DC-DC Converter with 12V Regulated Automotive Switching

Figure 1 

The project produces a DC-DC converter that is designed to accept a range of input voltages that are present in the electrical system of a car while providing a regulated 12V output.

Batteries have a nominal voltage around 13.8V which can often drop down to 6 or 7V for a short time when the engine is being started and it could reach peaks somewhat above nominal voltage as well. The converter is needed to be a buck-boost configuration because the input voltage can be either above or below the desired output of 12V. This particular design is a SEPIC topology that uses a UC3843 controller chip.

There are two main conductors normally seen in a SEPIC design. Two 12uH inductors are present, one each on the input and output. In this case, they are hand-wound on a common core with coupling to improve efficiency. It provided 75% efficiency after prototyping in a perfboard with both the transistor and diode mounted to a heatsink.

To turn the DC-DC ON and OFF, the power was connected to the chip to a switched 12V line from the shutdown controller with the jumper being removed.