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Basic transformer question - number of turns depends on frequency?(Not turns ration, number of turns

Discussion in 'Electronic Design' started by Al Borowski, Apr 9, 2004.

  1. Al Borowski

    Al Borowski Guest

    Hi,

    For a university assignment, I have to design and construct a switchmode
    regulator (flyback type). It has to drop 10V to 5V, using a certian
    chip. I can choose the frequency to be between 20-100kHz.

    Before I dive into the equations, could someone please tell me if the
    number of turns required on the transformer depends on the switching
    frequency? For instance, if a higher frequency means less turns, then
    I'll go for it. If the number of turns is independant of frequency then
    I'll use 100kHz for minimum ripple.

    thanks alot,

    Al
     
  2. Ian Stirling

    Ian Stirling Guest

    Oh dear.
    You really need to read your books/notes a bit more.

    Saturation in transformers is a bad thing.
    If the magnetic flux in the transformer goes over a limit, it
    stops behaving as a transformer.
    Look up saturation, and "leakage inductance"

    All else being equal, higher frequencies mean lower peak fluxes, as
    the energy transfer happens more often, so there is lowere energy to
    be stored in each transfer.

    A lower inductance is usually used at higher frequencies, so there are
    fewer turns.

    Look up how inductance varies as number of turns.

    If all else fails, read the datasheet.
     
  3. Al Borowski

    Al Borowski Guest

    Thanks for the reply.

    As I said, I haven't done *any* reading on this yet :) I just wanted a
    quick answer before I 'hit the books'. Luckily I have plenty of time
    before this is due.
    I'm OK with these - I think I understand the theory. Starting tomorrow
    I'll get stuck into the assignment, but I first thought I'd ask here -
    because I knew someone could tell me the answer right away :)
    I thought so, which explains why the core size can be smaller, right?
    Thanks! That makes sense.

    No worries, I plan to do so.

    cheers,

    Al
     
  4. Ian Stirling

    Ian Stirling Guest

    Post in case of any problems...

    There seem to be a lot of people who post saying "help me, I've
    got 12 hours to complete the assignment and haven't read any books,
    or done any work, can you please do all of the work for me".

    Good luck.
     
  5. I read in sci.electronics.design that Ian Stirling
    ..net>) about 'Basic transformer question - number of turns depends on
    frequency? (Not turns ration, number of turns)', on Fri, 9 Apr 2004:
    Indeed. Al is a student, and an Australian student to boot. Yet he gave
    a polite reply to a rather stern post, and he's got plenty of time for
    his project. This man will go far!
     
  6. Genome

    Genome Guest

    message | Hi,
    |
    | For a university assignment, I have to design and construct a
    switchmode
    | regulator (flyback type). It has to drop 10V to 5V, using a certian
    | chip. I can choose the frequency to be between 20-100kHz.
    |
    | Before I dive into the equations, could someone please tell me if the
    | number of turns required on the transformer depends on the switching
    | frequency? For instance, if a higher frequency means less turns, then
    | I'll go for it. If the number of turns is independant of frequency
    then
    | I'll use 100kHz for minimum ripple.
    |
    | thanks alot,
    |
    | Al
    |

    There is no 'right' answer to your question.

    In an 'ideal' world higher frequency would equate to an overall smaller
    inductor with, perhaps, fewer turns.

    Unfortunately

    At low frequencies.Core losses are less so you are saturation limited,
    0.3T. Skin and proximity effects are less so you can use larger wire
    sizes. At high frequencies. Core losses are greater so you are loss
    limited, <0.2T Skin and proximity effects are greater so you have to use
    wire ropes.

    Discontinuous operation gives you high peak and RMS currents with higher
    copper losses implying thicker wire and more winding area. Continuous
    operation has lower RMS currents and losses but needs more inductance,
    more turns, and suffers from the right half plane zero.

    These are just a few of the design decisions you will have to make and
    justify. There are probably more and at the end of the day you have to
    get out your pen and paper and run through some iterations.

    I hope you have some good sums.

