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flyback cores question

Discussion in 'Electronic Design' started by rick, Apr 5, 2004.

  1. rick

    rick Guest

    I have quite a few somewhat interesting cores from x-ray flyback transformers
    that I would like to use in some experiments. It would be nice to be able to
    categorize them somewhat, so I could compare predicted and actual results. Is
    this a realistic (sane) thing to do, or are cores like this so cheap and readily
    available that I would be better off tossing these in the garbage and getting
    something with specs?

    I have a picture of one of the cores here:

    http://www.skyko.com/xfrmer/flyback.JPG

    The plastic discs that make the gaps are 0.043 inches thick. The pieces are
    0.783 inches in diameter and the overall length is 5.21 inches and the width is
    2.52 inches. It seems to be a pretty decent sized core, IMO. The primary was a
    0.325 inch wide, 0.050 inch copper strip wrapped 5 times around the core, while
    there seemed to be multiple secondaries of many turns of extremely fine wire.
    Anyway, it was easy and non-destructive just to slip the windings off the core.

    I don't have any books yet on flybacks, smps design, etc. I was thinking about
    ordering A. Pressman's "Switching Power Supply Design". I am interested in high
    voltage (3kV to 10kV) medium low current (10mA to 60mA) switching power
    supplies. I am not sure how much his book goes into this type of supply.

    Thanks,

    Rick
     
  2. Rick,
    Power ferrite core materials are more similar than
    different. Saturation flux densities, as operating
    frequency, increase slowly with the years. If you
    have a very old core, maybe it's a 3C8 material and
    limited to a couple hundred KHz. Since it's gapped,
    as are all flyback cores, the permeabilty is
    irrelevant.

    Also, since a lot of HV flyback power supplies are
    run in so-called discontinuous inductor current mode,
    i.e., the inductor current goes to zero and stays
    there for part of each switching period, all you need
    to know is how many volt seconds it takes to saturate
    the core. To the extent that everybody's input voltage
    is about the same, that fixes the frequency.

    Don't throw away the windings. Those HV secondary
    windings are often wave-wound to reduce the capacitance,
    so I'll use those too. Don't worry if the windings
    aren't optimally wound for your application, all that
    does is limit how much current you can pump through
    the thing.

    --Mike
     
  3. With that much gap, the core properties do not matter much, except for
    saturation flux, and that doesn't differ, much from one power ferrite
    to another, so you should get very representative results for that
    core size, regardless of the material you might replace it with,
    later.

    Keep in mind that 10 ma at 10 kV is 100 watts of continuous power,
    with the instantaneous power during flyback operation being something
    like 4 times that. So you are talking about pretty beefy stuff.
     
  4. R.Legg

    R.Legg Guest

    Looks like UR64/40/20

    http://www.ferroxcube.com/prod/assets/urcores.pdf
    You should not disassemble it unless you are convinced that its
    original configuration will not serve the intended purpose. If it was
    not impregnated, the wire and stiffer insulation layers might also be
    reusable, if of suitable guage for this or other tasks.
    It's a fairly large part for this power level, but this should only
    make HV isolation a little easier to achieve.

    Reading up is always a good idea. If the existence of the part is the
    only reason for your interest, then reading is mandatory - as you have
    yet to set an application to aim for. Without this goal, you have no
    starting point for a design procedure.

    RL
     
  5. Bill Sloman

    Bill Sloman Guest

    Farnell certainly stocks a useful range of transformer cores, and they
    aren't particularly expensive. I'd go for something where you can get
    a data sheet.
    I can't find much in the way of explicit references in my copy. One of
    the numerous problems with this sort of supply is that the secondary
    windings have relatively high inductance and self-capacitance, and
    tend to be resonant at relatively low frequencies.

    Peter Baxandall invented his class-D oscillator precisely to deal with
    just this problem, and while Pressman does discuss resonant
    converters, he doesn't seem to discuss that particular configuration,
    which does not appear to be well known in the U.S. Jim Williams of
    Linear Technology uses pretty much exactly the Baxandall circuit in
    his famous application note 65 on driving fluorescent back-lighting in
    lap-top computers

    http://www.linear.com/pdf/an65f.pdf

    but describes it as a "Royer" inverter, despite the fact that Royer's
    inverter was not resonant.

    The sort of high voltage inverter you are talking about is quite
    difficult to design and build - protecting the secondary side
    connections from arcing over is an art in itself. Cambridge
    Instruments built their own high voltage (30kV)power supplies for
    their electron microscopes for some years, but by the time I started
    working for them in 1982, they'd gone over to buying them in from
    people who had specialised in making these sort of supplies.
    Brandenburg and Hunting HiVolt come to mind.
     
