How to repair 5v switching power supply
1. Fuse burning or frying Mainly check the large filter capacitor on the 300V, the diodes of the rectifier bridge, and the switching tubes. Problems with the anti-interference circuit will also cause the fuse to burn and blacken. It should be noted that the fuse burnout caused by the breakdown of the switch tube will generally burn out the current detection resistor and the power control chip. NTC thermistors are also easily burned out together with the fuse.
2. No output, and the fuse is normal. This phenomenon indicates that the switching power supply is not working or has entered a protection state. First of all, it is necessary to 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 connected to the starting pin are leaking. If the power control chip is normal at this time, the above inspection Faults can be found quickly. If there is a starting voltage, measure whether the output terminal of the control chip has high or low level jumps at the moment of starting up. If there is no jump, it means that the control chip is broken, there is a problem with the peripheral oscillation circuit components or the protection circuit, and the control can be replaced first. Chip, and then check the peripheral components; if there is a jump, it is generally a bad or damaged switch tube.
3. There is output voltage, but the output voltage is too high. This fault generally comes from the voltage regulation sampling and voltage regulation control circuit. The DC output, sampling resistor, error sampling amplifier such as TL431, optocoupler, power control chip and other circuits together form a closed control loop, any problem will cause the output voltage to rise.
4. The output voltage is too low In addition to the voltage regulator control circuit will cause the output voltage to be low, there are also the following reasons that can also cause the output voltage to be low:
a. The switching power supply load has a short circuit fault (especially the DC/DC converter short circuit or poor performance, etc.). At this time, 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 disconnected load circuit is normal, it means that the load is too heavy; or if it is still abnormal, it means that the switching power supply circuit is faulty.
b. The failure of the rectifier diode and filter capacitor at the output voltage end can be judged by the substitution method.
c. The performance degradation of the switch tube 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.
d. The switching transformer is not good, which not only causes the output voltage to drop, but also causes insufficient excitation of the switching tube, resulting in repeated damage to the switching tube
e. The 300V filter capacitor is bad, resulting in poor load capacity of the power supply, and the output voltage will drop when the load is connected.
