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Snubber circuit design

Discussion in 'Electronic Design' started by Ignoramus13229, Oct 13, 2005.

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  1. I am trying to make a Dc to AC inverter using IGBT.

    There is a big inductor before the bridge. I do not yet know its
    unductance, but I will measure it.

    I want to safely handle switching the circuit off (both possibly
    during active switching, as well as if the bridge is simply turned
    off).

    I can try doing two things.

    1. Place an appropriately sized capacitor and resistor between the DC
    power rails coming in.

    2. Also place a big ass varistor between power rails. It could be
    rated for appropriate amperage. I can see that there are varistors
    rated for quite large currents.

    I read a little about varistors here:

    http://www.radioradar.net/docs/varistor.php

    It appears, from my current thinking as well as previously made
    suggestions, that doing both things together makes sense. For short
    interruptions done during routine switching (if they occur at all),
    the capacitor and resistor would be used, and for anything worse than
    that, the varistor would save me.

    If that is the case, I would like to see some guidelines on picking a
    varistor, given certain inductance (to be found out) and interrupting
    voltage. One thing that I am not so certain about is the breakdown
    voltage. I figure that for a 85 OCV welder, and 1,200V IGBT, it would
    be sensible to select breakdown voltage of, say, 300 volt or
    thereabouts.

    I will try to measure inductance tonight.

    i
     
  2. Terry Given

    Terry Given Guest

    that means you have to give the inductor current somewhere to commutate
    to, whilst the bridge is off.
    thats not gonna work so well. that R carries the full load current, so
    will need an impressive peak pulse power rating.


    an RCD snubber, with a monstrous D, is probably what you want.
    you can read more languages than I can.

    they may not mention the most important part - the wear-out
    characteristics of MOVs. basically they can clamp a finite amount of
    Joules, then they go short circuit. Then (often within a few hundred
    microseconds) they go *BANG*

    applications using MOVs to clamp repetitive surges are generally best
    suited for breaking MOVs.

    find a manufacturer (eg Seimens, or whatever they are called now) that
    actually makes the things, and read their app notes.
    Cheers
    Terry
     
  3. Chris Jones

    Chris Jones Guest

    I just had a look at the Epcos application notes. They specifically
    recommend using their varistors for clamping spikes due to switching off an
    inductive load. Take a look: (sorry for the long URL, I'll break it across
    a few lines, since I don't know if I'll mess things up otherwise.)

    http://www.epcos.com/web/generator/Web/Sections/ProductCatalog/
    NonlinearResistors/Varistors/PDF/
    PDF__Applications,property=Data__en.pdf;/Applications.pdf

    Certainly, if a varistor is used to divert a lightning strike then at best
    only one operation can be expected before the varistor becomes useless, but
    I do not see any mention of a slower wear-out mechanism in the Epcos
    application notes, except the warning that the varistor must not be allowed
    to reach an excessive temperature.

    I believe that the OP intends to put a large motor-run type capacitor in
    parallel with the varistor so that it will only be in unusual circumstances
    that the varistor will carry the full welder current.

    Chris
     
  4. I thought that it could simply flow into the capacitor through the
    resistor.

    There are two kinds of events, really, a transient switch off during
    bridge switching, and a complete shutdown (turn off) of the
    bridge. Note that most complete shutdowns would happen while NOT under
    power, for obvious reasons. I cannot be switching AC switch off while
    welding.

    I suppose that shutdowns due to arc extinguishing are rare and softer.

    For transient switch offs...

    Suppose that, for worst case example, 300 A is flowing into the
    circuit. Then a switching even occurs and the circuit opens for 2
    microseconds (a very long time for IR22141SS that can likely be
    reduced severalfold).

    That means that if the current continues to flow without any
    reduction, into the capacitor, without change, the charge of the
    capacitor would be

    300 * 2E-6 = 0.0006 coulomb.

    A 10 uF capacitor charged to 0.0006 coulomb would have a voltage
    across its leads of 0.0006/10e-6 = 60 volts. That's quite survivable,
    especially in an RCD snubber.

    That makes perfect sense. I have the snubber diagrams right in front
    of me, RCD has a diode in parallel with resistor. Right?

    What kind of diode should serve as that D. It should survive currents
    up to, say, 300-400 A and voltages of a few hundreds of volts.
    Also, if I read you correctly, a MOV is not necessary, is that right?

    I also measured inductance of my reactor. It seems to be about 1 mH.

