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Surge Pulse Clamping with Ceramic Capacitors

Discussion in 'Electronic Design' started by Klaus Kragelund, May 29, 2013.

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  1. Hi

    For an application I need to protect the electronics of an RS485
    interface from surges (8/20us 1kV pulse)

    The normal way be to use a transzorb, in a SMA housing or even a SOT23
    device.

    That is not possible since we have to be able to withstand up to 30V
    DC on the bus also (that is handled by a special RS485 IC)

    The problem is that the selected breakdown voltage of the transzorb
    therefore is high (>30V) and a lot of energy is dissipated into the
    transzorb

    In another product I have used a diode from the affected node to a
    ceramic capacitor with a bleeder resistor in parallel to clamp the
    energy and dissipate the energy into the bleeder and that worked fine.
    The diode sees very little energy and the capacitor is just charged
    during the pulse

    I never saw any problems doing that, but I would like to know if
    anyone here has tried the same and has any inputs into failure cases
    or even a better way to clamp the pulse?

    One "feature" of the diode-capacitor clamp is that closely spaced
    pulses will eventually destroy the capacitor, but anyhow closely
    spaced pulses in a tranzorb will also destroy that one....

    Thanks

    Klaus
     
  2. Joerg

    Joerg Guest

    What, you also? I am just looking at RS485 protection for an industrial
    system.

    Normally you need special ones with low capacitance or TVS in series
    with regular diodes. Ordinary transzorbs with their huge capacitance can
    muffle the data signal because of the lowpass effect. They'd "kill it
    dead", as John Wayne would have put it.

    It works but can blunt the edges of your signal in times when the cap is
    not yet charged at all. You can also limit the charge on the cap via a
    big TVS in parallel to it.

    Why not clamp directly to the 30V DC rail? You just have to make sure
    that this rail won't start to float up when lots of pulses appear and
    the load on the DC rail is too light. A TL431 can help with that
    although it'll be marginal in this case with its 36V limit. Maybe
    another homemade shunt regulator.
     
  3. Yes, we are using a low capacitance type (in other case put a diode in
    series to quinch the tranzorb capacitance)
    I have no room for it, don't even have room for 3 SMAs

    The 30V is only available since some idiot would connect 30V power to
    the A/B line of the RS485 interface

    One way to do it to add a set of biasing resistors, so one cap is
    charged to +12V and another to -7V (the RS485 CM range) and connect
    that surge capacitor to the bus via the diode.

    Another idea is to use the Bourns CDSOT23-SM712, specifically designet
    for surge protection for RS485 devices, but add ceramic caps in series
    with each line connection so it can tolerate 30V without creating
    wonderful smoke, but will be able to clamp surge pulses without
    affecting the high speed bus.

    Regards

    Klaus
     
  4. Like this:

    www.electronicsdesign.dk/tmp/RS485_cap_protection.pdf
     
  5. Joerg

    Joerg Guest

    Almost the same here :)
    One way to handle that would be to sense A and B. The instance either
    one goes beyond 12V cut the data line electronically or shunt to ground
    with Polyfuses in line. I don't like the Polyfuse approach because they
    become hot, if possible I'd try to electronically switch open. Two
    back-to-back P-channels or something like that, maybe use an array if
    space is really tight. That avoids dissipation issues.

    That would be a very nice way to do it. In my case I don't have any
    negative supply, and also very little space. But you'd have to make sure
    it can't run away. So either the dividers have to contain low enough
    resistor values or there have to be clamps. With the chance of some dude
    connecting a hard 30VDC that's next to impossible. Better to use an
    electronic protection like above.

    Sure, but then you'd have DC drift on the bus. Might be ok but it can
    get iffy.
     
  6. Robert Macy

    Robert Macy Guest

    what's the impedance of the pulse? if low, wont work.

    or, rather how many joules do you expect in the pulse?

    I once had to protect telco equipment whose pulse went to around 500V,
    BUT could supply around 500A With protection like you describe you
    didn't lose a tranzorb, you'd lose the PCB!
     
  7. Joerg

    Joerg Guest


    But that would not protect against a hard 30VDC applied because an
    installer miswired something. It could cause your RS485 chip to go PHUT
    unless it has internal protection against this.
     
  8. Tim Williams

    Tim Williams Guest

    Can give latches a try. There are SIDACs made for protection duty.
    That'll short the line down to a few volts within a microsecond or so (if
    your bitrate isn't too high, you could filter out the remaining fractional
    microseconds of overshoot). Should still work if the line remains
    active -- RS485 levels of 200mV won't keep a SIDAC on, so you could use
    two (one per line to ground) or three (full delta).

