What is the reason why switching power supply has no voltage output?

May 16, 2023

The importance of switching power supply is self-evident, but once there is a problem, it is difficult to solve. What is the reason why there is no voltage output in the switching power supply?

1. The reason why the switching power supply always has no voltage output

(1) The collector of the switch tube does not get enough working voltage

(2) The switching tube base does not get the starting voltage and the related circuit leakage

(3) The positive feedback element of the switching tube fails

2. Methods and steps to determine the fault

The primary task of maintenance of such faults is to determine which part of the drum barrier is in the above three parts. The specific method is to measure the base voltage of the switch tube collector. There may be the following situations:

(1) The switching tube collector voltage is 0V and 1.4 times lower than the mains. The switching tube has no normal working voltage. If there is 1.4 times the voltage, the switching tube collector has a normal working voltage, indicating that the circuit works normally.

(2) the base voltage of the switching tube is 0V(including the boot moment). This situation indicates that the starting circuit does not provide the starting (on-off) voltage to the switching tube base, or the breakdown of the relevant components between the base and the emitter should be checked the starting circuit and the switching tube emitter and related components. If the voltage is 0.6~0.7(including a few moments of opening), it indicates that the starting circuit and the emitter element of the switch tube are normal. If the voltage is above 0.7V, it indicates that the starting circuit is normal but the emitter junction or element of the switch tube is disconnected or the resistance value becomes large.

(3) The switching tube has the on-off condition: the base voltage of the switching tube is 0.6~0.7V, the collector voltage is greater than 250V, indicating that the switching tube has the working condition. The fault is in the positive feedback circuit, including the positive feedback resistance, capacitor, continuous current diode and the positive feedback winding of the switching transformer and the connecting plate between them.

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