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Surge Capable Chip Resistors

Discussion in 'Electronic Design' started by D from BC, Feb 18, 2007.

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  1. D from BC

    D from BC Guest

    Problem: Smallest chip resistor that can handle a one shot pulse of
    170VDC for 20mS. R=22k. Pulse is square and doesn't repeat for hours.

    Research effort:
    I looked at a few datasheets without surge data..
    Then I found this:
    http://www.ohmite.com/catalog/pdf/tfs.pdf
    This is smd ?? It's got leads..
    This will work, but what else is there.?

    Any suggestions...
    D from BC
     
  2. Eeyore

    Eeyore Guest

    For pulse operation you really need to consult the manufacturer.

    Graham
     
  3. 170V and 22k is 1.3W, which is far over the power rating for
    small SMD resistors. You could use Panasonic's thick-film ERJ
    2010 (0.5W, risky) or 2512 (1W) types - they allow 2.5x over-
    power. Or better (and smaller if mounted standing up), use a
    through-hole with appropriate construction (not a film type).

    The Ohmite TFS part you found will be expensive and painfully
    hard to get (Mouser). It has substantial excess capability
    (3kV and 6J, vs your 170V and 26mJ). I wouldn't bother.
     
  4. Ken Smith

    Ken Smith Guest

    www.irctt.com/pdf_files/PWCR.pdf

    There are two problems with these:
    1) Actually getting them
    2) Paying for them
     
  5. Ben Jackson

    Ben Jackson Guest

    Do you need it to properly resist for that pulse, or just survive?
    Something like a ESD protection diode or TransZorb could probably
    save the resistor. EDS diode data probably doesn't cover over the
    us-range, but picking a random TransZorb datasheet on Vishay's site
    shows graphs into the 10s of ms.
     
  6. D from BC

    D from BC Guest

    It's needs to somewhat resist properly..
    R=22k+/-5k and the resistor can be nonlinear. (Oops left out these
    possible useful details..)
    The resistors function is just to limit current to about 8mA during
    the 20mS 170VDC pulse.
    I may be able to use a device other than a resistor but can it compete
    with 2512 footprint?? 6.4mmx3.2mm

    D from BC
     
  7. John  Larkin

    John Larkin Guest

    Probably 0603. Certainly a 1206 wouldn't notice such a pulse. The
    thermal tau is much longer than 20 ms.

    It takes a couple of seconds for 1600 volts to blow a several-megohm
    0603 resistor, suspended in free air.

    We recently tested various precision (0.05% thinfilm) resistors for
    power capability, measuring resistance and thermal imaging. Turns out
    that the pads are dominant heat dissipation mechanisms. Soldered to
    big slabs of copper foil (which I assume you can't do) an 0603 can
    dissipate about as much power as a 1206, half a watt at reasonable
    hot-spot temperatures.

    But why don't you just try it? Find the destruct limit and then back
    off by 5:1 or whatever. We do that a lot; sometimes we decide that we
    can overload parts a lot (common for passives), sometimes we decide we
    can't even run them at the mfr's ratings (typically semiconductors.)


    John
     
  8. D from BC

    D from BC Guest

    Thanks..I'll use the ERJ 2512 1W.
    If space allows, I might resort to paralleling chip resistors.

    But I'm curious if a current limiter transistor circuit can beat the
    2512 (6.4mmx3.2mm) footprint ?
    At this scale, maybe a transistor is better at handing a power pulse
    beyond it's continuous power rating compared to resistors..?.
    I don't know much about hot spots with pulse overloading devices.
    I'll guess resistor wins..
    I'll check this out later. For now a resistor is fine.
    D from BC
     
  9. D from BC

    D from BC Guest

    This is certainly food for thought..
    Testing time vs trusting data sheets.
    Heat sinking copper area vs resistor footprint.
    Survivability vs reliability.
    Interesting...

    I think this is going to be my plan...
    If I can't squeeze in a 2512 in the pcb design, I'll drop the chip
    size down to 1206.
    That way when pcb size goes down...risk goes up.. :)

    Based on the above...sounds like a metal core pcb could provide some
    good pulse overpower tolerability with tiny chip resistors.

    Thanks for the thermals.. :)
    D from BC
     
  10. John  Larkin

    John Larkin Guest

    Datasheets almost never give typical destruct limits, and usually
    don't give thermal taus or SOAR equivalents (except for some fraction
    of power transistors, and not even all of them.)

    And blowing up parts is fun.
    Really, this modest pulse power is zero risk on a regular thickfilm
    1206.
    In 20 milliseconds, most of the hot-spot heat will remain trapped in
    the part, so the extra foil won't help much. The thermal mass of a
    larger resistor will absorb a shot of joules better than a small one.

