The switching power supply works like this: 220V alternating current, after rectification and filtering, becomes 3 10V direct current. This DC voltage is applied to the collector of the switching tube through the primary of the switching transformer, and at the same time, the base of the switching tube is biased, which will make the switching tube conductive. Because the switching tube is turned on at the moment of power-on, a current is generated at the primary of the switching transformer connected to the collector of the switching tube, which generates a magnetic field on the magnetic core of the switching transformer, and the magnetic field generates a voltage on the feedback coil, which is applied to the base of the switching tube. This positive feedback voltage makes the switch tube more conductive, the magnetic field increases and the feedback voltage increases. The switch tube is further conducted until it is saturated. After saturation, the current flowing through the collector of the switch tube is the largest. But the change amplitude of current decreases. Until it does not change. The primary current of the switching transformer remains unchanged, the induced magnetic field disappears and the feedback voltage disappears. Due to the disappearance of the feedback voltage, the base of the switching tube quickly returns to the conduction state from the deep saturation state, and the collector current decreases rapidly. The reduced current produces a magnetic field opposite to the above on the magnetic core, and the opposite magnetic field also produces a feedback voltage opposite to the above on the feedback coil. This voltage is applied to the base of the switch tube to accelerate the switch tube from on to off. After the switch tube is turned off, the current disappears, the feedback voltage disappears, and the switch tube is biased by DC. This process is constantly circulating, which is the working principle of switching power supply. Of course, there is also a process of stabilizing the output voltage, which is omitted here for simplicity.