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English2. The failure of the filter capacitor or rectifier diode at the output voltage end can be judged by the replacement method.
3. The performance of the switch tube is degraded, which causes the switch tube to fail to conduct normally, which increases the internal resistance of the power supply and reduces the load capacity.
4. The poor switching transformer will not only cause the output voltage to drop, but also cause insufficient excitation of the switch tube, which will damage the switch tube.
5. The 300V filter capacitor is not good, resulting in poor load capacity of the power supply, and the output voltage will drop as soon as the load is connected.
The high output voltage generally comes from the voltage-stabilizing sampling and voltage-stabilizing control circuits. In the closed control loop formed by the DC output, sampling resistor, error sampling amplifier such as TL431, optocoupler, power control chip and other circuits, any problem in any of these parts will cause the output voltage to rise.
The fuse is normal and no output voltage indicates that the switching power supply is not working or has entered a protection state. The first step is to check the value of the start-up voltage of the start-up pin of the power control chip. If there is no start-up voltage or the start-up voltage is too low, check whether the external components of the start-up pin and the start-up resistor are leaking.
If the power control chip is normal, the fault can be quickly found through the above monitoring. If there is a start-up voltage, measure whether there is a jump between high and low levels at the output end of the control chip at the moment of booting. If there is no jump, it means that the control chip is damaged, the peripheral oscillation circuit components are damaged or the protection circuit is faulty. Chip, check peripheral components, check one by one; if it is jumping, in most cases, the switch tube is bad or damaged.
If the fuse is burned out or blown up, check the rectifier bridge, diodes, switch tubes, and large filter capacitors on 300 volts. It may cause the fuse to burn and become black, or it may be caused by a problem with the anti-jamming circuit. It is especially worth noting that the fuse is burned due to the breakdown of the switch tube, which usually burns out the power control chip and the current detection resistor. The thermistor is also easy to burn out along with the fuse.
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