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Instability Problems with UDN Series Power Drivers?

Discussion in 'Electronic Design' started by W Letendre, Feb 9, 2006.

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  1. W Letendre

    W Letendre Guest

    Shipped a bunch of robot controllers to a Far East customer, using opto
    isolators driving UDN2982 high side drivers to light up customer I/O
    panel. Rig worked fine here, using our own I/O panel for test, but, at
    site, see bags of noise on customer's I/O lines. Noise persists, even
    with our control circuitry powered down, as long as customer is
    providing 24 volts to isolated side of our I/O PCB. Isolated side I/O
    cisrcuit as shown:

    +24 VDC
    ___
    |/------------| |
    Opto | |/--*-----------*
    |\>-------| |`.|
    |\>----------| >------- Out
    PS2806 |.'| UDN2982
    |
    -----
    ---
    -

    O'Scope doesn't show any noise to speak of on customer 24 VDC power
    supply line, but, can eliminate noise problem with RC filter between
    PS2806 isolator and UDN2982 driver. Curioser and cusrioser! Anyone here
    seen or heard of unusual noise susceptability in these chips? Have used
    them, off and on, for years as alternative to ULN2806, for customers
    requiring high side drive...

    W Letendre
    Electrical Engineer
    Danaher Motion
    Precision Systems Group
    7C Raymond Ave
    Salem, NH 03079
    USA
    URL: www.DanaherMotion.com
    eMail:
    VOX: 603.893.0588 X246
    FAX: 603.893.8219
     
  2. Maybe coupling between input and output of optoislator, due to layout
    issues or?

    Suggest earth on the nonisolated side may be part of the problem.


    Best regards,
    Spehro Pefhany
     
  3. W Letendre wrote...
    I'm looking at those two "floating" nodes and wondering
    what pulls them down so the OFF condition has some RFI
    noise immunity. The '2982 has a rather high Zin and no
    internal pulldown resistor. The PS2806 base also has a
    high Zin, and no internal pulldown resistor, but at least
    its area is small (I prefer optocouplers that give you
    access to the PT base so you can add an "off" bias-current
    resistor the PD has to overcome before the part comes ON).

    OK, so at a minimum I'd add a nice 3.3k or so here,

    .. PS2806 |
    .. |/------------+
    .. ---| |`.|
    .. |\>-+------| >------- Out
    .. | |.'| UDN2982
    .. '--/\/\---+
    .. 3.3k |
    .. gnd
     
  4. Darlington opto's have a very high current transfer ratio, so there
    might be a bigger leakage current than you expect, and already raising
    the UDN2982 input to a dangerous level. I'd place a resistor between
    input and ground. 10K perhaps.

    And delivering 24V into the UDN2982 when the opto is on, seems a bit
    much as well. At an estimated 4mA, that is 100mW per input. Or 800mW
    for the entire device. I'd put a 5V regulator in it, to feed to high
    sides of the opto's.
     
  5. Gack. Look at that input circuit. Yes, a resistor to ground on each
    input.

    http://allegromicro.com/datafile/2981.pdf
    Good point.


    Best regards,
    Spehro Pefhany
     
  6. W Letendre

    W Letendre Guest

    Did try adding current shunt resistor, as well as series voltage drop
    resistor (to limit UDN ship input voltage to legal max of 15 VDC)
    without any relief from noise problem. Capacitive bypass worked, but,
    damned if I know why. This is circuit that appeared to work:

    +24 VDC
    _____
    |
    |/------------|----------------------------*
    Opto | |/--| |
    |\>-------| |`.|
    |\>------/\/\/\----*----------| >----- Vout
    PS2806 4.7K | |.'|
    | | UDN2982
    ----*----- |
    | | |
    | | -----
    < ----- -
    10 K > -----
    < | 0.1 uF
    | |
    | |
    ----*-----
    |
    -----
    ---
    -

    Seems to require that cap to isolated GND; still get noise without it.

    W Letendre
     
  7. Tell us more about the layout. How far from the PS2806 to the UDN2982?


    Best regards,
    Spehro Pefhany
     
  8. W Letendre

    W Letendre Guest

    Lateral seperation ~ 15 mm. Current layout uses DIP parts; revising to
    SOIC, as some of the parts on PCB are being discontinued in through
    hole.

    Board is 4 layer, with internal split plane, internal and isolated
    grounds. Split between planes runs down center of opto isolator chip.
    Trace placement rules are 10 mil trace, 10 mil seperation (0.25 mm,
    give or take). Would have thought that was due diligence against
    coupling problems on PCB, but, here I am scratching head and hoping not
    to be on an airliner to South Korea next few days...

    May be worth noting that the internal and isolated power supplies are
    distributed on trace layers, not as planes. Also, bypass caps were not
    placed so generously as I would have liked; layout artist omitted
    bypass altogether on isolated power supply, which has been object of
    considerable (so far fruitless!) suspicion so far.

    W Letendre
     
  9. Well, it sounds about perfect to me, even with that high-impedance
    input node, and you say it doesn't solve the problem with 10K on the
    input. <cough> umm.. despite what you say below, you don't happen to
    have a *power* plane that isn't split or something
    blindingly-obvious-in-retrospect like that? If you hold the bare board
    up to the light you can see through it where the optos are, right? And
    the power planes (if any are present) are not joined somewhere else?
    I'm just thinking that power planes are normally specified as
    negative, so it's possible the board could have an internal floating
    plane by default.
    Sounds like it ought to be sortable remotely. But, hey, I'd be happy
    to go fix it for you for free (out-of-pocket costs only). Faster than
    you can say "kamsa haminda". ;-)
    None of that sounds serious enough to be causing the problems, with
    the possible exception of that power supply. Maybe it has junk on it
    that's getting coupled through the darlingtons. A regulator would deal
    with that potential issue.


    Best regards,
    Spehro Pefhany
     
  10. W Letendre

    W Letendre Guest

    Hmmmm. Gettin' me paranoid, now. Took a look at bare PCB, to see if fab
    shop made internal planes wrongly. They look OK....

    RC filter solution seems to work, and may be what I send out to field,
    but, it would be a comfort to know why! Still, won't miss going to
    Korea. Lovely country, but, the fabs there all cut their cleanroom
    "bunny suits" for guys 10 or 20 cm shorter than me. Nothing like an 8
    hour shift wearing a uniform with a built in "wedgie" to make one
    question the joy of traveling!

    W Letendre
     
  11. Guest

    The oscilliscope might not be fast enough to follow a really fast
    narrow spike, big enough to avalanche the input Darlington in the opto
    - this could dump enough charge into the base junction of the
    opto-transistor to (briefly) turn on the UDN2982 driver.

    IIRR opto-transistors have rather high base-collector capacitances, and
    your might be able to store quite a lot of charge.

    You could also conceivably have a healthy flash X-ray source in the
    vicinity, which could also dump charge carriers in the input transistor
    ....
     
  12. Interesting idea.

    Personally, I avoid using photo Darlington optos in this sort of
    situation- you can readily get 100%-200% min CTR phototransistor optos
    and Darlingtons are pretty sluggish. They're a good compromise over
    the photodiode+transistor 6N136 type when only modest speed is
    required.


    Best regards,
    Spehro Pefhany
     
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