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2MHz toroidal core?

Discussion in 'Electronic Design' started by Fred Bartoli, Jan 24, 2006.

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  1. Fred Bartoli

    Fred Bartoli Guest

    Does anyone know of a toroidal core, AL about 1000nH/T that'll be OK at
    2MHz/15mT

    The few cores I found in that AL/material range are not available in
    toroidal shape.

    Any hint?
     
  2. Ken Smith

    Ken Smith Guest

    Thats a lot of Al at 2MHz. Are you sure you need that much?

    Is this for a lossy element or a low loss one? If you want lossy look for
    type 77 material.

    You can stack toroids to make them into thicker cores. This raises Al.
     
  3. Fred Bartoli

    Fred Bartoli Guest

    Yup. Almost that much.

    That's for a low noise resonant converter some of us seem to have at the
    moment, and will power my low noise preamplifier head.

    The PS have to pass 10-12W with ultra low common mode current, +/-15V
    output.
    I now have a proto running on my bench, made from... unknown salvaged EMI
    management ferrites. They are 20x10x10mm (Od,Id,h) and I measured them at
    800 Mu_i. I also now have drawn the Mu'/Mu" curves, so that'll help
    identifying the manufacturer.

    At 2MHz, 2x6 turns it's 36uH and 100mA magnetizing current. I don't want
    much more and I can't have more turns (leakage inductance will be
    unmanageably high).
    And to render the thing a bit more involved, the low common mode current
    target excludes de facto the MnZn ferrites:
    the core needs to be high resistivity to avoid primary-core-secondary
    capacitive coupling. The ones I have
    Right now I have 0.35pf primary/secondary parasitics which I manage to lower
    down to about 4fF (yes, not pF) common mode coupling with some careful
    shielding and a bit of tricks.

    Too bad I only have 2 cores: I just needed 10 to build the 5 PSU I need.

    Losses are not that critical due to the relatively low power, but I don't
    want 5W either.

    Yup but that'll rise the primary/secondary parasitics too and I can't afford
    that. And clean winding is already difficult enough with one core. With two,
    arghh...
     
  4. Paul Mathews

    Paul Mathews Guest

    Have a look at earlier threads on how to minimize leakage inductance.
    You can go a long way with that.
    Paul Mathews
     
  5. Fred Bartoli

    Fred Bartoli Guest

    I'm very well aware of the way to reduce leakage inductance. Unfortunatly
    this invariably leads to higher interwinding parasitics which is a
    definitive no-no here.

    Think:
    Vout=15V, F=2MHz -> dv/dt>188V/us

    I want common mode currents under 1uA, preferably 1/3ua

    This translates to under 5fF interwinding capacitance, or capacitance
    dissymetry if you want to play the balancing game.

    Try to achieve that with interwound windings.
     
  6. Ken Smith

    Ken Smith Guest


    If it wasn't for your interwinding capacitance issue, I'd suggest twisting
    the windings together.

    First a couple of things I'm sure you've already thought of:

    The leakage inductance makes this:

    L1 T1 L3
    ------))))----+----- --------------)))))----
    ! ! !
    ) ) (
    L2 ) ) (
    ) ) (
    ! ! !
    --------------+----- --------------------

    T1 is a ideal transformer.

    I assume that the primary of this is effectively across the capacitor of a
    tuned circuit. (At least at higher power) This would be a normal way to
    do things.

    If the leakage inductance L3 can be predicted, you could place a capacitor
    in series to cancel it. This makes it a double tuned system but the Q of
    the secondary side will be quite low so it shouldn't be a major problem.

    You could put a capacitor across the secondary and make it a highish Q
    tuned circuit. Unfortunately for your application, I don't think this is
    in the cards.



    Now for my real suggestion:

    I've made lower interwinding capacitance at the cost of leakage inductance
    by linking cores with single grounded turn. I'm thinking that you may be
    able to do the same in a more extreme form like this:

    Rod soldered into hole
    /
    *************
    * * * <- Plumbing pipe cap
    *! * !*
    ! * ! <- copper tube
    ! TTTTT ! <- T= core
    !---- * ----! <- copper disk (washer)
    ! TTTTT !
    ! * !
    *! * !*
    * * *
    *************

    By turning the single grounded turn into totally enclosing shorted copper,
    the leakage inductance may be within reason even with the larger number of
    turns that the lower Al cores need.
     
  7. Fred Bartoli

    Fred Bartoli Guest

    Ken, you've almost described what I'm going to do (well have already done on
    the proto), but with some minor modifications.

    Effectively 2 cores, with a "one" turn in between.
    The first core is pushpull hard driven at 100% duty ratio.
    Then the series of both cores leakage inductances (about 500nH each xformer,
    seen from the 'single turn' side) is resonated out with a cap.
    The secondary side of the second xformer has 2 small parallel caps to limit
    dV/dt and thus parasitic currents closed on the shield.

