Four kinds of switching power supply common faults and maintenance skills

Switching power supply is an indispensable part of various electronic equipment, and its performance is directly related to the technical indicators of electronic equipment and whether it can work safely and reliably. Because the key components inside the switching power supply work in the high-frequency switching state, the power consumption is small, the conversion rate is high, and the volume and weight are only 20%-30% of the linear power supply, so it has become the mainstream product of the regulated power supply. The maintenance of electrical faults of electronic equipment, in line with the principle of easy to difficult, basically starts with the power supply, and after confirming that the power supply is normal, the maintenance of other parts is carried out, and power supply faults account for most of the electrical faults of electronic equipment. . Therefore, understanding the basic working principle of the power supply at the beginning, familiar with its maintenance skills and common faults, is conducive to shortening the maintenance time of electronic equipment failures and improving personal equipment maintenance skills.

1. No output, the fuse is normal

This phenomenon indicates that the switching power supply is not working or has entered a protection state. First, measure whether the start-up pin of the power control chip has a start-up voltage. If there is no start-up voltage or the start-up voltage is too low, check whether the start-up resistor and the components connected to the start-up pin are leaking current. If the power control chip is normal, the above check Faults can be found quickly. If there is a start-up voltage, measure whether the output of the control chip has high and low level jumps at the moment of power-on. If there is no jump, it means that the control chip is broken, the peripheral oscillation circuit components or the protection circuit have problems, 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.

2. Blown or blown fuse

Mainly check the large filter capacitor on the 300V, the diodes of the rectifier bridge and the switch tube, etc., the problem of the anti-interference circuit will also cause the fuse to burn and turn black. It should be noted that the fuse burning due to the breakdown of the switch tube will generally burn out the current detection resistor and the power control chip. Negative temperature coefficient thermistors are also easy to burn out along with fuses.

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

This kind of fault generally comes from the voltage-stabilizing sampling and voltage-stabilizing 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 lead to an increase in the output voltage.

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 for the output voltage to be low:

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 of the switch tube is degraded, 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.