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paralleing bridge rectifiers

Discussion in 'Electronic Basics' started by Jon Slaughter, Apr 15, 2007.

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  1. Anyone see any issues with paralleling two or more bridge rectifiers to
    increase the total current capacity and reduce each components individual
    heat signature?

    Thanks,
    Jon
     
  2. How will you ensure equal sharing of the current?
     
  3. I don't have to ensure equal sharing... I just need to ensure that the max
    rating isn't exceeded. I would imagine that between two identical diodes
    that they would share approximately the same current.
     
  4. Jamie

    Jamie Guest

    yes.
    it won't work.
    which ever bridge starts conducting first will be the one
    doing most of the work.
    it's not what you want to do.
    diodes have a region of no current flow. When it starts flowing,
    the resistance between the joints decreases as current increases.
    so if one should start before the other, then it will be the
    one doing the work.
    For power systems, you need to have a single bridge large enough
    for the job, or join them together via power resistors which most likely
    is not what you want due to power loss.
     
  5. I wouldn't assume that. If you run each via a one foot length of wire you'll
    probably be closer to sharing as it will act as a resistor.
     
  6. This is not true. There is always current flowing. (diodes are not ideal
    switches)

    The point is not that one will start before the other but if they will
    approximately equalize. I don't need to push each diode close to its max
    but, say, if I had 100 diodes each with a max of 1A and I paralleled them
    then I would expect I could run a few amps through them with no problem.
    Now if one blew then maybe there would be a cascade effect and obviously the
    more than blew the more that will blow.


    e.g., it doesn't concern me if one diode uses 80% of its max and the other
    50% as long as its well below 100%. Ofcourse if one diode went bad then you
    would be running 100% of the current through just one diode. I could put a
    fuse on each diode to prevent this(that is, let me know when one goes bad)
    or even maybe add some current limiting device to each diode?

    that is, maybe a foldback current limiter on each branch that is maybe 80%
    of its max?


    ultimately the best solution is to have the proper components but I don't. I
    have many 4A bridge rectifiers but no 12A ones. Now I can use the 4A but
    rather get about 8A out of it. Maybe paralleling 3 or 4 of them would be
    safe? putting fuses on each one for 4A would be even safer.

    Ofcourse if this runaway current draw that your talking actually happens
    then it won't work. That is, if only one diode will conduct at the start
    while all others will not then it will obviously draw all the current
    through it. But if there is some sort of distribution that is approximately
    average then I can deal with it and it will work for my circumstances.

    Thanks,
    Jon
     
  7. Maybe theres some way to use negative feedback between two bridges so that
    the current between them will equalize? Something like a comparator that
    adds resistance to one branch if it has more current than the other and vice
    versa.

    Of course it would be more work and money than just buying the proper bridge
    but would be interesting to see.

    Thanks,
    Jon
     
  8. john jardine

    john jardine Guest

    For low powers they reckon on average you'll get a 30:70 balance on two
    diodes running in parallel. This implies something like 3 bridges needed to
    double the current. (you only see a 60mV change in a diodes ON voltage for
    it to be passing 10x the original current)
    But ... As as current goes up the diodes start generating noticable heat and
    the loadsharing starts getting much worse. Resulting in the initial 'best
    diode' in the batch hogging more and more of the current the hotter it
    becomes and the hotter it becomes then the more current it hogs and on so
    on.
    That best diode is losing 2mV forward voltage drop for every degreeC
    increase in it's temperature which means the other poorer diodes with their
    higher mV on-voltages don't get a look in as they are running cool with low
    currents and higher forward voltages and can only remain that way.
    Low diode currents gives some semblance of order. Beyond a critical current
    and a clear winner emerges by positive thermal feedback even though all the
    diodes may be physically separate. (a chaotic function?)
     
  9. As replied, you can't rely on better than about 30% to 70%
    sharing, if connected directly together, and mounted on a
    common heat sink. If you can add a low resistance (one that
    will drop about a tenth of a volt at average rated current,
    which will drop more at instantaneous peak current) you can
    force the sharing to be much closer. You can put the
    resistor in either AC leg, but you need to put it in the
    same leg of both bridges. Now you have the temperatures of
    two more components to worry about. But there may be an up
    side, besides the sharing. They will tend to slightly lower
    the peak current in total, helping the transformer and
    capacitors reduce their RMS current for a given DC average
    output current. The other down side is the slightly lower
    output voltage (that should be less than a volt).
     
  10. jasen

    jasen Guest

    only thermal runaway and/or current hogging.

    If you arrange some series resistance to balance the current
    (winding resistance should be enough) all should be fine.

    Bye.
    Jasen
     
  11. Rich Grise

    Rich Grise Guest

    Buy the bigger rectifier. Under load, any amount of difference in their
    forward voltage will cause positive feedback. As the diode heats up, its
    forward voltage decreases, increasing its share of the current, and you
    get thermal runaway. One hogs all of the current, melts, and then the
    other gets stuck with all of the current, and melts.

    Or, you could use current-balancing resistors, which will waste power,
    but could protect the diodes; but bottom line, it'll be cheaper in the
    long run to just spring for the bigger bridge.

