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When the power supply is under load and the DC voltage at each output terminal cannot be measured, it is considered that the power supply has no output. At this time, you should first turn on the power and check the fuse, and analyze the scope of the fault through the blown fuse.
1) The fuse is blown and turns black
It indicates that there is a serious short circuit, and the rectifier, filter and power inverter circuit should be inspected.
(1) The AC filter capacitors C3 and C4 are short-circuited due to AC surge voltage breakdown. Some ATX power supply AC filter circuits are more complex, so check whether there are any short-circuited components.
(2) A diode in the AC main circuit bridge rectifier circuit breaks down. Cause of damage: Since the DC filter capacitors C5 and C6 are generally large-capacity electrolytic capacitors of 330μF or 470μF, the instantaneous charging current can reach more than 20A. Therefore, an instantaneous large-capacity surge current can easily cause a rectifier with slightly poor performance in the rectifier bridge to burn out. In addition, the AC surge voltage will also breakdown the rectifier diode and cause a short circuit.
(3) The DC filter capacitors C5 and C6 in the rectifier and filter circuit break down or even burst. Cause of damage: Because the withstand voltage of large-capacity electrolytic capacitors is generally about 200V, the actual working voltage reaches about 150V, which is close to the rated value. Therefore, when the input voltage fluctuates or some electrolytic capacitors are of poor quality, capacitance breakdown may easily occur. In addition, when the electrolytic capacitor leaks electricity, it will become severely heated and burst.
(4) The power switching transistors VT1 and VT2 and the commutation diodes VD1 and VD2 in the DC conversion circuit are broken down and damaged. Cause of damage: Since the output voltage after rectification and filtering is generally as high as about 300V, and the load of the inverter power switch tube is an inductive load, the voltage peak formed by the leakage inductance may be close to 600V, while the withstand voltage Vceo of VT1 and VT2 is only about 450V. . Therefore, when the input voltage is high, some switching tubes with low voltage resistance will be broken down. Therefore, you can choose a power switch tube with higher voltage resistance.
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