Switching power supply has the advantages of high efficiency, light weight, small size, wide range of voltage regulator and so on. Since its appearance in the middle of the 20th century, the development is extremely rapid, and it has been widely used in computer, communication, aerospace, office and household appliances and so on. It has the potential to replace linear voltage regulator power supply. Improving circuit integration is one of the pursuits of switching power supply. For small and medium power switching power supply, it is to realize single chip integration. Switching integrated voltage regulator refers to the control circuit, power switching tube and protection circuit integrated in a chip, and the switching power supply composed of switching integrated voltage regulator is called monolithic switching power supply. American PI company's single chip switching power supply series is its remarkable representative.

The 250W switching power supply circuit designed by TopSwitch-Gx is shown in Figure 1. The DC voltage is added to the drain D of Topswitch-gx through the primary side of the transformer; If the frequency selection terminal F and the limit current setting terminal X are connected to the source pole S, the functions of both ends are useless, that is, the limit current is not set externally, but the self-protection current ILIMIT is set internally, and the switching frequency is 132K. The control pole is connected to the optical coupling LTV817 and receives feedback signals to control the duty ratio of the internally integrated high voltage power MOS tube. The line detection terminal L is connected to the positive end of the DC high voltage input through a 2MΩ resistor to realize the line voltage detection of overvoltage and undervoltage feedforward. Capacitor C1 is a high-frequency filter capacitor. Transient voltage suppressor P6KE200 and ultrafast recovery diode BYV26C constitute a clamp circuit, in which a RC absorption circuit (consisting of R2, R3 and C6) is inserted. In this way, the voltage can be clamped at 200V in addition to absorbing part of the energy leakage. Can make the switching power supply in the case of starting or overload TOPSwitch -- GX internal integrated MOS tube drain voltage does not exceed 700V; The secondary output of high frequency transformer T1 is rectified by MURl640CT and filtered by C9, C10 and C11, and then filtered by magnetic beads L1 and C12 to get the output voltage. VD4 and C14 constitute the soft start circuit.

Monolithic switching power supply has monolithic integration, relatively simple peripheral circuit, better performance index, can achieve complete isolation with no power frequency transformer, etc., it is our ideal choice for designing the switching power supply below 290W.