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Smps Toroids on a Ground Plane

Discussion in 'Electronic Design' started by D from BC, Apr 18, 2007.

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  1. D from BC

    D from BC Guest

    I recall somewhere an author recommended that the inductors be placed
    on a ground plane..

    That's probably nice for little inductors..Like buttons.
    But my toroid is 1/2" tall and about 1.5" wide..

    I don't believe putting this toroid flat down on a ground plane is
    going to help reduce EMI emissions. Correct?
    All the copper windings are too far from the ground plane..

    Would it help to Faraday shield the toroid to reduce EMI... ?
    (I'm trying to avoid killing my neighbors AM radio reception.)
    Could I blanket my toroid with some foil and ground the foil?
    Or don't bother...?

    Toroid Conditions
    f=100khz
    I=2A average, 200mA ripple
    V=170V peak, 0.4 duty square wave (hard switching)

    This toroid is in a earth grounded metal box. The smps power ground is
    not connect to earth ground. The toroid is part of an offline
    unisolated smps..If the two grounds connect.. .Poof!!!
    So ..I'm guessing a faraday shield will be connected to power ground.
    That would make the shield "hot" relative to earth ground. Shocking to
    touch when live but safe in a closed earth grounded box.
    D from BC
     
  2. In my experience toroids don't emit much anyway; their flux is
    contained in the core. Have you any reason to think otherwise? I would
    pay more attention to the PCB layout, e.g. minimising the area of high
    dI/dT current loops. Also make sure input and output wires are
    filtered.
     
  3. D from BC

    D from BC Guest

    Yes..I know toroids are good at keeping in a magnetic fields but
    there's an electric field too ..
    When high switching voltages exist across a toroid..can the electric
    field be a problem...?
    I don't know much about electric field interference.
    D from BC
     
  4. Paul Mathews

    Paul Mathews Guest

    Yes, indeed. Most inductors in switching circuits get connected to a
    high dv/dt node at one end and a lower dv/dt node at the other.
    Placement of the high dv/dt pin(s) should be away from potential EMI
    and cross-talk victims. Many inductors can also be placed in either of
    2 orientations on the PCB (due to pin symmetry), and sometimes it
    makes a big difference which placement you happen to use. This has to
    do with the way that the 1st turn on one end comes from inside the
    core and vice versa, which means that the highest dv/dt locus moves a
    bit. There is also a extra 'turn' on most toroids that generates field
    that is not in the core at all. This is the turn that the current
    takes as the coil turns progress around the core. We'll probably have
    Mr. Sloman chime in on this fine point. A few standard toroid
    inductors are wound with a crossing turn halfway around, so that this
    effect is mostly cancelled. You can also have inductors wound with 2
    windings and accomplish the crossover turn on your PCB. In any case,
    the ground place can help by concentrating the E field in a region
    around the high dv/dt turns, thereby reducing parasitic currents to
    other structures. Of course, this can also 'inject' noise into your
    ground system...push down here and it pops up there.
    Paul Mathews
    Paul Mathews
     
  5. D from BC

    D from BC Guest

    Good colloquial "push down here and it pops up there" :)

    Crossing turn halfway around?? I don't understand yet..
    Maybe after some more coffee.... :)

    Still thinking...
    D from BC
     
  6. Joerg

    Joerg Guest

    Probably Whidbey Island speak for "twisted windings". Bifilar, trifilar
    and so on.
     
  7. Guest

    Probably not. "Non-progressive" windings, like the Ayrton-Perry
    "bootlace" winding technique, can be applied to single wire windings
    or to bifilar, trifilar and rope windings.

    Check out "Coaxial AC Bridges" by B P Kibble and G H Rayner, ISBN
    0-85274-389-0, which includes a lot of stuff on winding fancy
    inductors and transformers for high precision work. The stuff about
    non-progressive windings is in section 4.2.

    http://www.npl.co.uk/electromagnetic/publications/guides/ac_bridges.html
     
  8. Joerg

    Joerg Guest

    Ah, the secrets of RF engineering. Thanks for explaining. That seems to
    be one of those books from the days when engineers were self-taught and
    knew their stuff by heart.

