Switching power supply protection tutorial
Switching power supply is an indispensable part of various electronic equipment, and its quality is directly related to the technical indicators of the electronic equipment and whether it can be operated safely and reliably. Since the main internal components of the switching power supply work in high-frequency switching conditions, 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 now become the mainstream product of the stabilized power supply. The maintenance of electrical defects of electronic equipment is based on the principle of easy to difficult. Basically, the power supply is first started. After the power supply is confirmed to be normal, other parts of the maintenance are carried out, and the electrical shortcomings of the power supply account for the largest part of the electrical shortcomings of electronic equipment. most. Therefore, understanding the basic operation principle of the initial power supply, understanding its maintenance skills and common shortcomings will help shorten the maintenance time of electronic equipment shortcomings.
Disadvantages of switching power supply and maintenance methods
1. No output stabilizer tube is normal. This phenomenon indicates that the switching power supply is not operating or has entered a protection condition. The first thing to do is to measure whether the recommended pin of the power control chip has a recommended voltage. If there is no recommended voltage or the recommended voltage is too low, check whether the resistor and the external components of the pin are leaking. At this time, if the power control chip is normal, it can be activated by the above inspection. Find the shortcomings. If there is a recommended voltage, measure whether the output terminal of the control chip has high or 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 are defective, and the control can be replaced first. Chip, then check the peripheral components; if there is a jump, it is generally a bad or damaged switch tube.
2. For stable burning, first check the large filter capacitors on the 300V, the diodes of the rectifier bridge, and the switching tubes. Problems with the anti-interference circuit can also lead to stable burning and blackening. Need to pay attention to: the stable burning caused by the breakdown of the switch tube will generally burn out the current detection resistor and the power control chip. Negative temperature coefficient thermistor is also very simple and stable to be burned out.
3. There is an output voltage, but the shortcoming of the output voltage is too high generally comes from the voltage stabilization sampling and voltage stabilization control circuit. The DC output, sampling resistor, faulty sampling amplifier such as TL431, optocoupler, power control chip and other circuits together form a closed control loop. Anything wrong will cause the output voltage to rise.
4. The output voltage is too low. In addition to the voltage stabilization control circuit will cause the output voltage to be low, there are also some of the following reasons also cause the output voltage to be low:
- Switching power supply load has short-circuit shortcomings (especially DC/DC converter short-circuit or poor function, etc.). At this time, all loads of the switching power supply circuit should be disconnected to distinguish that the switching power supply circuit is still a load circuit with shortcomings. If the voltage output of the disconnected load circuit is normal, it means that the load is too heavy; or it is still abnormal, indicating that the switching power supply circuit has shortcomings.
- The failure of the rectifier diode and filter capacitor at the output voltage can be judged by substitution method.
- The decrease of the function of the switching tube will inevitably lead to the failure of the switching tube to conduct normally, which will increase the internal resistance of the power supply and decrease the load capacity.