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air core transformer efficiency

Discussion in 'Electronic Design' started by Fuxue Jin, Oct 7, 2003.

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  1. Fuxue Jin

    Fuxue Jin Guest

    Hi, everyone,

    Looking for a solution to improve an air core transformer efficiency.

    The transformer can not have a regular soft magnetic core, must be air.
    Primary to secondary is about 1:2 to 1:3. Coil is wound on a bobbin or
    tube with about .5 to 1" diameter.

    In order to improve the efficiency, what's the key factor that needs to
    be considered or adjusted? Turns ratio, coil size, operating frequency
    or something else?

  2. Ian Stirling

    Ian Stirling Guest

    Why can't you do a better core?
    What's it for?
    You'll get better coupling by winding a layer of primary, a couple of
    layers of secondary, ...
  3. Ian Stirling

    Ian Stirling Guest

    Why can't you do a better core?
    What's it for?
    You'll get better coupling by winding a layer of primary, a couple of
    layers of secondary, ...

    Operating frequency needs to be high, look into things like Litz wire.
  4. Fuxue Jin

    Fuxue Jin Guest

    The transformer is used in a medical device under very strong magnetic
    field, any core will be saturated.
  5. Ian Stirling

    Ian Stirling Guest

    If you can put an (three?) extra coils round the core to eliminate the
    external field over the small volume, that might be easier.
  6. R.Legg

    R.Legg Guest

    Wind in a toroidal shape on a non-magnetic, non-conductive former.
    This allows all windings to have equal coupling (maximum Lm) and
    assists in reducing concentration of external stray field.
    Section-wind if capacitance is a problem.

  7. Tim Shoppa

    Tim Shoppa Guest

    Is this for signal or for power transmission? I'm guessing power, seeing
    your goal of efficiency. And I'm guessing that another goal is

    One old-time trick for HV isolation in very hostile environments are motor-
    generator pairs connected by a (potentially very long) insulating rod.
    If you've already got a "free" high magnetic field then the magnets
    commonly associated with motors and generators might be entirely
    superfluous :)

  8. R.Legg

    R.Legg Guest

    Are you talking about an existing external field; is the part intended
    to generate a strong magnetic field; or is this field unavoidable in a
    transformer structure.

    You are being vague.

  9. Would encasing the coil in mu metal allow the use of iron?
    hank wd5jfr
  10. Fuxue Jin

    Fuxue Jin Guest

    This is a very interesting suggestion.

    The transformer is used as (more or less) a normal power transformer to
    drive a low dropout regulator. The whole final unit is under a strong
    megnetic field. Also a 5000V HIPOT between primary and secondary is

    I don't expect much high efficiency out of it, but want to do the best
    if there is any solution.

    Thanks for the posting!
  11. Fuxue Jin

    Fuxue Jin Guest

    According the people who have tried before, mu metal didn't provide
    enough shielding effect to isolate the core.
  12. John Larkin

    John Larkin Guest

    What's the frequency? This would be fairly easy at a high (10KHz or
    better?) frequency, but it's difficult to pass much power at 50/60 Hz
    with an air-core transformer.

    Considered photovoltaics?

  13. Tim Shoppa

    Tim Shoppa Guest

    My guess was that the device was in the vicinity of a superconducting
    magnet, e.g. a NMR (aka MRI) scanner.

  14. Transformers transform impedance, voltage, current, isolate and some are
    tuned. Which one of these is your objective and can another circuit be
    substituted to achieve your desired objective?
    Hank wd5jfr
  15. Fuxue Jin

    Fuxue Jin Guest

    Have tested at 40KHZ, and would have a test result later at 100KHZ. I
    expect the efficiency will increase at higher frequency.
  16. legg

    legg Guest

    Why is the power supply in the machine?
    Can't you just route double-insulated DC from a safe source to your
    circuit, for local regulation?

    You'd find that EMI was reduced, if you did.


  17. How much power do you need? What is your input voltage, and what is your
    output voltage?

    There was a thread here a couple of years back from a guy trying to make an
    air core power transfer system (titled Energy transmission with coils). The
    thread inspired me to play with the stuff myself (though reading back my
    contributions to his thread show my own ignorance of the subject at the
    time), and based upon my experience I would say:

    You must use a full bridge or half bridge topology to drive the primary if
    you want anywhere near decent power output and efficiency. Do not use a
    single switch converter topology, it puts allot of stress on the switch and
    will not yeild much power output at all. You should use as close to 50%
    duty cycle each direction as feasible, but make sure to insert some dead
    time in the gate drive signals to prevent cross conduction.

    Generally speaking the higher the frequency the better, but with some
    limitations. The problem with air core transformers is they have such
    pitifully small magnetizing inductances. As a result, the magnitizing
    currents ramps up to extremely large values very quickly. In order to keep
    the magnetizing currents reasonable, you can either increase the number of
    turns used (to increase the inductance) or increase the frequency. If you
    increase the number of turns used, you inevetably must use a longer wire
    which has more resistance (both DC and AC). So you get a reduced
    magentizing current, but you end up with more winding resistance which
    causes more dissipation for a given amount of current. The net result is
    they, to a degree, cancel each other out. So you really need to use a quite
    high frequency to get good results.