    When I was a kid an old engineer showed me how to build inductive
    wideband distributors and combiners. Whenever I asked him why this or
    that had to be exactly as shown his answer was: "It just don't work no
    other way".
     
  9. Paul Mathews

    Paul Mathews Guest

    Imagine a toroid with 10 turns. Beginning on the inside of the core,
    wind 5 turns, covering about 160 degrees around the core and ending
    outside the core. Then, cross over the the core, that is, route the
    wire on a diameter right over the core central axis to a point near
    the first turn (about 10 degrees to the bare side of the 1st turn) and
    continue the remaining 5 turns. You'll end up with a crossing turn
    that goes over the top of the entire core, with 5 turns progressing CW
    around the core and 5 turns progressing CCW (or ACW for some folks).
    There are many variations on this approach that can be used to
    accomodate multiple windings, minimize parasitic capacitance, etc. A
    few suppliers of power toroids do this as standard practice.
    Paul Mathews
     
  10. Paul Mathews

    Paul Mathews Guest

  11. Paul Mathews

    Paul Mathews Guest

    You have the choice whether to mount a toroid horizontally or
    vertically and how to orient the high dv/dt vs low dv/dt ends. The
    geometry scales: At an equal number of core diameters, the ground
    plane has equally beneficial effects, regardless of core size.
    High dv/dt end will generate parasitic currents according to the
    relation: I = C dv/dt. These currents WILL flow, regardless, so
    minimize C and dv/dt and provide the shortest possible intentional
    paths for such currents. Shields are meant to perform this function,
    but you must have a low impedance connection to the common node of the
    source of the dv/dt, otherwise the shield itself will have significant
    dv/dt. The highest dv/dt is often associated with switching elements
    and associated ringing, so rectifier technology, gate drive, and
    snubbers are all very important. Shields with large dimensions
    relative to the inductor (if you have the room for them) will have a
    low capacitance and corresponding low parasitic currents. However, the
    larger dimensions then present a challenge to make a sufficiently low
    Z connection to the common point. Faraday shields are supposed to
    completely enclose a field generator and theoretically neutralize
    magnetic and electrostatic fields. A properly wound toroid has very
    little external field, so Faraday shields are seldom seen except where
    very high isolation from magnetic fields is required.

    Positioning a shield very close to a switchmode toroid is an excellent
    way to generate very high parasitic currents to the shield. Unless the
    shield is very well connected to a common return point for the
    generated currents, the shield itself will have high dv/dt and radiate
    to other structures. See earlier comments.
    Safety capacitors can be used to direct parasitic currents through
    particular paths, returning them to their sources without much
    affecting leakage between mains and other conductively accessible
    parts. Of course, this is why there are limits on how much capacitance
    you can use for the purpose. Advanced safety topic: study carefully.
    See some interleaved comments above:
    Paul Mathews
     
  12. Joerg

    Joerg Guest

    Thanks, Paul. In noise critical apps toroid winding can become an art.

    The closest I came was while fishing with a former boss of mine. Didn't
    catch anything, we should have motored on to Langley for a nice
    breakfast and a stroll.

    Some of my co-workers at ATL in Bothell were from Whidbey Island. Quite
    a commute every day with the ferry ride and all. Many had an old car on
    the other side (Mukilteo?) and a bicycle on Whidbey Island. Whenever
    they talked about the island it sounded like they lived in paradise.
     
  13. D from BC

    D from BC Guest

    ok..I got the structure now..
    To help me understand what's going on I'm going to try this trick:

    I'm going to pretend the wound toroid is a power reostat.
    Let's say I put that in my smps..(but it still acted like an inductor)
    In my app..one end of the inductor is 170VDC and the other has 300Vpk
    100Khz square wave.
    Moving the wiper along the core (normally wound) and the wiper has an
    increasing square wave amplitude. I'm imagining the electric field is
    like this too.

    Now I switch to that special winding technique..
    Turning the reostat from the beginning and the waveform amplitude
    starts off small...increases....jumps to max...then decreases to half.

