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MOS optocouplers...

Discussion in 'Electronic Design' started by Arie, Apr 28, 2008.

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  1. Arie

    Arie Guest

    A rather strange effect puzzled us today...

    We use a MOS optocoupler to switch a RS485 terminator. The production test
    sometimes failed and indicated the terminator was on while it should be off.
    When probing the drive to the optocoupler, it sometimes switched on or off,
    depending on when or what we were measuring. Very erratic behavior.
    Comparing the production with the prototypes - the proto was fine, but it
    had another brand of MOS switch. Checking the datasheets of the old and new
    part - should be OK. Measuring again: sometimes we even found a slow drift
    of the ON resistance (while the LED drive clearly was off).

    Suddenly an idea hit us - the new coupler (Avago ASSR-1410, SMD) package was
    white, the older one was black. Now I've used white packages a lot, never
    had a problem with Sharp etc. But it just might be... OK, pulled over a
    halogen lamp: MOS went on. Cardboard over the coupler: MOS went off.

    Aha! It was just the interception of the outside light (cloudy sky) by our
    hands when measuring and probing that changed the operation! And the
    production test was done in broad light, not in the closed cabinets.

    Has anyone ever seen this behavior in MOS switches? I've use many types,
    never a problem. Must be something in that Avago design with not enough
    filler in the white epoxy.

    Arie de Muynck
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