Switching power supplies have outstanding advantages such as high efficiency, light weight, small size, and wide voltage regulation range. Since their advent in the mid-20th century, they have developed extremely rapidly and have been widely used in computers, communications, aerospace, office and household appliances. There is a great potential to replace the linear regulated power supply. Improving the integration of circuits is one of the pursuits of switching power supplies. For small and medium power switching power supplies, it is to achieve monolithic integration. The switching integrated voltage stabilizer refers to the integration of control circuit, power switch tube and protection circuit in one chip, and the switching power supply composed of the switching integrated voltage stabilizer is called a monolithic switching power supply. The monolithic switching power supply series of American PI Company is its notable representative.

    The 250W switching power supply circuit designed by TOPSwitch-GX is shown in Figure 1. The DC voltage is applied to the drain D of TOPSwitch-GX through the primary side of the transformer; the frequency selection terminal F and the limit current setting terminal X are connected to the source S, then the functions of the two ends are useless, that is, the limit is not set from the outside. Current, the self-protection current ILIMIT is automatically set internally, and the switching operating frequency is 132K; the control pole is connected to the optocoupler LTV817, and the feedback signal is accepted to control the duty cycle of the integrated high-voltage power MOS tube; the line detection terminal L passes a 2MΩ The resistor is connected to the positive terminal of the DC high voltage input to realize the line voltage detection of overvoltage and undervoltage line voltage feedforward. Capacitor C1 is a high-frequency filter capacitor; transient voltage suppressor P6KE200 and ultra-fast recovery diode BYV26C form a clamp circuit, and an RC absorption circuit (composed of R2, R3 and C6) is connected in series, so as to absorb part of the leakage inductance In addition to the energy in the power supply, the voltage can also be clamped at 200V, so that the drain voltage of the MOS tube integrated in the TOPSwitch-GX can not exceed 700V when the switching power supply is started or overloaded; the secondary output of the high-frequency transformer T1 passes through the MURl640CT. After rectification and filtering by C9, C10 and C11, and filtering out the switching noise through magnetic beads L1 and C12, the output voltage is obtained; VD4 and C14 constitute a soft-start circuit.

    Monolithic switching power supply has significant advantages such as monolithic integration, simpler peripheral circuits, better performance indicators, and complete isolation without power frequency transformers. It is an ideal choice for us to design switching power supplies below 290W.