    DNA
     
  7. Yes, the higher the freq, the less turns you need.
    This because there is less inductance needed for a low (zero load)
    current in the transformer.
    You can calculate that from the switch time and L with i = t / L
    Beware of saturation.
     
  8. Robert Baer

    Robert Baer Guest

    Turns per volt depends on the frequency, the core material, and the
    power level (ie: the size of the core).
     
  9. Al Borowski

    Al Borowski Guest

    Thanks to everyone who replied... clearly I need to bone up on
    electromag theory :)

    cheers,

    Al
     
  10. I read in sci.electronics.design that Al Borowski <[email protected]
    ..student.qut.edu.au> wrote (in <4077cc91$0$16606$
    ..net.au>) about 'Basic transformer question - number of turns depends on
    frequency? (Not turns ration, number of turns)', on Sat, 10 Apr 2004:
    You may find this easily-memorable equation useful:

    E = BAWN

    E = voltage (volt)
    B = induction ('flux density') (tesla)
    A = area of cross-section of core (m^2)
    W [omega] = angular frequency (rad) = 2 [pi]f
    f = frequency (hertz)
    N = number of turns.
     
  11. R.Legg

    R.Legg Guest

    You may find this easily-memorable equation useful:
    ......if you can figure out whether it's peak to peak, peak, average,
    or RMS for the flux quantity.

    For voltage, rms or instantaneous values are assumed, unless stated
    otherwise. This is not so for flux density.

    RL
     
  12. <>) about 'Basic
    transformer question - number of turns depends on frequency? (Not turns
    ration, number of turns)', on Sat, 10 Apr 2004:
    It is so for ALL quantities (that can have p-p, peak, average and r.m.s
    values - you don't normally get peak-to-peak d.c. resistance!). Why
    should there be an exception for B?If you understand SI units, it is obvious that E in that equation is
    r.m.s. if B is r.m.s.
     
  13. R.Legg

    R.Legg Guest

    It is B that is not obvious.

    Of actual interest is Bpk, as this will tell you if the circuit is
    linear (nonsaturating), and will also allow determination of core
    loss, per typical published mfr data that refers to the peak value.

    Bpk is also the product of V.t - the result of an average voltage, not
    an rms one. At least, that is my understanding.

    RL
     
  14. <>) about 'Basic
    transformer question - number of turns depends on frequency? (Not turns
    ration, number of turns)', on Sun, 11 Apr 2004:
    But if you put Bpeak in the equation, you lose the easily remembered
    form and you have to explain the 1/sqrt(2) factor. It's even worse of
    you 'simplify' it by also including the numerical value of 2[pi]. You
    get a factor of 4.44 to explain away.

    Anyone who doesn't understand peak and r.m.s. values has no business to
    be designing anything, let alone wound components.
     
  15. Genome

    Genome Guest

    | <>) about 'Basic
    | transformer question - number of turns depends on frequency? (Not
    turns
    | ration, number of turns)', on Sun, 11 Apr 2004:
    | >It is B that is not obvious.
    | >
    | >Of actual interest is Bpk, as this will tell you if the circuit is
    | >linear (nonsaturating), and will also allow determination of core
    | >loss, per typical published mfr data that refers to the peak value.
    |
    | But if you put Bpeak in the equation, you lose the easily remembered
    | form and you have to explain the 1/sqrt(2) factor. It's even worse of
    | you 'simplify' it by also including the numerical value of 2[pi]. You
    | get a factor of 4.44 to explain away.
    |
    | Anyone who doesn't understand peak and r.m.s. values has no business
    to
    | be designing anything, let alone wound components.
    | --
    | Regards, John Woodgate, OOO - Own Opinions Only.
    | The good news is that nothing is compulsory.
    | The bad news is that everything is prohibited.
    | http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

    The 'easily' remembered form for those designing switch mode power
    supplies, or otherwise, is;

    N = VinTon/B.Ae

    It's just a statement of Faraday's (apostrophe included) law in other
    terms that B is proportional to the integral of the applied volt
    seconds.

