The Switching power supply 30V manufacturer tells you that the 220V mains input is converted into 300V DC voltage after EMI and rectification filtering, and then added to the C pole (collector) of the switching tube after passing through the primary of the switching transformer.

At the same time, the 300V after rectification and filtering passes through a starting resistor (starting circuit). The Switching power supply 30V manufacturer tells you that when the oscillating circuit is reached, the oscillating circuit will start to work, and the switching tube will be switched on and off, repeating continuously. The induced electromotive force is generated, and the current generated by the induced electromotive force is rectified and filtered to become the voltage I need!
At the same time, a part of the current after the transformer secondary rectification and filtering is sampled, compared and amplified, and fed back to the oscillation circuit through the optocoupler for adjusting the oscillation frequency. The Switching power supply 30V manufacturer tells you that the protection circuit is associated with the oscillating circuit. After the protection circuit finds an abnormality, it will also transmit information to the oscillating circuit to stop the oscillating circuit and stop working to prevent the expansion of the fault. The single-tube self-excited switching power supply circuit and principle are relatively simple.
It is recommended that you take a moment to look at the actual product. Only when you understand the principle and its components, the subsequent troubleshooting analysis will be solved.