Switching power supplies have outstanding advantages such as high efficiency, light weight, small size, and wide voltage regulation range. Since their emergence in the mid-20th century, they have developed extremely rapidly and have been widely used in computers, telecommunications, aerospace, office and home appliances, etc., largely replacing linear regulated power supplies. Improving circuit integration is one of the pursuits of switching power supplies, and for small and medium power switching power supplies, it means achieving monolithic integration. A switching integrated regulator refers to integrating the control circuit, power switch tube, and protection circuit into a single chip, and the switching power supply composed of a switching integrated regulator is called a monolithic switching power supply. The monolithic switching power supply series of Power Integrations (PI) in the United States is a significant representative of this.   
    The 250W switching power supply circuit designed with TOPSwitch—GX is shown in Figure 1. The DC voltage is applied to the drain D of TOPSwitch—GX through the primary winding of the transformer; the frequency selection pin F and the current limit setting pin X are connected to the source S, thus the functions of these two pins are not used, meaning the current limit is not set externally, and the internal self-protection current ILIMIT is automatically set, with the switching frequency being 132KHz; the control pin is connected to the optocoupler LTV817 to receive feedback signals for controlling the duty cycle of the internally integrated high-voltage power MOSFET; the line detection pin L is connected to the positive terminal of the DC high-voltage input through a 2MΩ resistor to achieve line voltage detection for overvoltage, undervoltage, and line voltage feedforward. Capacitor C1 serves as a high-frequency filter capacitor; the transient voltage suppressor P6KE200 and the ultra-fast recovery diode BYV26C form a clamping circuit, with an RC snubber circuit (composed of R2, R3, and C6) inserted in series, which not only absorbs part of the energy in the leakage inductance but also clamps the voltage to 200V, ensuring that the drain voltage of the internally integrated MOSFET in TOPSwitch—GX does not exceed 700V during startup or overload; the secondary output of the high-frequency transformer T1 is rectified by MURl640CT and filtered by C9, C10, and C11, then further filtered by magnetic bead L1 and C12 to remove switching noise, yielding the output voltage; VD4 and C14 form a soft-start circuit.   
    The monolithic switching power supply has significant advantages such as monolithic integration, simplified external circuitry, superior performance indicators, and the ability to achieve complete isolation without a mains-frequency transformer, making it an ideal choice for designing switching power supplies below 290W.