    Anything else is just pants

    DNA
     
  16. Genome

    Genome Guest

    |
    | (in
    | | <>) about 'Basic
    | | transformer question - number of turns depends on frequency? (Not
    | turns
    | | ration, number of turns)', on Sun, 11 Apr 2004:
    | | >It is B that is not obvious.
    | | >
    | | >Of actual interest is Bpk, as this will tell you if the circuit is
    | | >linear (nonsaturating), and will also allow determination of core
    | | >loss, per typical published mfr data that refers to the peak value.
    | |
    | | But if you put Bpeak in the equation, you lose the easily remembered
    | | form and you have to explain the 1/sqrt(2) factor. It's even worse
    of
    | | you 'simplify' it by also including the numerical value of 2[pi].
    You
    | | get a factor of 4.44 to explain away.
    | |
    | | Anyone who doesn't understand peak and r.m.s. values has no business
    | to
    | | be designing anything, let alone wound components.
    | | --
    | | Regards, John Woodgate, OOO - Own Opinions Only.
    | | The good news is that nothing is compulsory.
    | | The bad news is that everything is prohibited.
    | | http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
    |
    | The 'easily' remembered form for those designing switch mode power
    | supplies, or otherwise, is;
    |
    | N = VinTon/B.Ae
    |
    | It's just a statement of Faraday's (apostrophe included) law in other
    | terms that B is proportional to the integral of the applied volt
    | seconds.
    |
    | Anything else is just pants
    |
    | DNA
    |
    |

    And a better one for those designing flyback converters is.....

    N = L.Ipk/Bpk.Ae

    These sums are incestuous.

    DNA
     
  17. legg

    legg Guest

    Which isn't made any easier if the limitations to the formula aren't
    pointed out. If you remove the equals sign and just stick an
    approximation in the formula, it would serve just as effectively as an
    educational tool, without promising exact solutions.

    Why not just

    Bpk = vt/NA ?

    Teslas
    volts
    seconds
    turns
    meters^2

    This allows the end user to work out his own waveshape effects, based
    on the learned or worked out relationships between peak, rms and
    average of the waveform in question.

    At the college level, you can throw in the integral symbol in the
    appropriate position, to allow for flashy numerical solutions that
    include the phase angle that your initial reference to radian
    measurement inanely suggests.

    Just because 2xpixf (W in BAWN) is easy to remember, doesn't mean you
    should chuck it in everywhere, as a sign that 'this is where there's a
    relationship but I can't be bothered to go into it now'.

    RL
     
  18. (in <dwhec.1539$>) about 'Basic
    transformer question - number of turns depends on frequency? (Not turns
    ration, number of turns)', on Sun, 11 Apr 2004:
    So is your equation unless you define the variables. I know that YOU
    know what they are, but a lot of people don't.
     
  19. (in <>) about 'Basic
    transformer question - number of turns depends on frequency? (Not turns
    ration, number of turns)', on Sun, 11 Apr 2004:
    Because it isn't as easily remembered.

    You are clearly determined to have the last word, so have it.
     
  20. Genome

    Genome Guest

    | I read in sci.electronics.design that Genome <>
    wrote
    | (in <dwhec.1539$>) about 'Basic
    | transformer question - number of turns depends on frequency? (Not
    turns
    | ration, number of turns)', on Sun, 11 Apr 2004:
    | >The 'easily' remembered form for those designing switch mode power
    | >supplies, or otherwise, is;
    | >
    | >N = VinTon/B.Ae
    | >
    | >It's just a statement of Faraday's (apostrophe included) law in other
    | >terms that B is proportional to the integral of the applied volt
    | >seconds.
    | >
    | >Anything else is just pants
    |
    | So is your equation unless you define the variables. I know that YOU
    | know what they are, but a lot of people don't.
    | --
    | Regards, John Woodgate, OOO - Own Opinions Only.
    | The good news is that nothing is compulsory.
    | The bad news is that everything is prohibited.
    | http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

    Ooooops, flak fall out alert.......

    Give me the address of your local drinking hole and the name of the
    landlord and I'll post a fiver for you and your good one to have a pint.

    DNA
     
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