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Some ESD protection questions (spark gaps, earth/ground connectionissues)

Discussion in 'Electronic Design' started by Christian Walter, Aug 27, 2007.

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  1. Hello,

    I just wanted to ask the more experienced people here for some advice. I
    am currently working on a redesign of an existing board and I want to
    improve some ESD aspects of the board because we had some problems in
    this area in the past. The system is built from two components where the
    PCB in questions contains some micro controllers and a USB equipment and
    the other PCB is the power supply unit and contains additional internal
    modules.

    I have to protect some I/O ports and an USB interface. I have chosen to
    use a TVS Diode Array (Littlefuse SP0502BA). I have chosen to place the
    diode array as near as possible to the I/O connectors of the system.
    What I am unsure about is whether to connect the common pins of the
    diode array. My first choice would be to use the earthing because if I
    connect it directly to the ground plane any discharge current will take
    the path from the I/O connector over the diode. From there to the ground
    plane and the PCB FCC connector cable to the power supply board and then
    to earthing. In my opinion this can cause some signal and system
    stability problems although the device on the PCB will not be damaged.
    Does anybody support this claim?

    The next question I would like to ask what you people think about spark
    gaps as a cheap alternative ESD protection for onboard I/O ports. That
    is I would add add a signal layer which surround the PCB connections
    using a very short distance (maybe 8 thou which is easily doable in a
    manufacturing house) and connect this to plane to the earthing. This
    would effectively limit the voltage to maybe 1-2kV.

    What I have also read is that a small guard ring at the edges of the PCB
    (connected to ground) can help to improve ESD performance. Has anybody
    used and tried this?

    I would be very grateful for some advice because I am still a bit new in
    the field. Another question is how I can verify my ESD protection
    without having to go to a special laboratory because of costs.

    Kind regards,
    Christian Walter
     
  2. Paul Mathews

    Paul Mathews Guest

    Envision a complete circuit between the ESD source and your product:
    This must include a complete return path. Often, this involves the
    protective 'earth' or grounding system. Your goal is to divert as much
    of the ESD energy as possible away from your circuits, both to protect
    circuit elements and to prevent disruption of equipment operation. If
    you insulate your product completely, there will be no ESD to the
    equipment, but this is impractical in cases where conductive parts
    must be exposed, particularly if those conductive parts cannot be
    connected to earth ground. The best defense, then, is to provide
    deliberate adjacent paths to ground, for example, the grounded 'ring'
    around the perimeter of a circuit board. If possible, design the
    product so that grounded conductive parts extend out in such a way
    that any ESD conducts to ground rather than into your circuit.

    For any ungrounded paths into your circuit that may still take an ESD
    hit, you have these basic defenses to consider:
    a) series resistance
    b) series inductance
    c) shunt resistance and shunt breakdown devices such as zeners,
    transils, MOVs, spark gaps
    d) shunt capacitance

    For high speed signals, it may be impossible to incorporate any
    appreciable amount of series resistance or inductance or shunt
    capacitance without degrading the signal, so you're left with the low-
    capacitance tranzorb solution. The common terminal of the tranzorb(s)
    should be connected to earth ground with the lowest possible
    impedance. If this doesn't do the job, reconsider how you might
    prevent direct ESD to signal nodes: shielding, shrouded connectors,
    etc.

    Keep in mind that the high currents and fast risetimes associated with
    ESD will develop high potentials across stray inductances in the
    return path, AND there may be significant magnetic field coupling of
    ESD currents to adjacent circuitry. Therefore, loop area of the
    complete ESD current path should be considered.

    Circuit board spark gaps will produce widely varying results depending
    on humidity, dust buildup, and other factors, and I think your
    estimate of 1 to 2 kV for 8 mils is probably a bit low for most
    conditions. Spark gap tubes have the advantage over semiconductor
    devices of maintaining relatively shunt voltages even for very high
    current levels, so they are often used to absorb more powerful
    transients such as those resulting from indirect lightning strikes.

    In Summary:
    1) Design packaging so that product is either totally insulated from
    ESD or any discharges go preferentially to grounded structures,
    shunting all ESD current away from internal circuits.
    2) Series and and shunt any remaining filter nodes that must take a
    direct ESD hit.

    As you get farther into this subject, you'll also discover the
    difference between insulating systems that dissipate vs collect static
    charge and between deliberate low-impedance discharge of ESD versus
    dissipating or bleeding off charge to prevent high amplitude ESD.
    Paul Mathews
     
  3. whit3rd

    whit3rd Guest

    Surely USB has a grounded shell and provision that the ground
    connection
    mates first, then the power connections, then the signal? There's no
    real
    threat of high potentials/spark/currents there, unless your shield
    melts
    in a lightning strike. If you're canny about the other I/O ports,
    they can
    be pretty robust, too, and NOT because of TVS diodes or other addons.

    Spark gaps (commercially available ones, look like disk capacitors but
    with
    a little saw-cut slot) will take a lot of current and don't often
    fail, BUT
    they're intended for rather high breakdown thresholds. Unless you
    expect
    high currents at high voltages (like an isolated power supply that
    floats
    at a few hundred volts from ground), the spark gap is going to be too
    insensitive, won't protect delicate circuitry well. Low-pressure gas
    has better breakdown thresholds, and a Ne lamp can be a very
    effective clamp if you can tolerate the (about 100V) turnon threshold.

    Grounded-track-surround is a variant of a guard ring. Multiple guard
    rings
    connected to successive potentials is the hallmark of HV circuitry.

    Guard rings are effective if you expect tracking (conduction through
    foreign substances after your PC boards get dirty), and I'd expect to
    see them used a LOT where a circuit board is expected to have
    hundreds of volts per cm kinds of gradients. USB and ' I/O ports'
    isn't
    really in that range, usually. With multiple guard rings, connected
    with spark gaps, you can make your printed circuit board REALLY
    intimidating, even without international warning symbols. Techs will
    quake every time they open the box...

    Typical ESD testing is at low event energy, with voltages (2 kV) that
    can
    be handled with minor precautions. But, any controlled, safe
    environment
    for such testing IS a laboratory, by definition. If you don't want to
    'go out'
    to a laboratory, you build one for yourself.
     
  4. Chris Jones

    Chris Jones Guest

    You might be able to find some useful information on this page:
    http://www.emcesd.com/

    Chris
     
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