  6. That is the 4 I was thinking of. Both the switch and the rectifier
    has to deal with a current peak 4 times the average current, at least,
    and at least full voltage, so an instantaneous power or 4 times the
    average power, at least.
    I contributed a fair part of the design effort on a 4 phase, 1600 watt
    85 to 125 volt in, 42 volt out, power factor correcting commercial
    flyback design. (so about 4 of yours in parallel).

    A tiny bit of leakage inductance between primary and secondary do
    nasty things to the voltage requirements of the switch. Passing
    conducted and radiated noise specs is a bitch, too.
    I await its completion with an......

    ticipation.
     
  7. Genome

    Genome Guest

    | I have quite a few somewhat interesting cores from x-ray flyback
    transformers
    | that I would like to use in some experiments. It would be nice to be
    able to
    | categorize them somewhat, so I could compare predicted and actual
    results. Is
    | this a realistic (sane) thing to do, or are cores like this so cheap
    and readily
    | available that I would be better off tossing these in the garbage and
    getting
    | something with specs?
    |
    | I have a picture of one of the cores here:
    |
    | http://www.skyko.com/xfrmer/flyback.JPG
    |
    | The plastic discs that make the gaps are 0.043 inches thick. The
    pieces are
    | 0.783 inches in diameter and the overall length is 5.21 inches and the
    width is
    | 2.52 inches. It seems to be a pretty decent sized core, IMO. The
    primary was a
    | 0.325 inch wide, 0.050 inch copper strip wrapped 5 times around the
    core, while
    | there seemed to be multiple secondaries of many turns of extremely
    fine wire.
    | Anyway, it was easy and non-destructive just to slip the windings off
    the core.
    |
    | I don't have any books yet on flybacks, smps design, etc. I was
    thinking about
    | ordering A. Pressman's "Switching Power Supply Design". I am
    interested in high
    | voltage (3kV to 10kV) medium low current (10mA to 60mA) switching
    power
    | supplies. I am not sure how much his book goes into this type of
    supply.
    |
    | Thanks,
    |
    | Rick
    |
    |

    Very meaty....

    Converting to SI units gives the following approximate figures.

    Effective area, Ae = 310mm^2
    Effective length, Le = 392mm
    Length of gap, G = 1mm
    Primary copper area = 10mm^2

    If you assume it's something like 3C80 material then intitial
    permeability is ui = 2000. That gives an effective permeability of

    ue = ui/(1+G.ui/Le) = 327.

    5 turns of your copper gives

    Lpri = uo.ue.N^2.Ae/Le = 8.12uH

    If you've got a way of measuring inductance then you can reverse
    engineer for a closer figure on ui and natch it up. I thing it's a
    reasonable guess though.

    Assuming Bsat of 300mT then the peak current is

    Ipk = Bpk.Ae.N/Lpri = 57A (ouch)

    If the thing goes discontinuous 50% duty at 40KHz, not unreasonable,
    then

    VIN = Ipk.L/Ton = 37V

    Not too far away from a standard 48V bus allowing for regulation.

    This is in the realms of 500 Watts worth. I don't know about effciency
    of X-ray tubes but I'd expect them to be fairly poor

    10mm^2 of copper at 4A/mm^2 is worth 40A RMS. 57A worth of 50% duty
    triangle is worth 23A average. Not unreasonable given the likely AC
    losses. (skin + proximity effect)

    As to using one in anger........ not for the faint hearted.

    DNA
     
  8. rick

    rick Guest

    Dang! You are good. That looks spot on.

    Thanks for the info and tips. I sorta have an application in mind, but I just
    wanted to learn about boost converters too. Perhaps cutting my teeth on
    something less than 3KV at 50mA would be wise (but not as much fun?).

    I have a dual cathode, center anode CO2 laser tube that I would like to power.
    The tube is about 30 inches in length between the cathodes, with the anode in
    the exact center. I am not exactly sure what the design voltage is supposed to
    be, but I believe it is in the neighborhood of 5kv to 6kv at 50 to 60mA. I do
    know the tube should output about 70 watts, so that would be an input of 350
    watts at theoretical max. eff. of 20%. Depending on the switching frequency, I
    believe it is possible to get a good output from the laser just driving it from
    the rectified high voltage, possibly even without ballast resistors. Also, if I
    want to modulate the laser electrically, it would seem that a high frequency
    switching design would allow for easier/faster modulation than a 60hz rectified
    and filtered chunky tranformer design.

