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Balanced grounding between devices & fault

Discussion in 'Electronic Basics' started by Shmuel Davis, Aug 12, 2004.

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  1. Shmuel Davis

    Shmuel Davis Guest

    I have designed a communication device that is being powered by 24V.
    It has two types connectors. The first provides the power and control
    signals, while the second provides communication to the outside world.
    The device is contained within a metallic box that is completely
    isolated.

    It was found that some of the devices failed at initial power on, or
    due to a later cycling of the power. Those devices were found to pull
    the 24V DC rail down to < 5V DC and would draw in excess of 200mA.
    There is a possibility that the device fails to handle the system
    inrush current.

    In my analysis we found there to be a short between the +3.3V supply
    and GND. When the +3.3V LDO and the Altera gate array that was driven
    by the +3.3V was replaced, the device functioned correctly.

    On the input to the control signals, we are using a National
    Semiconductor LM2676 3A Step-Down Voltage Regulator to step down the
    voltage to +5V. We are then using a National Semiconductor LM3940 LDO
    Regulator to provide us with the required +3.3V for the Altera FPGA.

    All the communication signals from the outside world are floating and
    are passed through transformers and Holt transceivers that are powered
    by the above +3.3V supply.

    For those devices that the fault was found, the shield from the cables
    that provide communication to the outside world were not connected to
    the GND of the device. I have two devices where this connection was
    made and the fault did not occur.

    Any suggestions of might be the source of the fault ? Suggestions have
    been made that a GND imbalance between my device and other devices it
    is connected to, may have caused damage to the components of my device
    (possibly large currents into my device). However I don't understand
    how this can happen when there is no connection between the other
    devices and the GND of my device via the cable shield ? Can spiking
    occur across a SM0805 footprint that could have caused damage to the
    gate array if the voltage difference gets high enough between the two
    grounds ? Also another point: In a normal connection where the cable
    shield does balance the grounds between devices – why doesn't this
    balancing cause large currents to flow between the devices ?

    Does anyone have any other suggestions ?
     
  2. While you are looking for the cause of the problem, send the failed devices
    to Altera for a failure analysis. They might be able to point you to a
    specific pin.

    Good luck.
     
  3. This sounds like there is a destructive EMF developing between the signal
    lines and ground at power-on. Mismatched transmission line effects? Can you
    add a resistor in the lines and a TVS? Also, if it works and does not fail
    with the shield connected, why not leave it connected?
    It could cause large currents to flow if the source impedance of the
    impalance is low enough. However, the source impedance of the imbalance
    could be low enough to destroy a semiconductor without causing "large"
    currents to flow. What is the current due to ground imbalance?
     
  4. Ken Smith

    Ken Smith Guest

    Imagine a walk across a carpet and touch the box. A spark jumps from my
    hand to the metal box. Where does it go from there on its way back to the
    earth?
     
  5. Shmuel Davis

    Shmuel Davis Guest

    How would such an EMF cause this type of damage and how would your
    suggested solution solve the problem ?
    What is a TVS ? What would be the connection of this resistor/TVS -
    from shield to GND or some other form of connection (The signals on
    the bus are a differential pair).
    This was going to be one of the solutions. However, I wanted to try
    and come up with some sort of theory of what was causing the fault. I
    was also worried that someone had suggested that if we do short the
    shield to GND then we may cause large currents to flow, doing more
    damage than good (even though the devices with the jumper in it did
    not show this fault !)

    Is there any way to measure these phenomena you mentioned above ? We
    do not have access to the system that the devices were placed in so we
    can't replicate the precise circumstances under which the fault
    occurred (we are not able to measure the "current due to ground
    imbalance" and I was only suggestion that this might be a cause of the
    fault). Is there any way within an "artificial" labratory environment
    for us to narrow down what the source of the fault is (and thus the
    remedy ?)
     
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