24V to 12V switching power converter

When installing 12V electrical appliances (such as instruments, radios, fans, etc.) on 24V diesel vehicles, 12V three-terminal voltage regulators are often used. However, since the voltage drop on the voltage regulator reaches 12V, it consumes a lot of power, has a high temperature, and is easily damaged.

We designed a switching power transformer that can reduce 24V to 12V. The circuit is shown in the figure.

This circuit uses 555 as the pulse oscillator. The ⑤ pin of 555 is connected to a voltage regulator tube to obtain the +6V reference voltage, and the ② pin obtains the sampling voltage from the sampling circuit composed of R7 and R8. When the voltage of pin ② is less than +3V, pin ③ outputs a high level, causing BG3, BG1, and BG2 to saturate and conduct, supplying power to the load. At the same time, the power supply charges C2 through R6, when the potential of pin ⑥⑦ reaches +9V. If pin ② also reaches above +3V, pin ③ outputs low level, the capacitor is discharged through pin ⑦, and switch tubes BG3, BG1, and BG2 are all turned off. When the voltage of the capacitor connected to pin ② is lower than +3V, pin ③ outputs high level again. This is repeated repeatedly to stabilize the output voltage value at the rated output voltage of 12V.

The starting circuit is composed of R1, R2, C1, and D1, so that the circuit provides base current to BG3 when it is first turned on, prompting the switching tubes BG1 and BG2 to conduct, and provides working voltage to 555. When the load is short-circuited, 555 will lose Working voltage, thereby cutting off BG3, BG1, and BG2 to protect the circuit and switching tube.

The inductor L and freewheeling diode D2 can reduce the burden on the switching tube. L can be fully wound with E7 core and Φ1.0mm enameled wire. D2 uses a fast recovery diode or ordinary diode greater than 5A. If you need to output a larger current, the parameters of BG1 are Icm>30A, Vceo>100V, Pcm>150W; the parameters of BG2 are Icm>1.5A, Vceo>100V, Pcm>5W, then the output current can reach 10A. BG1 and BG2 can also be replaced by high-power PNP Darlington tubes.