Switching power supply with high efficiency, light weight, small size, wide voltage range and other outstanding advantages, since the mid-20th century came out, the development is extremely rapid, in computers, communications, aerospace, office and household appliances and other aspects have been widely used, a potential to replace linear voltage power supply. Improving circuit integration is one of the pursuit of switching power supply, for small and medium power switching power supply is to achieve 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 a single switching power supply. The American PI company's single-chip switching power supply series is a notable 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 end F and the limit current setting end X are connected with the source pole S, the functions of the two ends are useless, that is, the limit current is not set externally, and the self-protection current ILIMIT is set internally automatically, and the switching frequency is 132K; The control pole is connected to the optocoupler LTV817 and receives feedback signals to control the duty cycle of the integrated high-voltage power MOS tube. The line detection end 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. The transient voltage suppressor P6KE200 and the ultrafast recovery diode BYV26C constitute a clamping circuit, in which a RC absorption circuit (consisting of R2, R3 and C6) is inserted, so that in addition to absorbing part of the energy in the leakage induction, the voltage can also be clamped at 200V. Can make the switching power supply in the case of starting or overload TOPSwitch - GX 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.
Single-chip switching power supply has the advantages of single-chip integration, simple peripheral circuit, better performance index, can achieve complete isolation with no power frequency transformer, etc., which is the ideal choice for our design of switching power supply below 290W.