    Why a half/full bridge only? The novelty of the half and full bridge
    topologies (built using either MOSFETs alone or BJTs/IGBTs with antiparallel
    diodes) is they can readily recapture the energy stored in the magnetizing
    inductance and return it to the input DC bus capacitance. This is very
    important since leakage inductances and magnetizing currents would cause
    massive power dissipation.

    In my experimentation I found the best efficiency (for my setup) was
    obtainable with a switching frequency of around 300kHz. In my case I was
    using a half bridge from a ~155V DC supply rail (simple mains rectified and
    filtered), driving a transformer constructed by winding about 67uH (IIRC)
    worth of plain 22AWG wire on a plastic wire bobbin (one from the Radio Shack
    magnet wire 3 spool set). This served as the primary (which was to see
    about 78V RMS AC since it was being driving by a half bridge with near full
    duty cycle). I used an approximately 1:2 turns ratio, so the secondary had
    approximately 250uH (again IIRC) worth of the same 22AWG plain 300V
    insulated wire (also from Radio Shack). Together they just filled the
    bobbin to the max, all very nice and neat. The half bridge switches were
    IRF630N devices. With resistive loads of less than or equal to a plain 120V
    40W lightbulb, the whole arrangement could probably easily have operated all
    day with no extra cooling but plain small heatsinks. I would have estimated
    the efficiency around 80% perhaps (combined MOSFETs and transformer). I
    used a variac to turn down the DC bus voltage slightly (otherwise the lamp
    voltage would increase to around 130V, which was quite close to the unloaded
    secondary voltage) to keep the output at 120V. When using a 60W lightbulb
    the heat dissipation in the transformer and MOSFETs with small heatsinks was
    enough to likely need forced air cooling for continous operation. With a
    100W lightbulb operation for a few minutes was reasonable before things
    would get too hot and need to be shut down. Nevertheless the load
    regulation was suprisingly good as it seemed the output voltage would drop
    maybe 20V between no load and 100W lightbulb loaded. As such I could easily
    drive the 100W lightbulb at its full 100W rating at 120V.

    The transformer design is quite important. Not having to worry about
    saturation and core loss is nice, but ultimately DC and AC winding
    resistance become incredible problems. At low frequencies the magnetizing
    current could easily become substantially larger than the load current for
    any reasonably sized primary inductance. You can find out how much the
    current will swing using the basic E=L*(dt/dt) formula. Very high
    (relatively speaking) frequencies combined with small size litz wire will
    likely yeild the best possible combination and highest efficiency. My
    choice of 300kHz with 22AWG wire was not optimal. The skin depth at 300kHz
    is much smaller than 22AWG wire is thick. If you aren't already familiar
    with it, Texas Instruments magnetics design handbook will provide an
    excellent introduction to skin depth effects.

    It probably would yeild good results to try to size the magnetizing current
    to be around the same size as the load current itself. You do this by using
    the E=L*(di/dt) forumla along with your switching frequency and the formula
    for finding inductance of air core coils for given geometries (any good
    physics textbook) and numbers of turns.

    I'm not quite sure how your transformer would behave in close proximity to a
    superconducting inductor. I might imagine some big forces might get
    developed (else how do mag-lev trains work?). I'm afraid I lack experience
    with superconductors (the local Radio Shack doesn't stock room temp.
    superconducting wires yet, dang!). The toroid winding technique sounds like
    it might have some appeal, though I suspect you would have most
    disappointing results if you tried to wrap them in sections. You can do
    that with a nice high permeability core, but I hightly doubt that would work
    with an air core. Wrapping them right on top of each other aught to work
    fine though I should think. It seems to provide quite good coupling (k
    perhaps equal to or better than 0.9?) with solenoid types wrapped right on
    top of each other.
  18. Tim Shoppa

    Tim Shoppa Guest

    Would a hi-efficiency bulb/hi-efficiency photovoltaic combo be
    capable of an efficiency over 5%? I'm thinking that given current 30%-
    efficient bulbs and 2%-efficient photovoltaics that you'd get under

  19. John Larkin

    John Larkin Guest

    One can just resonate the primary to solve this dilemma.

  20. Tom Bruhns

    Tom Bruhns Guest

    Air-core transformers are common at HF, where cores can be pretty
    lossy. The efficiency can be quite good. I'd be trying for a few MHz
    at least. At HF, you can use tuned structures and avoid much of the
    usual capacitive switching loss you see in common LF power switchers.
    In the size you're talking about, an unloaded Q of a few hundred is
    reasonable at a few MHz, and if you have an unloaded/loaded Q ratio
    of, say, 20, then you've got about 95% efficiency. With tuned coils,
    you don't need really tight coupling, either.

    But even if you just use untuned switching, a few MHz isn't too
    bad...a while back I ran a small switcher at 4MHz with efficiency
    around 65%, with losses roughly equally divided among the switching
    mosfets, the ferrite-core transformer, and the rectifiers. It was a
    simple push-pull thing at around a watt. (I haven't seen mention of
    the power you need to transfer.)

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