the internal resistance of the power supply and reduce the load capacity.
④ Poor switching transformer not only causes the output voltage to drop, but also causes insufficient excitation of the switching tube and thus repeated damage to the switching tube.
⑤ The 300V filter capacitor is poor, resulting in poor load carrying capacity of the power supply, and the output voltage will drop as soon as the load is connected.
8. Maintenance of repeated failures of switch tubes
Repeated damage power switch tube (or thick film circuit, thick film circuit contains a switch tube) is the key and difficult point of the power adapter circuit maintenance, the following is a system analysis. The switch tube is the core component of the power adapter. It works in a high-current and high-voltage environment, and its damage ratio is relatively high. Once damaged, it is often not possible to remove the fault by replacing it with a new tube, or even damage the new tube. , It is more troublesome to troubleshoot the failure of this kind of repeated damage switch tube, which often makes it impossible for beginners to start.
