How to design the simplest and cheapest SMPS? - Page 1
Forum section: electronics / projects

author: Michael George
author_rank: Contributor
timestamp: June 08, 2016, 08:07:49 pm
content: I would like to design a switching mode power supply (SMPS) 12 V / 1 A.The efficiency is not important for me. I would like it to contain few components and cheap. I'm making it just for learning. Could you answer the following questions, please?What determine the voltage of the SMPS? Is it the transformer or the IC?How to describe the transformer to the seller? Should I mention the frequency?Some transformers have three coils. One coil on one side and two coils on the other side. What is the purpose of that?What is the cheapest and easy to use IC?Finally, Would you provide me a simple design, Please?Thank you very much,

author: Kleinstein
author_rank: Super Contributor
timestamp: June 08, 2016, 08:24:47 pm
content: The cheapest way is likely to buy one ready made, maybe as a wall-wart. A problem can be the low quality of some (many) cheap Chinese supplies - finding a good one (safe, not too noisy, not overheating and thus reasonable lifetime) can be a challenge.12 V and 1 A and isolated form the mains usually uses a flyback type converter.There are several cheap chips, like UC384x (more made for higher power) and TNY268 (includes power transistor). However there are many other chips, as small supplies is a big market. Very low cost versions might use an winding for feedback. Others use an optocoupler and often an TL431 for more accurate feedback.The third winding is often used to power the controller and reuse some energy.

author: rx8pilot
author_rank: Super Contributor
timestamp: June 08, 2016, 08:58:08 pm
content: What are you hoping to learn?You can get nearly all-in-one solutions but all the learning opportunities are hidden inside. You can go discreet (or nearly so) and expect an amazing SMPS learning experience without spending much at all. What side are you leaning toward.

author: tautech
author_rank: Super Contributor
timestamp: June 08, 2016, 09:42:11 pm
content: Take previous advice but if you must pursue this for interests sake then select a SMPS controller IC, one with a good datasheet and study it.It's not an easy task.....even those that have been professionally designed when broken can be a nightmare to fix.....go down this road and you'll soon learn all this.There's a whole field of magnetics and frequencies to consider not to mention the dangers of dealing with mains supply and the measures that need be added to capture both incoming mains and SMPS created mains disturbance.For good design parameters in any circuit you create must then be observed normally with a scope and in doing so the points that any signal of interest need be referenced against are often elevated from mains Gnd.....do you have an isolated channel scope or a differential probe.....and know how to use them safely?Take previous advice but if you must explore here's a common SMPS datasheet that show a little of what you can expect:http://www.onsemi.com/pub_link/Collateral/UC3842B-D.PDF

author: uncle_bob
author_rank: Unknown
timestamp: June 09, 2016, 12:31:53 am
content: HiThe days of designing a switcher from scratch have been gone for at least two and closer to three decades. All the ones you see today are built around a specific IC. You pick the IC and follow the data sheet recommendations. That hopefully gets you a working supply. The one you "design" looks a lot like the demo board they sell you / give you with the IC already on it.That's not to say it's easy or trivial to do. There is a lot of math, and a lot of analysis involved. The process is a bit different for each IC and often works around the limitations of that specific IC. The limitations are buried in the silicon where you can't get at them. They also may not be very will explained. Nobody ever puts up "our IC isn't quite as good as it might be" in their data sheet.Bob

author: dannyf
author_rank: Super Contributor
timestamp: June 09, 2016, 12:44:21 am
content: QuoteFairly simple:1) get an audio power opamp;2) configure it as an oscillator;3) put a HF transformer on its output;4) put a rectifier on the secondary of that transformer.you got yourself a simple smps.less than a handful of parts.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 09, 2016, 02:56:33 am
content: Simplest, cheapest, efficiency doesn't matter:Zener diode shunt regulator.It switches once, so it can be called a switching power supply with a low operating frequency and very poor switch efficiency.Tim

author: Circlotron
author_rank: Super Contributor
timestamp: June 09, 2016, 05:56:13 am
content: Make yourself one that runs off 12VDC on the input side, not rectified AC mains. Way way safer, and you can also then use low voltage mosfets that you might have laying around instead of having to buy high voltage ones. For the most part all the same principles apply. When you get that mastered then move on to stuff powered straight from the mains. Also, get yourself some goggles. SMPSs can explode in a variety of ways. I worked in a SMPS design lab for 10 years Jan 1996 - Dec 2005. Worked on tiny stuff up to 3kW resonant with planar transformers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 09, 2016, 08:58:56 pm
content: ^ You can save a lot of ills by using commercial power adapters. For example, you don't have to test your project against mains surges -- the adapter should handle that already.Tim