    MOVs handle gobs of energy, but of course, they are rather high
    capacitance, no good for a terminated line.

    There are low-capacitance TVSs, made from some sort of plastic I think,
    that'd do this sort of job, but I don't know that any will handle the kind
    of energy a 1kV spike will most likely deliver. Here's an example:
    http://www.littelfuse.com/products/...ata Sheets/Littelfuse_PulseGuard ESD_PGB2.pdf
    Yeah, no mention of avalanche energy... hmm...

    Other than that, diodes into a rail or conventional TVS -- as suggested --
    is your best alternative.

    Tim
     
  9. The electronic protection would probably be sensitive to ESD and
    burst, and is a pain to design with a possible 30V voltage applied
    The Bourns part has 7V/12V transzorbs back-to-back, so it will handle
    the CM range of the bus. A resistor should be added in the schematics
    in parallel with each capacitor

    Cheers

    Klaus
     
  10. The impedance is 40ohms, so the peak current as test reveals is about
    20A
    I have a surge test earlier passed using the CDSOT device, without the
    requirement for 30V applied

    Regards

    Klaus
     
  11. The RS485 IC has +/-60V protection rating, so its ok

    Regards

    Klaus
     
  12. .... resistor added to avoid stair casing of the voltage on the
    capacitor from multiple surges (discharge time of less than a second,
    the standard defines wait times between pulses of maximum 60 seconds,
    which of course does not mimic real life pulses)
     
  13. Joerg

    Joerg Guest

    Which one do you use? The ones on mine (clients's choice) have abs max
    ratings of -8V to +12V, and no internal circuitry given.

    Also, make sure for how long it can take that. Sometimes there is only a
    poly-resistor that eventually hisses out.
     
  14. Looks like a nice part, had not heard of them before. Seems it will be
    a little too big, AFAICS the same size as SMA tranzorbs.

    MOVs derate over time, every pulse applied reduces the breakdown
    voltage, so after some use the product self-destructs

    A ceramic TVS, like the CT0805K14 is better, so they say, but it has
    very soft knee and large capacitance

    Regards

    Klaus
     
  15. Joerg

    Joerg Guest

    You'd need a 40V or so TVS upfront but I don't see the 30V presenting a
    problem. I was thinking about something like this:

    http://www.vishay.com/docs/71433/71433.pdf

    Yes, it'll handle it alright but the bus will meander around a bit
    depending on the data transmitted. But how would it prevent 30VDC from
    being sent down a data line?
     
  16. LTC2862, can take 60V indefinitely, but you need to add clamping
    circuit to the VDD node since it will dump current into that node if
    it is in transmit mode and is subjected to back fed voltage.

    Cheers

    Klaus
     
  17. Our device will survive, but other devices on the same bus from other
    suppliers will probably be destroyed.

    A lot of the device I have reverse engineered has no protection at all
    and will be destroyed from this

    Cheers

    Klaus
     
  18. It would need levelshifting and another supply rail to control the
    FETs at all input voltages. be immune to bursts and allow to be turned
    on from regular signals from the RS485 device

    I tried to draw it up, became a nightmare

    Cheers

    Klaus
     
  19. Joerg

    Joerg Guest

    Wow, that sure is the Rolls-Royce of RS485 chips. With a corresponding
    price tag :)

    But isn't that bleed-through only an issue if the ground of the LTC2862
    has come off? Otherwise this would really be a problem because you'd
    just have moved the dissipation from one place to another. I think the
    only real protection for 30V continuously is some sort of cut-off,
    whether inside or outside a chip. At least a partial one where the
    current becomes very small.
     
  20. Joerg

    Joerg Guest

    Yeah, you do need a negative supply voltage. Sources and gates tied
    together, zener between them, resistor from gates to negative supply.
    The OVP trigger could then shunt gate-source via an optocoupler, or you
    could have a transistor at the base (in that case there needs to be also
    a gate-source bleed resistor).

    There may even be a ready-to-go OVP protector chip with all this in
    there, but probably expensive.

    Then there are always the Supertex high voltage mux chips but they will
    probably be too big. I wish they made something like this for RS485
    level trip points but haven't seen it yet:

    http://www.supertex.com/pdf/datasheets/MD0100.pdf
     
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