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Over Voltage Protection Crowbar Circuit

Discussion in 'Electronic Design' started by panfilero, Nov 30, 2012.

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  1. panfilero

    panfilero Guest

    I'm needing an over-voltage protection circuit to put at the output of a switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V.

    Here's the 3 options I was considering: For all the options the point is to blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output

    1. A Zener - I'm thinking these get rather bulky for higher current handling, and they're not very accurate

    2. A TVS - Probably better than the Zener option as far as current handling and bulkiness... I think TVS kills Zener for this application

    3. An SCR - I could trigger an SCR off of a LM431 to get an accurate trip voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop

    Anybody have any thoughts or recommendations for OVP circuits like this... I'm likeing the SCR option right now

    much thanks!
  2. Guest

    why not something like this:
    no need for fuses then

  3. Joerg

    Joerg Guest

    #3 is the only option I ever consider. Personally I like to have the SCR
    right at the circuit side of the fuse. If the fuse were at the input and
    the SCR were at the output you'd have the upper FET and the inductor in
    the path. This will slow down the fuse tripping and there is a chance
    that the FET grenades before the fuse trips, something that is generally
    not desired.
  4. panfilero

    panfilero Guest

    Thanks for the input, can anyone tell me why the SCR is a better option than the TVS? I was considering using one of these guys...
  5. Joerg

    Joerg Guest

    The SCR is much faster. Assume this scenario: The FB resistor comes
    unglued for some reason, gradually. Now the voltage slowly rises, the
    TVS begins to conduct, gets hot, gets hotter, starts to smell, smoke
    arises, a smoke alarm goes off, your lab begins to become drenched in
    sprinkler water, the bell rings at the local fire station, engine
    company 27 roars down the road with wailing sirens ...

    The SCR, in contrast, can be control by a "snap action" circuit. Voltage
    exceeds a certain threshold ... WHAMMO ... fuse blows, the day is saved.
  6. mike

    mike Guest

    Is this a dedicated supply where you KNOW what it's gonna be hooked to
    in EVERY case?
    There are times when it's not a good idea for the output to short.
    Like when you're charging a battery.
  7. Joerg

    Joerg Guest

    Sounds like the OP is using a buck. A sync buck would give at least some
    level of protection. A white-knuckle event is when the upper FET welds
    through. Eventually, depending on its control architecture, the lower
    FET will come on and then one can only hope that the fuse opens. A
    non-synchronous buck would not protect at all, the output would head
    straight up to Vin and most likely leave a path of destruction in
    anything connected.

    Belts-and-suspenders would be to sense the output and trip a beefy SCR
    right at the fuse.
  8. Guest

    Please remember to put some ferrites in series with the SCR gate

    I once made a power supply for a 100 W RF transmitter. When used with
    an indoor antenna, the RF signal could get into the protection circuit
    and trigger the SCR and blow the fuse.
  9. legg

    legg Guest

    With the battery attached, the overvoltage condition would be
    difficult to detect, until the battery was toast. A current regulator
    failure in this condition would likely blow the fuse without external

    If it's a back-up battery, it should be connected into the circuit in
    such a manner that source failure (including the shorted condition)
    doesn't interfere with the intended back-up function.

    There ARE issues with scr failure, when improperly sized for large
    storage capacity and supply/fuse combinations. Parts intended for
    crowbar applications have I^2t ratings that are considerably larger
    that jellybean devices. MCR67,68 and 69 were examples of parts
    designed for this use. The latter two are still available.

  10. Guest

    The dimensions of the human body match quite well with 1/2 or 1/4
    wavelengths at VHF frequencies, thus the radiation limits for humans
    are quite low at VHF frequencies.

    However, at HF frequencies, the limits are quite higher, so in reality
    much higher levels are allowed, so in places into which people are
    allowed to freely enter, any equipment should also tolerate these
    field strengths.

    The situation gets worse, when long wires are attached to the device,
    such as speaker cables in an audio amplifier or power supply wires,
    forming antennas much longer than the human body.

    For this reason audio amplifiers and DC power supplies needs some
    filtering in the external wires (or at least in the feedback
  11. mike

    mike Guest

    I spent a significant portion of my career trying to train that kind
    of thinking out of engineers.

    Some engineers leap directly to telling you why your example is wrong.
    Many times, I've had to go back and fix their designs.

    The ones you want working on your project use the example to consider
    what real users might do with the product and come up with a
    more robust design.

    I don't remember the exact one, but I once had a commercial bench
    power supply that
    I wanted to use for charging batteries. Turns out, if you turned
    off the power switch before disconnecting the battery, it made
    a LOT of smoke.
  12. Fred Abse

    Fred Abse Guest

  13. Fred Abse

    Fred Abse Guest

    Put a fuse at the *output* and use the usual SCR/zener circuit.

    Don't rely on the PSU input fuse. Too uncertain.
  14. Fred Abse

    Fred Abse Guest

    I agree. The biggest I've done was for a 100-amp bought-in power supply. I
    used fabricated copper busbars for that. It made a satisfying thump on
  15. Fred Abse

    Fred Abse Guest

    Not my experience. A big SCR driven by a sensitive one (like C106) with
    just an A-G zener is extremely robust. The only faults I've encountered
    with that sort of circuit (both mine and others') have been age-related
    false tripping at greater than ten years.
    I've used them (once). Bitches to keep stable.
  16. Fred Abse

    Fred Abse Guest

    Very common. Bench power supplies should *never* be used to charge
    batteries without a suitably rated diode in series.

    I've had it done to my bench supplies when I've been away. I better not
    find out who it was...
  17. Joerg

    Joerg Guest

    Hey, we have the absolute majority in the house now :)
  18. Joerg

    Joerg Guest


    This works, and fast, done it many times. One has to make sure that the
    SCR triggers with gusto and is big enough.
  19. Joerg

    Joerg Guest

    That was my point, ideally you don't want to have the inductor in the path.
  20. legg

    legg Guest

    I don't follow your reasoning.

    Is it the battery back-up, overvoltage protection method, or the power
    supply design you're questioning? The subject is crowbar ovp methods.
    You're point was the ovp condition occurring in a battery charging
    situation. I think was addressed.

    Power supply compatibility in parallel redundant or backup
    applications is a subject in itself. Misapplication of products, or
    their immunity to such conditions can also be an interesting
    discussion, along with the sad state of the equipment and the
    amazement or frustration of the misapplicator.

    Rest assured that you won't see a product that wasn't immune to the
    application of external voltages occupy or originate in my work area.
    This is, in fact, a crude test method to confirm the function of OVP
    circuitry, though alternative methods exist that may perform the same
    function automatically and with reduced hardware, in both design and
    in production test.

    I do not, however, assume this capacity in any equipment that is
    unfamiliar to me, until proven otherwise.

    The crowbar method of protection isn't always used, and it's external
    application in any black box situation should probably be evaluated.

    Your battery charging or back-up situation isn't exactly a black box,
    so you should be able to get your mind around it, with a little

    There's no reason why a commercial bench power supply should smoke on
    application of external voltage, within it's normal output compliance,
    save the naivete of it's designer, or the complaisance of it's

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