Switching power supply fault inspection method and fault repair ideas
Switching power supply is an essential component of various electronic devices, and its performance directly affects the technical indicators and whether it can work safely and reliably. Due to the fact that the key components inside the switching power supply operate in a high-frequency switching state, with low power consumption, high conversion rate, and only 20% -30% of the volume and weight of linear power supplies, it has become the mainstream product of regulated power supplies. The maintenance of electrical faults in electronic devices follows the principle of starting from easy to difficult, basically starting from the power supply. After confirming that the power supply is normal, maintenance of other parts can be carried out, and power supply faults account for the majority of electrical faults in electronic devices. Understanding the basic working principle of the power supply at the beginning, familiarizing oneself with its maintenance techniques and common faults, is beneficial for shortening the repair time of electronic equipment failures and improving personal equipment maintenance skills.
1. No output, the fuse is normal
This phenomenon indicates that the switch power supply is not working or has entered a protective state. Firstly, it is necessary to measure whether there is a starting voltage at the starting pin of the power control chip. If there is no starting voltage or the starting voltage is too low, it is necessary to check whether the starting resistor and external components of the starting pin are leaking electricity. If the power control chip is normal at this time, the above inspection can quickly detect the fault. If there is a starting voltage, measure whether there is a high or low level jump at the output terminal of the control chip at the moment of startup. If there is no jump, it indicates that the control chip is broken, or there is a problem with the peripheral oscillation circuit components or protection circuit. The control chip can be replaced first, and then the peripheral components can be checked; If there is a jump, it is usually due to poor or damaged switch tubes.
2. Insurance burns or explodes
Mainly inspect the large filtering capacitor, rectifier bridge diodes, and switch tubes on 300V. Problems with the anti-interference circuit can also cause fuse burning and blackening. It should be noted that fuse burnout caused by switch tube breakdown usually burns out the current detection resistor and power control chip. Negative temperature coefficient thermistors are also easily burned out along with fuses.
3. There is an output voltage, but the output voltage is too high
This type of fault usually 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, and any problem at any point will cause the output voltage to rise.
4. In addition to the voltage regulator control circuit causing low output voltage, there are also some reasons that can cause low output voltage:
a. When there is a short circuit fault in the load of the switching power supply (especially in the DC/DC converter), all loads in the switching power supply circuit should be disconnected to distinguish whether it is a fault in the switching power supply circuit or the load circuit. If the voltage output of the disconnected load circuit is normal, it indicates that the load is too heavy; Or if it is still abnormal, it indicates that there is a fault in the switch power supply circuit.
b. The failure of rectifier diodes and filtering capacitors at the output voltage terminal can be determined by substitution method.
c. The performance degradation of the switch tube will inevitably lead to the inability of the switch tube to conduct normally, resulting in an increase in the internal resistance of the power supply and a decrease in load capacity.
