Switching power supplies for microcontrollers and electronic devices

　　The power supply has the characteristics of stable output, adjustable voltage, small size, stable performance circuit, etc. The output current of the switching power supply is 1.8A; the output voltage can be set within the range of +15V ~ +5V; the input voltage is suitable for AC90V-240V50/60Hz power supply, so it can power electronic experimental measurement devices and can also be used to charge 6V/12V batteries.

　　Circuit Principle

　　This switching power supply circuit is a single-sided flyback type, and its core IC-U1 UC3844AN (8) is used as a pulse width controller. (PWM) (Note that the drawing method of the circuit diagram U1 uses the implicit method of power supply and ground in Protel, that is, the ⑦th pin of U1 is connected to the power supply VCC, and the ⑤th pin is connected to the ground). When the alarm is not triggered, power is provided to UI through R9. When the self-excited oscillation is formed, the pulse of the ⑧th pin of T1 is rectified by D3 and supplies power to U1. The ⑥th pin of U1 outputs pulse signals with different widths to control the switch of the power field effect tube M1. The sampling signal is taken from the photocoupler U2. Since the reference voltage comparator D5 is connected to the front end of the photocoupler , the output voltage is relatively stable. By fine-tuning R14 to change the sampling voltage, the output voltage can be changed.

　　Component and circuit debugging

　　The component parameters are shown in the circuit diagram, where the switch transformer T1: uses ER28x28 ferrite core, its primary winding ⑥-⑦ is wound with φ0,39mm enameled wire for 80 turns, winding ⑧-⑩ is wound with the same number wire for 8 turns, and secondary winding ①-⑤ is wound with +0.65mm enameled wire 2 strands for 12 turns. 2-3 layers of yellow wax cloth should be added between the primary and secondary to ensure good insulation. M1 needs to add a heat sink, D1 uses T0-220 package, and a small heat sink should also be installed. The entire circuit is installed, and the same-name end of T1 is correct, and the voltage can be output after power is turned on. T2 uses a magnetic ring of about φ10mm, and is wound with high-strength enameled wire (or plastic AWG24~26 wire) for 4-6 turns. R9 is a PTC self-resetting fuse, which can be used with 5-10A, 250VAC.

　　U2 is an optocoupler , but care should be taken that the pins are not connected incorrectly.