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AC->DC Flyback

Discussion in 'Electronic Design' started by Sylvain Munaut, Sep 20, 2004.

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  1. Hello,

    I've searched the web, but haven't exactly found yet what I'm looking for so maybe someone here has a direct link.

    I'm searching on how to design a "simple" flyback smps that would work on 110AC and 220AC (preferably without change), and has
    12V, 5V and 3.3V output. The total power would be < 100W, something like
    12V - 4A
    5V - 4A
    3.3V - 5A

    Also, I'm probably gonna need a "custom" transformer, any pointer on that ?


  2. Pooh Bear

    Pooh Bear Guest

    I *strongly* recommend that you visit Power Integrations website and check out their very informative application

    They include several fully engineered examples including a multi-rail supply not dissimilar to what you require.

    There is ( unusually for a semi maker ) also a great deal of info on designing the magnetics too, including a simple to use
    application that'll do most of the calculating for you and an app note discussion of safety related construction issues.

  3. Tam/WB2TT

    Tam/WB2TT Guest

    A small PC power supply has got to be cheaper than what you could build one

  4. Pooh Bear

    Pooh Bear Guest

    I see what you're saying - but not all PC supplies are 'universal voltage' for
    starters. I'm unsure whether the individual rail current demands would be
    compatible either.

    In fact most PC supplies use forward converter methodology rather than flyback

    I reckon that the OP wants to learn about flyback. It's interesting - I learnt
    a lot about SMPS design myself this way.


    p.s. 'top-posting' is somewhat frowned upon in Usenet - you're meant to add
    your comments *after* the original post, so as to preserve the 'timeline of
    thought' as opposed to *hey I know this and it's best imho and I'm gonna tell
    you first*
  5. Rich Grise

    Rich Grise Guest

    Whenever someone recommend bottom-posting, I wish they'd not neglect
    "Snip Mercilessly."

    Only quote the relevant parts of the post, that you're responding to.
    The rest is clutter, and very off-putting. I've been known to skip
    over posts where all I see is a full page of quoted crap that I've
    seen already, that's irrelevant to the particular answer.

  6. 110AC and 220AC

    There is really nothing simple about that - what do you need it for (If it
    is for a product, in almost any case it will be cheaper to buy the thing as
    a unit ready-made & tested from f.ex. Toko, If it is for learning I would
    choose something a little less deadly to start out with ;-)?

    Philips used to have some good design notes.

    The voltages you quote are common - you job will be *a lot* easier - and
    safer - if you at least choose a commodity transformer and design around
  7. Hi,
    PC supply has some drawbacks :
    - More power than needed ( often >= 200W )
    - More voltages so would lead to more complexity that needed
    - Probably need fans
    - I would need to remove it from it's casing to include it ...

  8. Thanks for theses pointers, I'll check them out in details.

  9. My purpose is dual ;)

    First, learn about design of <100W SMPS design. And from what I've read in many place, the
    flyback topology would be the more suited for that.

    Second, obtain a design that would be manufacturable at low-cost, to integrate in a project.
    So It also need to pass safety and emi tests.
    I can't hire someone to do the job since I must first present a working/manufacturable prototype
    and it's cost to the investors ...

  10. It didn't sound that complicated ;) I just looked at the Viper100 from ST
    and their design application and that looks like it could fit my needs with
    little external components.

    But at that level of power, it's working in continuous mode, I'm not sure to
    know all the implications of that.

    Prebuilt modules could do it for the final product (even if building one myself
    still interests me for learning). I've looked at TOKO and didn't find modules ...

    Yes certainly. But often in the app notes / design software given by pwm
    controller manufacturer, they don't give reference for a part but just
    number of turn / core / ...

    Where could I get standard transformers ? I may not be searching for the good
    word ...

  11. It *gets* complicated with alle the UL ... safety issues and ensuring that
    the design *really* works reliably under real line conditions - seem to
    remember from about 90 V to 320 ;-)
    A bit higher losses in the inductor and switch; regulation will be slower
    than discontionous *and* there may be instability if the operation causes
    mode switching between contionuous & dicontinuous operation.

    Normally one fixes the design to operate in one mode only.
    modules ...

    Well they did little half-open switchers a while back - but almost all
    products tend to get subsumed into other businesses at random. The best
    person to ask is a large supplier - usually, they keep track of "what is
    Look under "Magnetic Components"

    Philips Components used to - maybe that went to FerroxCube?
    Siemens - may be Matsushito(?) now.
    Toko, Panasonic, Vishay maybe.
  12. Joerg

    Joerg Guest

    Hi Sylvain,
    You could try companies such as Pulse Engineering, or scope out Digikey
    and others to see if they offer a suitable transformer from one of the
    Asian manufacturers. But when you have data about which core to use and
    the number of windings you might as well make your own for a lot less
    money. Usually for less than the cost of a glass of Jupiler beer in the pub.

    However, you need to understand isolation requirements for the wires,
    protection of the core from dielectric breakdown etc. Cores can be
    purchased for little money from ham radio shops. Often they will carry
    the Amidon brand.

    Regards, Joerg
  13. Pooh Bear

    Pooh Bear Guest

    Typically - yes.

    You'll find that Power Integrations offer good advice on safety and EMI issues too !

    You might want to purchase one of their eval boards to do that quickly.

  14. Pooh Bear

    Pooh Bear Guest

    Higher working flux in the core > larger losses.

    OEM modules are great for small quantity manufacture but about 3 times the cost of
    the parts if you're building it yourself.

    I'm not aware of any 'commodity transformer'. You have to design to suit.

    Most semiconductor manufacturers don't give much info about the magnetics. I had to
    work it out for myself first time I did a flyback design and it would have been much
    easier with the CAD tools that exist now.

    Are you looking for 'preferred cores' ? I suggest that you consider the ETD and EFD
    cores for a European design for starters.

    I've also found E cores quite popular too. E.g. used E30/15/7 in my latest design (
    35W ). Multi-output supplies need more pins on the coil former - this may influence
    your choice of core ( it did for me ).

    The magnetic design application I recommended from power Integrations includes a
    'stock list' of popular cores listed by region ( USA, Europe, Asia ).

    Siemens also do a useful magnetic design tool too. Errrr. that's EPCOS now.

  15. Genome

    Genome Guest

    Higher working flux in the core does not strictly translate to greater
    losses. It's the peak to peak flux excursion and the area enclosed in that
    loop that you are interested in.

    Operating discontinuous with Bpk of 200mT gives you about the same losses as
    operating continuous with Bmin 100mT and Bpk 300mT, the flux excursion is
    the same.

    Continuous operation introduces the right half plane zero into the loop gain

  16. Joerg

    Joerg Guest

    Hi Genome, Hi Sylvain,
    And that RHP issue is nothing to be sneezed at, especially if you need a
    really quiet output voltage. Discontinuous mode is generally considered
    to be easier if this is one of the first switchers you are designing.
    The price to be paid for a larger core and switch isn't so steep. Also,
    when you need a very wide load range it will go into discontinuous mode
    below a certain load anyways.

    Regards, Joerg
  17. Ken Smith

    Ken Smith Guest

    Often the price for the "extra" switch for discontinuous mode is zero. In
    continuous mode the switch has to turn on with a current flowing in the
    inductor. This adds to the switching losses.
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