Building a switch mode power supply

Over the last few decades, power supplies have changed quite a bit. In most small inexpensive consumer electronics of the past, you’d find a small transformer, and a linear voltage regulator. This worked quite well to achieve low voltage DC for the product to operate. There are however limitations with this kind of supply for some applications. The first being the size of the transformer. While linear transformers are relatively large in size, switch mode power supplies use much smaller transformers to achieve equal power delivery. The size of a transformer is directly equated with its weight, as well as cost. So the main benefits of reducing transformer size are going to be weight and cost reduction. The efficiency of a switch mode power supply is also going to be significantly better, so reduction of electricity costs are also associated with switch mode power supplies over a linear transformer type. There are of course cons, the main downside being complexity, and emf (noise).


Over time I’ve completed a handful of projects that required power from line voltage. Basically anything that wasn’t powered from USB or battery. Traditionally, I’ve used a switch mode power supply daughter board that I’d solder directly to the main board. This approach worked well, but it required sourcing small switch mode power supply (SMPS) boards, and relying on them not changing in order to maintain compatibility.


I decided that the best avenue would be to design my own switch mode power supply that I could incorporate into my designs. The first and probably most important task is to figure out how you’re going to do the switching. I decided to use a chip that would encompass most of the switching operation and contain the switching transistor within the chip. This would simplify things for my first shot at such a design. After looking around Digikey for awhile, I settled on the FSL206MR. I designed a circuit based around the chip on some perfboard for testing. Found some suitable transformers on ail-express, and had some boards made up.


The first revision had some issues, as I drew the schematic assuming the incorrect transformer winding polarity; an important aspect to pay attention to. The typical application diagram/schematic is an important tool to use when using an integrated circuit, and can usually be found in the datasheet. I made some small changes in regard to the voltage found on the transformer’s auxiliary winding, incorporating a Zener diode to clamp the voltage to the chip if necessary. This particular chip is also able to run without an auxiliary winding to power it, which was another reason I decided to go with it. If you’re interested in learning more about switch mode power supplies, how they work, and looking in depth at some various designs, there are a number of great YouTube videos, one youtuber in particular with such a focus is Diode Gone Wild.