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How to solve the common faults of switching power supply

Insurance tube blown
Mainly check the large filter capacitor on 300V, the diodes of the rectifier bridge and the switch tube. The problem of anti-interference circuit will also cause the fuse to blow and black. It is worth noting that the fuse blown due to the breakdown of the switch tube is often accompanied by damage of the overcurrent detecting resistor and the power control chip. The negative temperature coefficient thermistor is also easily burned out together with the fuse.
No output, but the fuse is normal
This phenomenon indicates that the switching power supply is not working or has entered the protection state after working. First, measure whether the starting pin of the power control chip has a starting voltage. If there is no starting voltage or the starting voltage is too low, check whether the starting resistor and the external components of the starting pin are leaking. If the power control chip is normal, the above check can be performed. The fault was quickly detected. If there is a starting voltage, then whether the output of the control chip has a high or low transition at the instant of power-on, if there is no jump, it means that the control chip is bad, the peripheral oscillator circuit component or the protection circuit has a problem, and can be replaced first. Control the chip, and then check the peripheral components; if there is a jump, the switch is generally bad or damaged.
Output voltage, but the output voltage is too high
In liquid crystal displays, such faults often come from regulated sampling and voltage regulation circuits. We know that DC output, sampling resistors, error sampling amplifiers (such as TL431), optocouplers, power control chips and other circuits together constitute a closed control loop, any problem will lead to an increase in output voltage.
Method skills:
For a power supply with an overvoltage protection circuit, the output voltage is too high first to cause the overvoltage protection circuit to operate. At this time, the overvoltage protection circuit can be disconnected, so that the overvoltage protection circuit cannot be used, and the power supply main voltage at the startup instant is measured. If the measured value is higher than the normal value by more than IV, the output voltage is too high. In actual maintenance, it is common to use sampling resistors, precision regulators, or optocouplers.
Output voltage is too low
According to the maintenance experience, in addition to the voltage regulator control circuit will cause the output voltage to be too low, there are some reasons that will cause the output voltage to be too low, mainly the following:
1 Switching power supply load has short circuit fault (especially DC/DC converter short circuit or poor performance, etc.). At this time, all the loads of the switching power supply circuit should be disconnected to distinguish whether the switching power supply circuit or the load circuit is faulty. If the voltage output of the load circuit is disconnected, it means that the load is too heavy; if it is still not normal, the switching power supply circuit is faulty.
2 output voltage terminal rectifier diode, filter capacitor failure, etc., can be judged by the substitution method.
3 The performance of the switch tube is degraded, which will inevitably lead to the switch tube not being able to conduct normally, which will increase the internal resistance of the power supply and reduce the load capacity.
4 Switching transformer is not good, not only causes the output voltage to drop, but also causes the switching tube to be insufficiently excited and thus repeatedly damages the switching tube.
The 5300V filter capacitor is poor, resulting in poor power supply load capacity, and the output voltage of the load will drop.

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