    But I'm still a little fuzzy on how this minimizes electric field
    interference with neighboring components...
    .....I'm gonna have to have some more coffee :)

    D from BC
     
  14. Paul Mathews

    Paul Mathews Guest

    Magnetic field effect: The parasitic turn from normal toroid winding
    around the circumference in one direction produces field outside of
    the core. The effect of the field depends on what loops might couple
    to it, and ordinary shielding is ineffective at preventing such
    coupling. You cancel this field with the crossover turn approach.

    Electric field effects: The usual winding approach brings opposite
    ends of the coil near to one another. High dv/dt of one end relative
    to the other, along with their close proximity, means current flow
    through a relatively high parasitic capacitance. As you suggest, dv/dt
    varies continuously around the turns. Using the crossover-turn winding
    technique, you end up with the low dv/dt end close to the middle of
    the coil rather than the end, where it is adjacent to half the dv/dt.
    It takes a complex model to approximate the overall effect, since
    you'd need to consider all of the turns and not just the ends.
    However, by simply measuring the self-resonant frequency, it's easy to
    demonstrate that the crossover-turn technique can raise resonant
    frequency by a factor of 4 or so. At the higher resulting ring
    frequency, you can use much lower snubbing capacitance, as an example
    of one benefit. These used to be considered 'RF' techniques, but
    switchmode power supplies have entered that realm.

    Side note: Best read for offline power switchmode is anything by
    Sanjayit Maniktala.
    Paul Mathews
     
  15. Joerg

    Joerg Guest


    In case someone uses a search engine the spelling of his name would be
    Sanjaya Maniktala.

    Also very good reading are the older Unitrode app notes, now TI and
    hopefully still on their server. If you have the "Unitrode IC Data
    Handbook" from around 1990 don't ever think about tossing it. Tons of
    valuable SMPS info in there.
     
  16. Totally agree Paul, his "Switching Power Supply Design + Optimization"
    (ISBN 0-07-143483-6) has a ton of info. Sorry to see him voted off Am.
    Idol.
    Harry
     
  17. D from BC

    D from BC Guest

    [snip]
    Resonance increased by a factor of 4 or more!!! Wow :)

    Thanks for the book suggestion.
    I'll check out some online bookshops..
    D from BC
     
  18. Paul Mathews

    Paul Mathews Guest

    The general technique of designing windings so that high dv/dt is
    maximally far away from low dv/dt can be applied in many different
    ways on various kinds of cores. 'Progressive' winding is a fairly well
    known example of this, but there are many other less well known
    methods, and you can easily dream up your own. Figuring out a way to
    make a technique mass producible is the real trick. In my experience,
    the majority of magnetics suppliers can't offer much creativity, and
    many don't really know much beyond the basics of how to use their
    winding machines.
    Paul Mathews
     
  19. MooseFET

    MooseFET Guest

    It is sometimes easier to design in two inductors in series than to
    get one made with special winding methods. On things like EFD
    formers, you can get the ones with multiple winding bays. You can
    wire the resulting sections in series on the PCB.

    You can also specify the winding to be done with "wire rope".
    Twisting 7 strands together isn't quite Litz wire but the high
    frequency Q is a lot better. Once a winder understands the concept of
    how the rope is done, they don't seem to have any problem with doing
    it for you. (At least, the winder I use)

    Also, if you are going after every % you can get, the snubber can be
    made so that some of the energy ends up back in the input supply.
     
  20. Tony

    Tony Guest

    Magnetic field effect: The parasitic turn from normal toroid winding
    Sorry to have come across this thread late, but what is the "crossover
    turn" technique?

    I've been trying two methods to minimize susceptibility to axial
    fields in Rogowski sensors and tranformers (toroids with non-magnetic
    cores):
    1) start with a circumferential turn in a groove machined into the
    former, then wind on a single layer winding in the opposite direction,
    or
    2) wind two layers, reversing the feed for the second layer.

    Both seem to be OK at reducing axial flux sensitivity, but still
    having trouble getting sufficient winding uniformity (another
    subject).

    But in general, I'd like to know about anything that could raise
    self-resonant frequency 2 octaves.

    Regards,
    Tony
     
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