# SMPS (LM2678) what transformer to use and why?

I have made a few power supplies in the last year (only linear ones using voltage regulators). I have always used a big simple step down transformer, from 230v down to 12/24v (depends on the supply i need).

There are a lot of circuits out there on how to make the power supply for the LM2678 but from what I've seen no one writes about the transformer. From what i know, since SMPS operates at a high frequency i can use a smaller transformer?

I have a lot of small yellow ones that looks like this:

But since i don't know much about smps transformer, and i really don't want to play around with the 230v ac to find out where the primary/secondary of the transformer is. I wanted to ask you guys if you could tell me a little more about what transformer i should use and why.

I first thought of using a step down transformer from 230v down to 6v ac, and then use a resistor to limit the current to like 10mA and use that to find the primary of the smps transformer because that would be safer right?

Any tips are appricated! Thank you.

PDF to the LM2678:

http://www.ti.com/lit/ds/symlink/lm2678.pdf

• Designing a mains powered SMPS is a tall order and "don't know much about smps transformer" puts you in a very bad position since you can easily kill someone with 230 Vac. Perhaps start with understanding the buck converter first and design your own inductor as an exercise? Oct 17, 2017 at 11:11

## 2 Answers

You are confusing a mains powered SMPS with a Buck converter.

The LM2678 is a buck converter, it uses an inductor but no transformer.

This is a typical implementation of a DCDC converter using the LM2678:

Note that there is no transformer. The inductor is the component marked "220" (for 22 uH).

For the LM2678 to operate you need a DC voltage up to 40 V. You can make that 40 V (or lower) form 240 V AC Mains voltage using a bulky 50 Hz transformer like you have done sofar.

Or you can use a Switched Mode Power Supply (SMPS) which could look like this:

Note that this has a (small) transformer like the one from your question.

• Does the first one come with a fortune cookie ? Oct 17, 2017 at 10:18
• @Trevor Probably it does ;-) Maybe it says so in writing but I cannot read Chinese. Oct 17, 2017 at 10:19
• "You are confusing a mains powered SMPS with a Buck converter." That explained it for me haha, thank you for the detailed answer + pics!!
– Xane
Oct 17, 2017 at 13:02
• Sorry for double comment. But Do you happen to know the name of such a SMPS in the second picture? Or maybe even a schematic for it? Thanks.
– Xane
Oct 17, 2017 at 13:09
• Yep, it's a flyback converter, if you Google "flyback converter circuit" and then search for images (instead of websites) you will see plenty of circuits. They're basically all the same: High voltage AC in => Bridge rectifier + smoothing capacitor => 200 - 400 V DC => Transformer + switching transistor => output uf transformer => rectifier + smoothing capacitor. Some also have an optocoupler for voltage feedback. Oct 17, 2017 at 13:13

The LM2678 is a DC-DC convertor with a max input of 40VDC. As such you still need a regular power transformer and rectifier, or other AC-DC convertor, before it.

The device does use high frequency to drop the output voltage from your transformer and rectifier down to your design level by regulating the current in an inductor on it's output. However, that does not affect the size of AC/DC converter you need before it.