    Hope This Helps!
    Rich
     
  12. John Fields

    John Fields Guest

    ---
    I think the best way would be to put Peltier junctions and
    thermocouples on each of the diodes and then as the diodes started
    warming up to cool them all off so they were all at the same
    temperature as the one with the lowest temperature which was being
    cooled the least.

    Either that or to run them in a bath of circulating deionized
    refrigerated water.
     
  13. Eric R Snow

    Eric R Snow Guest

    Greetings John,
    I built a power supply for a CNC machine retrofit using the
    original 3 phase xmfr and 5 full wave rectifiers. I used 4 1/2 of the
    5 because the xmfr was actually 9 windings wound on 3 cores. Anyway,
    the machine is only using the original contactors, spindle motor,
    servos and the xmfr that the machine came with new . All the other
    electric and electronic parts are new. I did not use any resistance
    with the rectifiers to force the diodes to share the current more
    equally because I didn't know I should. The machine runs fine and the
    conversion has hundreds of hours on it. I think the rectifiers are
    rated at 20 amps. The servos draw 15 amps max before the control shots
    down. This has only happened once when I jammed one of the axes. The
    xmfr is wired to the rectifiers with 10 gauge wires a little over 3
    feet long each.
    Some questions: How does the resistance help spread the load across
    all the diodes?
    Why should the resistance be located between the xmfr and the
    diodes?
    Why haven't the diodes in my setup been destroyed? Is this because
    of the long 10 gauge wires?
    Thank You,
    Eric
     
  14. Without drawing pictures, I am having a little difficulty
    imaging that schematic. Is the secondary array essentially
    3 Ys or 3 Deltas that all feed one DC supply? If this is
    the case, and you use separate rectifiers for each
    secondary, then the impedance of those secondaries acts as
    the current balancing resistance (and inductance), to help
    force all the rectifiers to share the load current.
    So the 20 amp rated rectifier is usually running well below
    its rating. That's good.
    It just adds additional voltage drop to he rectification
    that is proportional to the instantaneous current, unlike
    the very nonlinear and negative tempco drop of the diode
    junctions. It helps to make the overall drop more
    proportional to current and with a less negative tempco.
    It doesn't force perfect sharing, but only makes the sharing
    less unequal.
    I was afraid you would ask me that. Ideally, you would add
    a bit of linear (and optimally, with a positive tempco)
    resistance in series with each diode junction. I was
    picturing the parallel rectifiers being fed from a single
    secondary, and powering a single storage capacitor. So it
    may work as well in series with any of the 4 legs of the
    bridge, since each leg carries either all the AC current, or
    all the DC current.

    I was not taking the time to visualize all the permutations
    and their relative performance, but just told you about the
    first one I thought about. I stand by the requirement that
    if there is one AC source, and one DC load, the sharing
    resistors all need to be in the same led of the bridge.
    Perhaps it is because, most of the time, the average current
    is pretty low, and the diodes have a fair thermal mass to
    absorb the big thumps. Most rectifier diodes have
    impressive surge current ratings.

    And, yes, the wires are low value resistors with positive
    temperature coefficient.
     
  15. Eric R Snow

    Eric R Snow Guest

    Greetings John,
    I don't know what the configuration of the secondarys is. When
    connected to the original three servo amps the 9 taps from the xmfr
    were connected such that each winding had one wire going to each servo
    amp. I think. Anyway, what I did was connect those 9 taps to the
    rectifiers so each winding does have it's own rectifier. And the
    outputs from the rectifiers are all tied together. This is because the
    new servo amp configuration has all three amps on one board and they
    share the DC supply.
    Thanks for the explanations John. I will keep them in mind next time I
    need to build a similar power supply. And I'm sure there will be a
    next time.
    Cheers,
    Eric
     
  16. jasen

    jasen Guest

    If the three secondaries all produce the same voltage you should be safe.

    Bye.
    Jasen
     
  17. Andy Dingley

    Andy Dingley Guest

    If you're going to that sort of length, isn't there already some
    commercial technology for 100A+ rectifiers (from International
    Rectifier or the Hexfet people?) that uses a funny geometry so as to
    gain the right sort of temperature coefficient inherently. You can
    just parallel these things up as much as you like, they sort it out
    amongst themselves.

    As to paralleling a pair of rectifiers to share current on a
    breadboard lash-up, then this is an old hack and works fine -- just
    use a matching pair (same part, same manu.) and don't expect that the
    total rating will be twice that of a single.

    As to why you'd do it for production, then I don't know. High power
    rectifiers are cheaper than low power ($/A)
     
  18. John Fields

    John Fields Guest

    ....

    ---
    I guess I should have punctuated it like this:

    I think the best way would be to put Peltier junctions and
    thermocouples on each of the diodes and then as the diodes started
    warming up to cool them all off so they were all at the same
    temperature as the one with the lowest temperature which was being
    cooled the least. :^)
     
  19. redbelly

    redbelly Guest

    Uh, you're joking, right?

    Mark
     
  20. John Fields

    John Fields Guest

     
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