Designing a tiny 240VAC-12VDC Switch Mode Supply

Discussion in 'The Projects Forum' started by brbaker, Apr 28, 2014.

  1. brbaker

    Thread Starter New Member

    Apr 28, 2014
    I need a very tiny low power (1.5W) 12 volt mains driven supply. My design goal is to include a micro controller and the 12 volt supply into something the size of a normal wall charger pack.
    I am an experienced digital circuit designer but have never built a switch mode supply before and do not know were to start. There seem to be many different design approaches (buck, boost, flyback, ...) and I don't know the pros and cons of each approach.
    All designs I have seen are for very high power supplies. I need something low power, with a small form factor, and would survive a proper CE tick electrical certification.
    I have two questions:
    1. Can anyone point me to good resources to walk me through how to design a switch mode supply.
    2. If there are folk who can offer design services (paid or a collaboration) and have experience in this exact type of solution please let me know.
    Thanks and have a great day!
  2. t06afre

    AAC Fanatic!

    May 11, 2009
    Why do want to build it your self? It is very easy to get a premade CE marked power module of the shelf very cheap. Even with a very small foorprint
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    why do people always want to re-invent the wheel?....
  4. SgtWookie


    Jul 17, 2007
    Unless you believe that you can sell hundreds of thousands of your product, you really should use an off-the-shelf wall-wart type supply that already is CE'ed; otherwise your per-unit cost will be many times what it could be.
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
    As with any offline anything, it's all about the iron. Switching power supply transformer is a bear to design. There are online calculators to spit out the basic inductances; building one that works is a whole other thing. And they don't get easier at low power levels.

    Power Integrations has built their company on specialized power supply chips for exactly what you are asking about. Excellent app notes, design kits, etc. For some of their parts you can buy the transformers off the shelf without going through 0.5 years of development and certification.

    brbaker likes this.
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Offline SMPS now are so tiny now!

    I opened a 5v 1A SMPS that was similar size to a matchbox apart from the mains socket prongs. The board used a 8pin chip and didn't even use an optcoupler for voltage regulator feedback, it was all done via clever transformer sensing done by the dedicated 8pin IC.

    Out of curiosity I tried a few different resistive loads and it regulated quite well! No opto needed.

    Like others haver said, just buy a tiny SAFE regulated wallwart, and your project becomes about building the box, not the offline SMPS.

    With some wallwarts they use a larger enclosure and you might be able to fit your project inside the wallwart enclosure anyway. :)
  7. brbaker

    Thread Starter New Member

    Apr 28, 2014
    Thanks for the replies. Reusing a wall charger is not an option because I am making a product for sale (hence the need for a CE tick). I want my product to fit inside something like a wall charger and plug straight into the outlet...
    I have tried finding a tiny SMPS power supply I could OEM and include in the case but I can not find anything tiny enough. about 50mm square seems to be the best I can find. Yet as THE_RB says - you can get some unbelievably tiny SMPSs out there; but I just can't find one to OEM...
    Power Integrations seems to have the best resources so thanks AnalogKid.
  8. wayneh


    Sep 9, 2010
    Maybe you could buy one of these and tear it apart to see who made the guts. Reverse engineer.
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008

    Or just get the chip name from a small simple offline SMPS wallwart, then get the chip datasheet and the chip manufacturer might also have appnotes for PCB design and EMI reduction etc using their chip, and even may have online calculators for SMPS design using their chip.

    I don't want to sound rude but the questions you are asking are simplistic, and making a CE certified offline mains SMPS for consumer certification is NOT a simplistic task. How will you do the EMI testing etc?