    Rick
     
  9. rick

    rick Guest

    Thanks. That is a very interesting ap note.
    Perhaps that is why the whole assembly (flyback, diodes, caps) was immersed in
    oil in the x-ray power supply? I assumed it was because they were running in
    the neighborhood of 100kV or so. I had hoped something around 6kV might be
    quite a bit simpler.

    Thanks,

    Rick
     
  10. rick

    rick Guest

    Thanks for the calculations. I was not expecting someone to do that much work,
    but I guess Sunday night *is* pretty crappy for TV. :)

    It sounds like a core this size *could* support a 350 to 400 watt design then.
    Also, this gives me an idea what the untouched oil filled transformer/multiplier
    units might expect as a drive voltage. Not that I really have a use for 100kV
    at 5mA....


    Thanks,

    Rick
     
  11. legg

    legg Guest

    This thing has been assembled with added sections, to provide four
    gap locations and to increase the Le of the original assembly to
    approximately 500mm. (Assumed the tape measure is inches, as no
    reputable metric measure would use divisions of 8ths or 16ths)

    It might be assumed that the added sections are the same core
    material, from the physical appearance, but there's some pretty
    'ferrite-looking' iron/mpp dust ferrules out there.

    Depending on the number of spacers used (not mentioned), there may
    have been an air gap as small as 2mm (the spacer gap appears in both
    arms) or larger than 6mm if a spacer were located in each available
    position. If the spacers were only used as end washers, it could alsso
    have been solely dependant on the physical dimensions of a lower
    permeability insert.

    Equations that assume UR64 will be valid without the additional
    magnetic inserts, but should recognize the double gap dimension.

    Doubling the gap will halve the Lcalc value, reducing the supply
    voltage required in the end use for any specific Ipk.

    RL
     
  12. rick

    rick Guest

    It assembles just as pictured. There are only two 1mm thick spacers total.
    They each go in the place shown in the photo and everything else is bolted
    together with the brass rods. The length of the bolted together unit is about
    132mm.
     
  13. Terry Given

    Terry Given Guest

    good comments so far. Obviously you understand the 100W, but why 4 times?!?

    If using DCM (Discontinuous Conduction Mode - the inductor current returns
    to zero before the end of a switching period) with a 50% duty cycle [aim for
    this - it gives the best "balance" between primary and secondary RMS
    currents - low D gives high primary RMS current and low secondary RMS
    current (reflect it back to the primary for this comparison); high D is the
    opposite] then you can caluclate:

    Iavg = Pout/Vout
    The secondary current is a sawtooth, height Ipeak, width Toff. In one
    period, its average value must be the same as Iavg, i.e.
    0.5*Ipeak*(Toff/Tperiod) = Iavg

    then calculate Iavg. If Toff = Tperiod/2 (boundary of DCM, D=0.5) then
    Iavg=0.25*Ipeak i.e. Ipeak = 4*Iavg. For any other D use Ipeak =
    2*Iavg/(1-D) (for secondary currents; for primary use D not 1-D).

    Also the RMS current is Ipeak*sqrt(D/3) - easy enough to prove.

    This is actually the big downside to flyback converters - the output peak &
    RMS currents are quite high. Mind you I have built 400W 24V DCM flybacks
    running at 50% duty cycle - Iavg = 16A, Ipeak = 64A, Irms = 26A......but you
    need to pay close attention to diode & capacitor specs/temp.

    perhaps you should look at genomes website.....


    Cheers
    Terry
     
  14. Bill Sloman

    Bill Sloman Guest

    I've not had much to do with voltages around 6kV. From what I have
    seen I'd guess that all the high-voltage side of the power supply
    would be potted in de-aired potting compound with just a 6kV-rated
    coaxial connector sticking out.

    People like Radial and Greenpar make specialised high-voltage
    connectors, but my experience was that you had to go to the
    manufactureres to get them - the market was too small for even
    specialised distrubutors to find it worth their while to stock them.
    Pasternak Enterprises in California do seem to stock a bunch of SHV
    connectors, but they are only good to 5kV

    http://www.caton.com/21.htm

    One great advantage of using coaxial connectors and cables is that if
    a point in the circuit does arc over, the consequent huge burst of
    discharge current (as the cable capacitance discharges) does not
    induce large external magnetic fields. Cambridge Instruments electron
    microscopes for years had a nasty habit of blowing up board to board
    links when the 30kV across the electron gun flashed over, which went
    away when we went over to proper coaxial connectors on either end of
    the cable linking the high voltage supply to the electron gun.
     
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