author: uncle_bob
author_rank: Unknown
timestamp: June 09, 2016, 09:04:11 pm
content: HiIf you are talking about a direct line powered device with no isolating AC to DC bulk supply, that's not a good "first project". I would argue that it's probably not a good home project at all. I have seen a number of "professionals" get those designs wrong. The results can be catastrophic. The damage list can include injury and death.Bob

author: chris_leyson
author_rank: Super Contributor
timestamp: June 09, 2016, 09:29:40 pm
content: It depends on what voltage you are stepping down from, look at simple switchers fromNatSemiTexas. You even get a free on line design tool, Webench.

author: rx8pilot
author_rank: Super Contributor
timestamp: June 09, 2016, 09:34:08 pm
content: Still waiting to hear what the OP plans to learn.Learning about SMPS circuits or simply needing one to power other projects.

author: Brutte
author_rank: Frequent Contributor
timestamp: June 10, 2016, 08:02:27 am
content: Quote from: Michael George on June 08, 2016, 08:07:49 pmAny effort made so far? References, application notes, background? Are you expecting us to do more than you have done in the subject?

author: poorchava
author_rank: Super Contributor
timestamp: June 10, 2016, 08:19:51 am
content: Aside from the fact that making a 12V/1A yourself doesn't make economical sense, it's not that hard (this is not to say it's effortless, just not difficult). For those power levels you just need to follow manual for the particular IC. I can recommend Power Integrations products, all they are easy to use and all the documentation is focused on short time to market, which means that most of the stuff is being served to you on a silver platter.The fun starts when you get into higher power levels, where integrated-everything is not an option and there is little room for error (and errors are usually rewarded with an expensive fireworks show).Sent from my HTC One M8s using Tapatalk.

author: uncle_bob
author_rank: Unknown
timestamp: June 10, 2016, 03:57:47 pm
content: HiAbout the only option that has not been mentioned yet is to custom wind a transformer and do a 1950's style "switcher". It's fairly easy to understand. it's not hard to build, as long as you are used to designing and winding transformers from scratch. You also need a source for the transformer internal parts.====It would indeed be nice to get some feedback from the OP.Bob

author: mikerj
author_rank: Super Contributor
timestamp: June 10, 2016, 04:24:26 pm
content: Quote from: dannyf on June 09, 2016, 12:44:21 amThat is simply generating an AC voltage and stepping it up with a transformer, it is not how a switching regulator operates.

author: rob77
author_rank: Super Contributor
timestamp: June 10, 2016, 04:48:28 pm
content: if you you need a SMPS to power your project from mains then buy a ready to use SMPS module. if that's not hard enough then build your own around a single purpose smps IC.. i like the TinySwitch ICs by Power intergrations.. those are very easy to use.if you want to learn, then go for a low voltage design with old classics like UC3842 or TL494 - just google for app notes and examples and start playing around

author: samofab
author_rank: Regular Contributor
timestamp: June 11, 2016, 01:54:43 pm
content: If you really don't want to use off-the-shelf unit, I bookmarked one chip for a very simple simple SMPS with off-the-shelf transformer (Coilcraft/B0570-BL for 12v/1A). The chip series is NCP101x. As a matter of fact, I'm planing to design a max 5x5cm PCB for that chip - mostly to see how small I can make it.

author: Michael George
author_rank: Contributor
timestamp: June 11, 2016, 04:48:02 pm
content: Quote from: rx8pilot on June 09, 2016, 09:34:08 pmI would like to learn about SMPS and how they work.I downloaded the datasheet of UC3842 and TL494 and I feel like they require many components.I found a simple circuit of Tny267 and I'm going to read its datasheet.I downloaded the datasheet of NCP1010, I quickly read it and I found that it contains a simple design and I'm going to read it deeply later.Here is the simple circuit that is about Tny267:http://danyk.cz/impulz4_en.html

author: uncle_bob
author_rank: Unknown
timestamp: June 11, 2016, 04:56:40 pm
content: Quote from: Michael George on June 11, 2016, 04:48:02 pmHiTake careful note that the circuit you are looking at requires you to custom wind a power transformer. There are a number of ways to get that wrong. The results (as noted on the site) can be fatal. They also can be non fatal, but result in a really stinky fire ...Bob