Abstract : This paper introduces a design method for a single-channel 200W output switching power supply. It briefly describes the basic principle of pulse-width modulation (PWM) switching power supplies, elucidates the performance characteristics and pin functions of the TOPSwitch-GX series monolithic switching power supplies, presents a switching power supply design circuit based on the TOP249Y chip, and analyzes and explains the five external components—input rectifier and filter circuit, high-frequency transformer, output rectifier and filter circuit, feedback circuit, and protection circuit—using PI Expert software. Finally, it presents the measured output waveform. Keywords : Switching power supply; TOPSwitch-GX; TOP249Y; PI Expert 1. Introduction Power supplies are an essential component of various electronic devices, and their performance directly affects the safety and reliability of the entire system. Switching power supplies are favored for their low loss and high efficiency, representing the development direction of regulated power supplies. They have become the mainstream product in regulated power supplies and are widely used in computer equipment, electronic instruments, communication equipment, and home appliances. Monolithic switching power supplies (MSPS) boast advantages such as high integration, cost-effectiveness, minimal peripheral circuitry, and optimal performance indicators. They can be used to construct high-efficiency transformerless isolated switching power supplies and various special-purpose switching power supplies. Since their introduction in the mid-1990s, MSPS have demonstrated strong vitality and are now the preferred integrated circuit for developing medium- and low-power switching power supplies, precision switching power supplies, and switching power supply modules internationally. The switching power supplies constructed using MSPS are comparable in cost to linear regulated power supplies of the same power, while significantly improving power efficiency and greatly reducing size and weight, creating favorable conditions for the promotion and popularization of new switching power supplies. In the 1990s, Pixnet (PI) Corporation of the United States launched the world's first three-terminal isolated pulse-width modulation (PWM) MSPS, its first-generation product being the TOPSwitch-I series. The TOPSwitch-GX series, launched by PI in 2000, is the fourth generation of MSPS, with improved performance and efficiency, and a maximum output power of 290W. PI Expert is a computer-aided design software developed by PI for the TOPSwitch series chips. It is easy to use and can greatly reduce the workload of designers. This article describes the design of a single-channel 200W pulse-width modulation (PWM) switching power supply based on the TOP249Y chip, using PI Expert 6.0 software. Full text of the switching power supply design based on TOPSwitch-GX and PI Expert can be downloaded here.