The overall voltage conversion process of the switching power supply: AC alternating current → pulsating direct current → high frequency square wave → stable direct current. As shown below.
What is the circuit in front of the primary of the transformer? It is a rectifier and filter circuit. The AC power U1 or U2 input is rectified by the rectifier bridge to turn the AC power into a pulsating DC power; if there is no load, then a relatively stable DC power will be obtained after passing through the filter capacitor, because there is no load, the capacitor has no discharge circuit. The voltage across it will remain unchanged. If there is a load, a DC waveform with ripple will be output.
Then the obtained direct current is chopped at high frequency by the switch tube (PWM control IC), and a high frequency square wave is obtained, but this waveform is still in the form of direct current. How to convert the voltage through the transformer?
Its essence is to use the self-inductance effect of the inductor, and the inductor current cannot change suddenly!
When a 310V DC voltage is input to the primary of the high-frequency transformer of the switching power supply, the terminal voltage of the coil changes abruptly, but the self-inductance effect prevents the sudden change of the current, which will gradually increase the current. At that time, the switch tube is cut off. Then in this process, since the current has been slowly rising and has been changing, the magnetic flux of the transformer has been changing. With the change of the magnetic field, alternating current can naturally be induced in the secondary coil. Then, after the switch tube is turned off, the current of its primary coil is gradually reduced.
Because the switching frequency of the switch tube is very fast, it is 20KHZ~1MHZ. The above process is carried out rapidly, the inductance will never enter a steady state (current value is stable), and the transformer core will not enter a state of magnetic saturation. Therefore, when designing a switching power supply, it is necessary to pay attention to the selection of the switching frequency to avoid the occurrence of magnetic saturation.