Maintenance methods for switch mode power supply faults
Switching power supply is an essential component of various electronic devices, and its performance directly affects the technical indicators of electronic devices and whether they can work safely and reliably. Due to the key components inside the switching power supply working in high-frequency switching state, low power consumption, high conversion rate, and only 20% -30% of the size and weight of linear power supplies, it has become the mainstream product of voltage stabilizing power supplies. The maintenance of electrical faults in electronic devices, based on the principle of starting from easy to difficult, generally starts with the power supply. After confirming that the power supply is normal, other parts of the equipment can be repaired, and power supply faults account for the majority of electrical faults in electronic devices. Understanding the basic working principle of the starting power supply, familiarizing oneself with its maintenance skills and common faults, is beneficial for shortening the repair time of electronic equipment faults and improving personal equipment maintenance skills.
1. No output, the safety tube is normal
This phenomenon indicates that the switch power supply is not working or has entered a protective state. Firstly, 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 resistance and external components of the starting pin are leaking electricity. If the power control chip is normal at this time, the fault can be quickly detected through the above inspection. If there is a startup voltage, measure whether there is a high or low level jump at the output end of the control chip at the moment of startup. If there is no jump, it indicates that the control chip is damaged, the peripheral oscillation circuit components or the protection circuit are faulty. The control chip can be replaced first, and then the peripheral components can be checked; If there is a jump, it is generally due to poor or damaged switch tubes.
2. Safe burning or frying
Mainly inspect the large filtering capacitor, rectifier bridge diodes, switch tubes, and other parts on 300V. Problems with the anti-interference circuit can also cause fuse burning and blackening. It should be noted that fuse burning caused by switch tube breakdown usually burns out the current detection resistor and power control chip. Negative temperature coefficient thermistors are also prone to being burned out along with the fuse.
3. There is an output voltage, but the output voltage is too high
This type of fault usually comes from the voltage stabilization sampling and voltage stabilization control circuit. A closed control loop is formed by circuits such as DC output, sampling resistor, error sampling amplifier such as TL431, optocoupler, power control chip, etc. Any problem at any point will cause an increase in output voltage.
4. Low output voltage. In addition to the voltage stabilizing control circuit causing low output voltage, there are also some reasons that can cause low output voltage:
a. If there is a short circuit fault in the load of the switch power supply (especially in the case of a short circuit or poor performance of the DC/DC converter), all loads of the switch power supply circuit should be disconnected to distinguish whether the switch power supply circuit or the load circuit is faulty. If the voltage output of the disconnected load circuit is normal, it indicates that the load is too heavy; If it is still abnormal, it indicates that there is a malfunction in the switch power supply circuit.
b. The failure of the rectifier diode and filtering capacitor at the output voltage terminal can be determined by substitution method.
c. The performance degradation of the switching tube will inevitably lead to the inability of the switching tube to conduct normally, resulting in an increase in the internal resistance of the power supply and a decrease in the load capacity.
