The switching power supply has no DC output. ...

No DC voltage output or unstable voltage output.

If the fuse is intact, there is no DC voltage output at all levels under load. This situation is mainly caused by the following reasons: short circuit of power supply, failure of overvoltage and overcurrent protection circuit, failure of auxiliary power supply, failure of oscillation circuit, overload of power supply, breakdown of rectifier diode in high frequency rectifier filter circuit, leakage of filter capacitor, etc. After measuring the secondary component with multimeter, the breakdown of high frequency rectifier diode and load short circuit are eliminated. If the output is zero at this time, it is certain that the control circuit of the power supply is out of order. If there is voltage output, it means that the front-stage circuit is working normally, and the fault lies in the high-frequency rectifier and filter circuit. The high-frequency filter circuit is mainly composed of rectifier diode and low-voltage filter capacitor, in which the breakdown of rectifier diode will make the circuit have no voltage output, and the leakage of filter capacitor will lead to unstable output voltage. Using multimeter to measure the corresponding parts statically, the damaged parts can be detected.