Switching power supply is an indispensable part of all kinds of electronic equipment, its performance is directly related to the technical indicators of electronic equipment and can work safely and reliably. As the key components within the switching power supply work in the high-frequency switching state, small power consumption, high conversion rate, and the volume and weight of only 20% -30% of the linear power supply, so it has become the mainstream of the regulated power supply products. Electrical fault repair of electronic equipment, in line with the principle of easy to difficult, basically from the power supply to start, after determining the power supply is normal, and then other parts of the overhaul, and power supply failure accounted for the majority of electrical faults in electronic equipment. Therefore, to understand the basic working principle of the beginning of the power supply, familiar with its maintenance techniques and common failures, is conducive to shorten the electronic equipment failure repair time, improve personal equipment maintenance skills.
1. no output, the fuse is normal
This phenomenon indicates that the switching power supply does not work or enter the protection state. First of all, we must measure the power control chip start foot whether the start voltage, if no start voltage or start voltage is too low, it is necessary to check the start resistor and start the foot of the external components of leakage, at this time, such as power control chip is normal, the above checks can be quickly found fault. If there is a start-up voltage, then measure the output of the control chip in the boot moment whether there is a high, low level jump, if no jump, indicating that the control chip is bad, peripheral oscillator circuit components or protection circuits have a problem, you can replace the control chip, and then check the peripheral components; if there is a jump, generally bad or damaged switching tubes.
2. insurance burn or fried
Mainly check the 300V on the large filter capacitors, rectifier bridge diodes and switching tubes and other parts of the anti-interference circuit problems can also lead to insurance burn, black. It should be noted that: due to the switching tube breakdown caused by the insurance will generally burn the current detection resistor and power control chip. Negative temperature coefficient thermistor is also easy to be burnt together with the insurance. 3.
3. output voltage, but the output voltage is too high
This kind of fault generally comes from the voltage sampling and regulator control circuit. In the DC output, sampling resistor, error sampling amplifier such as TL431, optocoupler, power control chip and other circuits together constitute a closed control loop, any one of the problems will lead to increased output voltage.
4. low output voltage in addition to the voltage regulator control circuit will cause low output voltage, there are some reasons below will also cause low output voltage:
a. Switching power supply load has a short-circuit fault (especially DC/DC converter short circuit or poor performance, etc.), at this time, you should disconnect all the load of the switching power supply circuit, in order to distinguish between 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 switching power supply circuit failure.
b. Output voltage rectifier diode, filter capacitor failure, etc., can be judged by substitution method.
c. The performance of the switching tube decreases, which will inevitably lead to the switching tube can not be normal conduction, so that the internal resistance of the power supply increases and the ability to carry load decreases.
