Switching power supply for power supply and development equipment - Power supply circuit - Circuit diagram

Circuit principle: The switching power supply circuit is unilateral flyback type, and its core IC-U1 UC3844AN (8) is used as the pulse width controller (PWM). (Note that the drawing of the circuit diagram U1 uses the power and ground implied paintings in Protel. The method is that the {7} pin of U1 is connected to the Power Supply Vcc, and the {5} pin is grounded. When not oscillating, power is supplied to U1 through R9. When the self-oscillation is formed, the pulse of the {8} pin of T1 is rectified by D3 to supply power to U1. The {6} pin of U1 outputs pulse signals of different widths to control the power FET M1 switch. The sampling signal is taken from the photocoupler U2. Since the reference voltage comparator D5 is connected to the front end of the optocoupler, the output voltage is relatively stable, and the output voltage can be changed by fine-tuning the R14 to change the sampling voltage.

Component and circuit debugging: The component parameters are shown in the circuit diagram. Among them, the switching Transformer T1: uses ER28×28 ferrite core, and its primary winding {6}～{7} is wound with 80∮ of 0.39mm enameled wire, winding {8}-{10} is wound around 8 turns with the same wire, and the secondary winding {1}~{5} is wound with 12 turns of ∮ ∮ 0.65mm enameled wire. Two to three layers of yellow wax cloth should be added between the primary and secondary Power Supplies to ensure good insulation. M1 needs to add heat sink, D1 is packaged in T0-220, and small heat sink should be added. The whole circuit is installed, and the T1's same name is correct. After power-on, the voltage can be output. T2 uses a magnetic ring of about 10mm, and is surrounded by high-strength enameled wire (or plastic AWG24~26 wire) for 4-6 turns. R9 is a PTC self-resetting fuse. It can be used for 5~10A or 250VAC. U2 is an optocoupler, but care should be taken that the pins should not be misconnected.