Common fault maintenance of switching power supply

1) The fuse is burnt out

Mainly check the large filter capacitor on 300V, the diode of the rectifier bridge and the switch tube and other parts, the anti-interference circuit problems will also lead to fuse burning, black. It is worth noting that the fuse rupture caused by the breakdown of the switching tube is often accompanied by the overcurrent detection resistor and power control chip damage. Negative temperature coefficient thermistors are also easily burned out along with fuses.

2) No output, but the fuse is normal

This phenomenon indicates that the switching power supply does not work, or after working into the protection state. 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 resistance and the components connected to the starting pin leak. If the power control chip is normal at this time, the fault can be quickly detected through the above check. If there is a starting voltage, measure whether the output end of the control chip has a high or low level jump at the moment of starting up. If there is no jump, it indicates that the control chip is bad or there is a problem with the peripheral oscillating circuit component or the protection circuit. Replace the control chip first and then check the peripheral components. If there is a jump, generally bad or damaged switch tube.

3) There is output voltage, but the output voltage is too high

In liquid crystal display (LCD), this fault is often caused by the stabilized voltage sampling and stabilized voltage control circuit. We know that the DC output, sampling resistance, error sampling amplifier (such as TL431), optical coupler, power control chip and other circuits together form a closed control loop, any one of the problems will cause the output voltage increase.

Methods and skills:

For the power supply with overvoltage protection circuit, the output voltage is too high will first make the overvoltage protection circuit action, at this time, you can disconnect the overvoltage protection circuit, so that the overvoltage protection circuit can not be used, measure the power supply at the moment of the main voltage. If the measured value is more than IV higher than the normal value, the output voltage is too high. In actual maintenance, it is common to have poor sampling resistance, precision voltage regulator amplifier or optical coupler.

4) Output voltage is too low

According to maintenance experience, in addition to the voltage regulator control circuit will cause the output voltage is too low, there are some reasons will cause the output voltage is too low, mainly the following points:

① The load of the switching power supply has a short circuit fault (especially the DC/DC converter short circuit or poor performance, etc.). At this point, all 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 normal, it indicates that the load is too heavy. If the fault persists, the switching power supply circuit is faulty.

② The failure of rectifier diode and filter capacitor at output voltage end can be judged by substitution method.

③ The performance of the switch tube decreases, which will inevitably lead to the switch tube can not be normally switched on, so that the internal resistance of the power supply increases and the load capacity decreases.

(4) Bad switching transformer, not only cause the output voltage drop, but also cause the switch tube excitation insufficient resulting in repeated damage to the switch tube.

⑤ Poor 300V filter capacitance, resulting in poor load capacity of the power supply, once connected to the load output voltage will drop.