Switching power supplies are an indispensable component of various electronic devices, and their performance directly affects the technical specifications of the electronic devices and whether they can operate safely and reliably.
First, switching power supplies are an indispensable component of various electronic devices, and their performance directly affects the technical specifications of the electronic devices and whether they can operate safely and reliably.
Because the key components inside a switching power supply operate in a high-frequency switching state, it has low power consumption, high conversion efficiency, and its size and weight are only 20%-30% of that of a linear power supply. Therefore, it has become the mainstream product of regulated power supplies.
When troubleshooting electrical faults in electronic equipment, the principle of starting with the easy and progressing to the difficult is generally to begin with the power supply. Only after confirming that the power supply is normal can other parts be repaired. Power supply faults account for the majority of electrical faults in electronic equipment.
Therefore, understanding the basic working principle of switching power supplies and being familiar with their repair techniques and common faults can help shorten the repair time of electronic equipment and improve personal equipment maintenance skills.
1. No output, fuse is normal.
This phenomenon indicates that the switching power supply is not working or has entered a protection state. First, measure whether there is a startup voltage at the startup pin of the power control chip. If there is no startup voltage or the startup voltage is too low, check whether the startup resistor and the external components connected to the startup pin are leaking current. If the power control chip is normal, the fault can be quickly found through the above checks.
If there is a startup voltage, measure whether there is a high or low level transition at the output terminal of the control chip at the moment of power-on. If there is no transition, it indicates that the control chip is faulty, or there is a problem with the peripheral oscillation circuit components or protection circuit. You can replace the control chip first and then check the peripheral components. If there is a transition, it is generally due to a faulty or damaged switching transistor.
2. Insulate by burning or frying
The main areas to check include the large filter capacitor on the 300V line, the diodes in the rectifier bridge, and the switching transistor. Problems with the anti-interference circuit can also cause the fuse to blow or turn black. It's important to note that a blown fuse due to a short-circuited switching transistor will usually also damage the current sensing resistor and the power control chip. Negative temperature coefficient thermistors are also easily damaged along with the fuse.
3. There is output voltage, but the output voltage is too high.
This type of fault typically originates from the voltage regulation sampling and control circuits. The DC output, sampling resistor, error sampling amplifier (such as TL431), optocoupler, and power control chip together form a closed control loop; a problem in any part of this loop will cause the output voltage to rise.
4. Output voltage is too low
Besides the voltage regulation control circuit causing low output voltage, the following other reasons can also cause low output voltage:
a. If the switching power supply load has a short circuit fault (especially a short circuit or malfunction of the DC/DC converter), all loads in the switching power supply circuit should be disconnected to distinguish whether the fault lies in the switching power supply circuit or the load circuit. If the voltage output is normal after disconnecting the load circuit, it indicates that the load is too heavy; if it is still abnormal, it indicates that the switching power supply circuit is faulty.
b. Failure of the rectifier diode or filter capacitor at the output voltage terminal can be determined by substitution.
c. A decline in the performance of the switching transistor will inevitably lead to the transistor failing to conduct properly, increasing the internal resistance of the power supply and reducing its load-carrying capacity.
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