    Then the differences:
    primary side of the second core has a center tap to ground in order to
    balance parasitics currents to the primary side shield. Just a single
    grounded turn doesn't do. The secondary side is already balanced.
    The inter-cores loop has more than one turn to reduce loop current (10W have
    to pass there) and ease these parasitic currents balancing. Plus we need an
    even turn number to make a center tapped winding :)
    Then we need shielding between the "1 turn" primary and the isolated side
    secondary. 0.1pF capacitance is already way too much and if you carefully
    think about the voltage distribution along the turns, CM voltage can't be
    perfectly balanced, even in a perfect world.
    So we have complete shielding of both sides, with both shields crossing the
    second core between primary and secondary.

    Funnily enough, the shields, which have to extend right in the middle of the
    core (but be careful, don't close the loop :), see high induced voltage in
    the center leg (a few volts), and some adjustment is necessary to compensate
    for the minute dissymetry that will inevitably appear (flux leakages are
    pretty high) and create some unwanted common mode current.
    The rather crude proto (dead bug on a plain GND plane) show that the induced
    CM voltage into both shields has to be carefully equaled.
    Once that's done, the CM current is well below 1uA, down to the 200nA level
    on each harmonics. I probably can expect better with a clean PCB and tightly
    coupled tracks (this isn't well controlled now).

    Now, I've also found some available cores (43 material) so I should have a
    more definitive view of all this next week, before making a clean PCB.
     
  8. Ken Smith

    Ken Smith Guest

    A comment about "great minds" could have been put here but wasn't.

    Ok got it.
    Huh? Where's the cap? Is it in series with the single turn? At least
    that's what I think you said. That cap ends up wit a lot of current in it
    doesn't it?

    It needs to be mounted very near the cores and needs to have a very low
    ESR. I guess you could take my pumbing parts idea and put the cap on one
    of the pipe-caps. A ring of smaller values would work better than one big
    one.

    I assume you ground one point on this near the output side core. Did you
    also ground the core material too? Most cores are conductive. If you put
    a small blob of conductive epoxy onto them you can hold a fine wire in
    place to ground them.
    Do you really need those extra caps? It seems like it would be better to
    try to not put the sharp edges into the core.

    .... I've got to go make a salad for dinner so more later
     
  9. Fred Bartoli

    Fred Bartoli Guest

    Yup, it's there. About 2.2A RMS and 20V RMS "only", but still within reach
    for good reasonably sized capacitors.
    That's also one reason why I only have a 2:1 and 1:2 ratio.

    I just hate plumbing :) Seriously, I'm not sure I got all the picture of
    what you drafted.

    ....
    Getting back to your drawing
    ....

    Hmmm, yes, I didn't understand but now I am. That's clever and will indeed
    help keep the leakage inductance low, but alas this won't solve the
    capacitive coupling at all.
    It will obviously provide low coupling from one core side to the other, but
    to the detriment of the capacitance to the *common* shield which is the
    copper tube, washer,...
    And now, where do you reference all the plumbing?
    You've just replaced the interwinding capacitive coupling by a worse
    (parasitics will be higher) winding to other side ground coupling.

    See below.
    I think you're speaking of the single turn there. You can ground it as near
    to the core as you like, even in the core's symetry plane and have it
    perfectly balanced, there are still the 15V secondaries voltage swings
    that'll induce CM currents.
    You *have* to have 2 shields between the windings (keep in mind the fF order
    of magnitude), one referenced to each side, to keep the windings capacitive
    current from flowing across the barrier.

    Yup, *most* cores are conductive. That's why I banned MnZn and use NiZn
    ones. Of course, one can "ground" the core, but which side? And what to do
    with the other side winding parasitics to core?
    I've ordered some MnZn cores too to play with, but this is just to confirm
    they won't suit and I'm ready to bet they will be really bad.

    But you're giving me some idea to maybe further reduce the residual
    capacitance that crosses the barrier through the core cross section.


    Yup. You can do whatever you want to 'not put the sharp edges into the
    core', the leakage inductance does its work and gives you nice ones on the
    output.
     
  10. Ken Smith

    Ken Smith Guest

    It depends on what coupling you are worried about. The copper plumbing is
    at (or very near) some voltage ground. You get a large capacitance to
    "ground" but remove all the capacitance to the AC voltage on the primary.

    The capacitor in at the input side's pipe cap. The ground connection is
    at the output sides pipe cap.

    Perhaps it is worse for your actual situation. I had assumed that a
    capacitance to ground was OK to add. If not, oh well, the idea cost you
    nothing and was worth every penny.

    Yes, I was thinking in terms of the single turn system I had used in the
    past. I didn't care so much about currents. I needed a total capacitance
    of the system under 1pF. I used a thinish copper wire and thickish
    insulation to keep the copper far from the windings. There was also a gap
    between the two cores.
    Hummm... Yup your right.
    Yes, for your application you need more copper parts.

    I'm not sure I understand this comment. In the two core case, I assumed
    that each core would be grounded to the local ground.


    I just had another though.

    Would multiple thin link windings be better than one thicker one?
    Unfortunately, the capacitance function has a log of the diameter in it so
    this may not work.

    How about 3 cores?
     
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