Why should the switching power supply feedback the winding power supply?
Of course, you can use an external power supply to separate the power supply to the switch chip. But that way, the cost is high.
The chip needs a DC power supply to work. At the beginning of the boot, most of the power supply through a power resistor will be 300V DC high voltage step-down to provide to the chip. When the chip outputs multiple pulses, the feedback winding can provide normal power. Because in the normal work, the chip power consumption is also taught. Direct use of resistance buck, it is easy to lead to work instability. Therefore, it is the type of power supply feedback circuit to send the way to work.
The switching power supply is divided into self-excited and it is excited. The self-excited need is only the feedback winding to provide an excitation pulse signal. And it is excited because it requires independent control chip to work.
