Four switching power supplies often make faults and maintenance tips

Switching power supply is an indispensable part of various electronic devices. Its performance is directly related to the technical indicators of electronic devices and 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 electronic equipment electrical faults is based on the principle of easy to difficult, basically starting with the power supply. After confirming that the power supply is normal, the maintenance of other parts is performed, and the power failure accounts for most of the electrical equipment electrical faults. . Therefore, understanding the basic working principle of the starting power supply, familiar with its maintenance skills and common faults, will help shorten the maintenance time of electronic equipment and improve personal equipment maintenance skills.

1. No output, insurance pipe is normal

This phenomenon indicates that the switching power supply is not working or has entered the protection state. First, measure the start-up voltage of the power control chip's start-up pin. If there is no start-up voltage or the start-up voltage is too low, check whether the external components of the start-up resistor and the start-up pin are leaky. If the power control chip is normal, check the above. You can quickly find the fault. If there is a starting voltage, then whether the output terminal of the measurement control chip has a high or low level jump at the moment of starting, if there is no jump, it indicates that the control chip is bad, the peripheral oscillation circuit element or the protection circuit has a problem, and can be replaced first The chip, then check the external components; if there is a jump, it is generally a bad or damaged switch.

2. Burn or fry

Mainly check the large filter capacitor on the 300V, rectifier diodes and switch tubes and other parts, anti-jamming circuit problems will lead to insurance burn, black. It should be noted that the current detection resistor and the power supply control chip are usually burned out due to the breakdown of the switch tube. Negative temperature coefficient thermistors are also easily burned together with the fuse.

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

This fault usually comes from the regulator sampling and regulator control circuit. In the DC output, sampling resistors, error sampling amplifiers such as TL431, optocoupler, power control chip and other circuits together constitute a closed control loop, any one of the problems 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 is low, there are some of the following reasons will cause the output voltage is low:

a. The switching power supply has a short circuit fault (especially a short circuit or poor performance of the DC/DC converter). At this time, all loads of the switching power supply circuit should be turned off to distinguish whether the switching power supply circuit or the load circuit has a fault. If you disconnect the load circuit voltage output is normal, indicating that the load is too heavy; or still not normal that the switching power supply circuit is faulty.

b. Output voltage side rectifier diode, filter capacitor failure, etc., can be judged by substitution method.

c. The drop in the performance of the switch tube 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 carrying capacity.