A simple and easy switching power supply

The schematic diagram of the switching power supply is shown in the figure. Although the voltage stabilization accuracy is not high, it can meet general requirements, and the circuit is simple, using conventional components, and the cost is extremely low. The output allows open circuits and short circuits.

After the mains power is rectified by D1 and filtered by C1, a DC voltage of about 300V is applied to the ① pin of the transformer (the upper end of L1). At the same time, this voltage is biased by R1 to V1, causing it to conduct slightly, and current flows. L1, at the same time, the upper end of the feedback coil L2 (pin ③ of the transformer) forms a positive voltage. This voltage is fed back to V1 through C4 and R3, making it more conductive and even saturated. Finally, as the feedback current decreases, V1 quickly exits saturation and Cut off, this cycle forms an oscillation, and the required output voltage is induced on the secondary coil L3. L2 is the feedback coil and also forms a voltage stabilizing circuit together with D4, D3 and C3. When the voltage on C5 of coil L3 increases after being rectified by D6, it also shows that the voltage on the negative electrode of C3 after L2 is rectified by D4 is lower. When it is as low as about the voltage stabilization value of voltage regulator tube D3 (9V) When D3 is turned on, the base of V1 is short-circuited to ground, turning off V1, and ultimately reducing the output voltage. In the circuit, R4, D5, and V2 form an overcurrent protection circuit. When some reasons cause the working current of V1 to be too large, the voltage transformer generated on R4 is added to the base of V2 through D5, V2 is turned on, and the base voltage of V1 drops, causing the current of V1 to decrease. The theoretical voltage regulation value of D3 is 9V+0.5~0.7V. In actual application, if you want to change the output voltage, you only need to replace D3 with a different voltage regulation value. The smaller the voltage regulation value, the lower the output voltage, and vice versa. high.

When making your own, the high-frequency transformer is the key. You can make it with reference to the following parameters: Use an E-shaped 4X4mm high-frequency magnetic core. L1 is wound with 160 turns of 0.15mm enameled wire; L2 is wound with 10 turns of 0.15mm enameled wire; L3 is wound with 12 turns of 0.39mm enameled wire. . When winding the transformer, film tape must be used for interlayer insulation, and a layer of film tape must be placed between the two E-shaped cores to prevent magnetic saturation. Pay attention to the phase relationship when connecting the circuit, otherwise it will not vibrate (the coil in the picture has a black dot as a mark). When actually winding the transformer, L1 connected to V1 is wound on the innermost layer of the skeleton. Its lower end (pin ② of the transformer) is the starting end, and its upper end (pin ① of the transformer) is the power supply end (300V DC). Then wind the feedback coil L2 and the output coil L3. The advantage of this is that the distributed capacitance between the feedback and output coils and the V1 collector (which has a higher pulse voltage) will be greatly reduced, which helps to improve performance.