Four typical switching power supply circuit designs

　　Selection of the working mode of the switching power supply:  In the switching power supply circuit, there are four basic types: single-ended flyback, single-ended forward, half-bridge and full-bridge. For switching power supplies below 100 W, single-ended flyback converters are mostly used. The transformer in the flyback power conversion circuit not only plays an isolation role, but also has the function of energy storage. The structure of the flyback power conversion circuit is relatively simple, the output voltage is not limited by the input voltage, and it can also provide multiple voltage outputs. The TOPSwitchⅡ series is applied to single-ended flyback converters. Typical usage is shown as follows:

Figure a

Figure b

Figure c

Figure d

　  Circuit analysis: (a) The bias coil is directly used as the input of the TOPSwitchⅡ control electrode through a current limiting resistor; (b) A voltage regulator is added on the basis of (a), which is an enhanced version of (a); In (c), the output voltage acts on the TOPSwitchⅡ control electrode through an optocoupler , which improves the output voltage feedback accuracy; (d) A precision reference TL431 is added on the basis of (c), so that the output voltage regulation accuracy and load regulation rate can obtain higher accuracy. The effects of the four conversion circuits are shown in the table:

　　The single-ended flyback switching power supply is adopted. The biggest feature of this feedback control circuit is: when the input voltage and load current change greatly, it has a faster dynamic response speed, automatically limits the load current, and has a simple compensation circuit. The flyback circuit is suitable for low-power switching power supplies.