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    Switching power supply has the advantages of high efficiency, light weight, small size, wide voltage regulation range, etc. Since the advent of the mid-20th century, it has developed extremely rapidly, and has been widely used in computer, communication, 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, monolithic integration is achieved. The switch integrated voltage regulator refers to the integration of the control circuit, the power switch tube and the protection circuit in one chip, and the switching power supply composed of the switch integrated voltage regulator is called a single-chip switching power supply. The US single-chip switching power supply series of PI Company is a remarkable representative.

    The TOPSwitch-GX designed 250W switching power supply circuit is shown in Figure 1. The DC voltage is applied to the drain D of the TOPSwitch-GX via the primary side of the transformer; the frequency selection terminal F and the limit current setting terminal X are connected to the source S, and the functions of the two ends are useless, that is, the limit is not set from the outside. Current, the internal self-protection current ILIMIT is automatically set, the switching operating frequency is 132K; the control pole is connected to the optocoupler LTV817, and the feedback signal is received to realize the control of the internal integrated high-voltage power MOS tube duty cycle; the line detection terminal L passes a 2M The resistor is connected to the positive terminal of the DC high voltage input to achieve 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 a RC snubber circuit (composed of R2, R3 and C6) is connected in series, so that part of the leakage inductance can be absorbed. In addition to the energy in the middle, the voltage can be clamped at 200V, so that the drain voltage of the TOPSwitch-GX internal integrated MOS transistor does not exceed 700V when the switching power supply is started or overloaded; the secondary side output of the high-frequency transformer T1 passes the MURl640CT The rectification and C9, C10 and C11 filtering, and then filtering the switching noise through the magnetic beads L1 and C12, the output voltage is obtained; VD4 and C14 constitute a soft start circuit.

    The single-chip switching power supply has the advantages of monolithic integration, simpler peripheral circuits, better performance specifications, and complete isolation with no power frequency transformers. It is an ideal choice for designing switching power supplies below 290W.