    I used this technique

    http://et.nmsu.edu/~etti/fall96/electronics/induct/induct.html

    and used both high and low Ohms on my decade resistor. High ohms were
    useless, I did it more for curiosity.

    i
     
  5. You are 100% right. I will use a relatively large capacitor, which
    should be no problem with an RCD type snubber (if I am not
    mistaken). It would not make starting conditions severe and would not
    make starting welding arcs make a sound of a .22. Then the varistor
    would be for catching emergency situations, etc, and not strictly
    necessary.

    i
     
  6. Terry Given

    Terry Given Guest

    not strictly speaking, no.
    not necessarily much use for your inductor, as it only pumps very low
    current thru the inductor. If the core material permeability varies
    strongly with H, then this will give an incorrect reading - invariably
    an over-estimate. google groups "splat test" for a trick that doesnt
    have that problem.
    Cheers
    Terry
     
  7. Got it. Are any particular ones more suitable? I looked at digikey and
    found too many dazzling choices.

    I see. It's more for protection when "shit happens", like electrical
    ground. It does not normally see any use.


    CORRECTION, it is about 2-5 mH, see my another post Measured
    INDUCTANCE of my reactor. I used some wrong data when I made my
    previous post.
    Thanks. I looked up the splat test. I am not quite sure how would I
    measure current with my oscilloscopes. Tek 2465 and 475. I kind of
    like this idea though, that's definitely the way to go.

    I did some calculations of capacitance of snubbers for given
    parameters. The I used formula from Fuji

    Cesp = L*Io^2 / (Vcep - Ed)^2

    L inductance
    Io normal current
    Vcep snubber peak voltage
    Ed DC supply voltage

    The results are mind boggling. My calculations suggest that I need
    about 50,000 uF (!) for 250V caps. Either my math is wrong somewhere,
    or else I am missing something very big.

    The above calculation applies to complete turnoff of the H
    bridge. Which should almost never happen under load. I assume that
    conditions of arc extinguishing are much milder.

    Complete turnoff at high current, I think, should happen rarely. Maybe
    never. In those instances, a snubber may provide appropriate
    protection.

    In regular operation, turn off time would be very short, perhaps 0.5
    uS or so. To absorb 0.5 uS's worth of high current, I need a lot less
    capacitance. (microfarads)

    So, I can say that I am quite confused.



    i
     
  8. Terry Given

    Terry Given Guest

    pass. its gotta carry the right amount of current, and have an
    appropriate voltage rating - at least peak snubber cap voltage.

    use a pulse generator (555 timer) and a decent FET, a current shunt and
    a low-voltage supply with shitloads of capacitance - ensure Ecap >>
    Einductor at measured current.
    the latter.

    inductor energy Eind = 0.5*L*I^2
    initial cap energy Ecap_init = 0.5*C*Vinit^2
    final cap energy Ecap_final = 0.5*C*Vfinal^2

    Efinal = Einit + Eind

    0.5*C*Vfinal^2 = 0.5*C*Vinit^2 + 0.5*L*I^2

    C*Vfinal^2 = C*Vinit^2 + L*I^2

    C*[Vfinal^2 - Vinit^2] = L*I^2

    C = L*I^2/[Vfinal^2 - Vinit^2]

    so yeah, there's a bit of a mistake in your numbers, because the
    equation is wrong.
    yeah Q = CV = It, can assume I is constant. So 200A for 0.5us = 100uC =
    100V across 1uF. You've also got stacks of volts to play with....
    the typo in the fuji app note doesnt really help, does it :)
     
  9. Got it.

    I think that I can buy 15 Panasonic ERZV20D271 varistors, they would
    provide max voltage of 225 VDC, clamping voltage of 455 VDC, and 135
    joules each. I would parallel them to get appropriate joule rating.

    The hope is that these varistors would never actually get a chance to
    conduct current. The H bridge should not be stopped at full load
    unless someone moves the current switch handle during welding, which
    is unlikely. I hope that interruptions of the welding arc would happen
    more slowly and thus the highest peak voltages/currents would not
    occur. I may be mistaken.

    For a diode, I could use a 300U60AMA diode, 300 A, 600 V rated. It's
    kind of pricey, maybe I can parallel several cheaper diodes? Such as
    3-4 Fairchild diodes RURU10060?
    I see. I may do that. Good idea.
    Thanks, you are right. Still, the required capacitance is tremendous.
    Very nice to know, makes life easier.

    Thanks Terry. Your thoughts about component selection for the snubber
    (see my text in the middle of the message)will be appreciated...

    i
     
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