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ESD protection and transient voltage suppression devices

Discussion in 'Electronic Design' started by mj, Dec 27, 2012.

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  1. mj

    mj Guest

    Are ESD protection devices for signal lines and transient voltage suppression diodes for power lines same? For ex, if I have a data line that is 5V logic level and a power input of 5V to the board can I use the same type of devices for both?

    I have a regulated 5V DC coming in to the board via 2-3 mtr cable. Can I use the ESD protection diodes specified for 5V data lines here instead of transient voltage suppression devices specified especially as for data lines?

    I understand low capacitance required for high speed data lines. Is low capacitance of esd devices specified for data lines an issue for power lines?

    Do I need unidirectional or bi-directional for 5V DC power line?

  2. Guest

    There're the same, but different. Both are Zener-like devices but on
    different scales and tuned for different purposes.
    Possible, but probably not. There are many strategies for dealing
    with ESD. Some use a series impedance with a capacitor, or clamp. The
    series impedance limits the energy from an ESD strike so a smaller
    device can be used. There is a tradeoff that has to be made to select
    the appropriate device. The chances are that your diodes specified
    for 5V data lines won't be robust enough to protect a power input,
    where you may or may not be able to add sufficient impedance to limit
    the energy.

    They make hundreds of different devices for a reason.
    Low capacitance generally means low energy capability. Power lines
    generally require higher energy capability.
    Usually, unidirectional devices are used for DC inputs. You only want
    the Zener action in the direction of the applied voltage. In the
    reverse direction the TVS diode is a diode, so will protect against
    excursions below ground, which will also destroy devices. There isn't
    any reason to limit the negative excursion to the positive supply
    voltage. The reverse diode will also help protect against supply
    reversal (assuming you have a protection device to limit current).

    Bidirectional TVS diodes are most often used for differential data
    lines, where there is no reference to ground. The TVS diodes are
    selected using the maximum common mode voltage.
  3. mj

    mj Guest

    Thanks a lot. That makes it pretty clear.
  4. miso

    miso Guest

    Diodes are more rugged when forward biased than reverse biased, so I
    wouldn't go bidirectional if you don't need that feature.

    The machine model only has a 200pF cap, but nearly no resistance. The
    human body model is 100pF with a 1500 ohm resistor. For power supply
    lines, the capacitance on the line itself should be enough to limit the
    voltage. Just think of it as a charge transfer design problem, i.e.
    conserve charge and you can predict the final voltage.

    Generally the machine model is the one that gets you.
  5. There is of course the other option of using a double diode such as
    a BAV99 to clamp the signal just above the power supply or just below
    ground. Then you only need to ensure the supply can't be pulled too
    high. Usually the decoupling capacitors achieve this, but you if you
    want add a high-power clamping device to the power supply just in

    The nice thing about double diodes on signal lines is that they have
    low capacitance but can pass considerable current for short periods.
    It is a good idea to have a little series resistance between the diode
    and the outside world and some more between the diode and the
    device input to limit the current that flows during an esd event.

  6. thumper

    thumper Guest

    Today's on-chip ESD protection networks are only 100 fF or so for high
    speed paths but we're doing good to survive 2kV from a HBM
    (human-body-model) discharge. Fast I/O has diodes to supply and gnd and
    then supply to gnd clamp circuits where the higher capacitance is
    tolerated. 8-12 Gbps I/O seldom use series resistance in my (perhaps
    limited) experience.
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