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; If the frequency selection terminal F and the limit current setting terminal X are connected to the source pole S, then the functions of both ends are useless, that is, the limit current is not set externally, and the self-protection current ILIMIT is automatically set internally. The operating frequency of the switch is 132K; The control pole is connected to the optocoupler LTV817 and receives feedback signals to control the duty cycle of the internally integrated high-voltage power MOSFET; The line detection terminal L is connected to the positive end of the DC high-voltage input through a 2M Ω resistor to achieve line voltage detection of overvoltage and undervoltage line voltage feedforward. Capacitor C1 is a high-frequency filtering capacitor; The transient voltage suppressor P6KE200 and the ultra fast recovery diode BYV26C form a clamp circuit, and an RC absorption circuit (composed of R2, R3, and C6) is connected in series. In addition to absorbing energy from some leakage inductance, the voltage can also be clamped at 200V, which can ensure that the drain voltage of the MOSFET integrated inside TOPSwitch GX does not exceed 700V when the switching power supply is started or overloaded; The secondary output of high-frequency transformer T1 is rectified by MURl640CT and filtered by C9, C10, and C11, and then filtered out by magnetic beads L1 and C12 to obtain the output voltage; VD4 and C14 form a soft start circuit.

Single chip switching power supplies have significant advantages such as single-chip integration, simpler peripheral circuits, better performance indicators, and the ability to achieve complete isolation without power frequency transformers. They are our ideal choice for designing switching power supplies below 290W.