    John
     
  11. This can be true for constant power-handling, which has much
    higher energy-handling capability than the transient energy-
    handling case.

    The issue for short power transients, of course, is the thermal
    mass of the power-dissipating element, which can be a thin film
    without much thermal mass. Additional thermal mass located at
    a distance isolated by a thermal-resistance pathway, necessarily
    comes into play later. But 20ms is a fairly long time frame as
    these things go.
    Testing in your exact setup is not a bad idea, because the
    datasheets are often excessively conservative. But you do
    want to use a huge safety margin like 5x, as John suggested,
    because of production variability, and because there may be
    some long-term reliability issues that may not show up in
    short-term testing. I've seen too many failed SMD resistors
    in commercial equipment to be sanguine about this issue.
    I'd try 2010 resistors before 1206, or use two 1206 parts.
    That would be expensive! And likely not so effective for
    short very time scales.
     
  12. John  Larkin

    John Larkin Guest

    Any idea what the thermal tau of typical surfmount resistors is like?

    I did measure the thermal tau of a thinfilm platinum-on-ceramic RTD,
    which is probably a reasonable model for a surface-mount resistor.
    It's easy to fire a shot of energy into an rtd and then measure the
    resistance versus time, directly measuring the film temperature. I
    have that data at work and I'll try to find it monday/tuesday and
    post.

    I don't think we've ever seen a surface-mount resistor fail, unless
    some unusual thing overloaded it, or, in a few cases, it cracked from
    some stress. They seem pretty tough.
    I'd tend to agree. Heatsinking won't help much in 20 ms.

    John
     
  13. Gibbo

    Gibbo Guest

    I was just doing a *very* similar thing (190 volts, 100uS) and came
    across a datasheet in my collection. I can't remember where on the
    Philips site I found it so I've stuck up here.......

    http://www.smartgauge.co.uk/bits/misc/resistor_thick_film.pdf

    Some handy graphs.
     
  14. Ken Smith

    Ken Smith Guest

    How about:

    http://www.supertex.com/products/selector_guides/102
    and a resistor?
     
  15. D from BC

    D from BC Guest


    I just mentioned metal core pcbs because I make my own..
    I was just trying to find another excuse to use it.. :)

    So... it's not very effective to heat sink (by Cu trace or mcpcb) a
    2512,2010 or 1206 chip resistor in this case to handle a 26mJ energy
    burst..
    I'll go for that. :) Thanks..

    I guess my weak spot in heat dynamics is showing :)
    All I know is this:
    Heat has a propagation time.
    Materials have thermal resistance.
    Most materials increase in resistance with heat.
    Materials decompose if spot is hot enough.
    Heating an object is like charging a capacitor???
    At first I didn't quite understand what was meant by tau...It clicked
    in about 1 hour later... :)
    Gotta do the google heat review someday...
    D from BC
     
  16. Gareth

    Gareth Guest

    Welwyn have some data for short pulses here:

    http://www.welwyn-tt.com/products/pulse-data.asp

    http://www.welwyn-tt.com/pdf/application_notes/pwc_appnote.pdf

    --
     
  17. D from BC

    D from BC Guest

    I noticed a SOT 23 in the table..
    Found app note on:
    http://www.supertex.com/pdf/app_notes/AN-D25.pdf
    mmmm depletion mosfet I limiting...There's an animal I haven't played
    with yet..

    I'm going to dodge the Supertex reading& math for now...
    As posted here, I should be ok with a 1206 chip in my app..
    A SOT23 + small chip resistor at first guess has a larger footprint
    than the 1206.

    By the way, the resistor I'm looking for is part of a smps startup
    cct.. But I like my homebrew cct. more than the Supertex sol'n.

    Thanks...I might use DMOS someday...
    D from BC
     
  18. John  Larkin

    John Larkin Guest

    Join the club. Even if you know the theory, the practical issues -
    finding reliable data, modeling 3D distributed diffusive systems,
    estimating reliability vs temp - are still fuzzy. [envision global
    warming rant here]. At least most thermal conduction systems are
    linear, sort of.

    Yes. There's a direct similarity between thermal and electrical
    circuits. You can model thermal things with Spice, making the simple
    equivalences

    1 volt == 1 degree C

    1 amp electrical == 1 watt thermal

    1 farad == 1 gram aluminum

    1 ohm == 1 degc/watt

    1 second == 1 second

    which is good to about 5%. There's no thermal equivalent of an
    inductor. The bitch is that, while circuits are usually lumped,
    thermal systems are often distributed, so diffusion equations apply.
    Yuk.

    Thermal time constant, how fast a thing transitions to its
    steady-state temperature. Can range from nanoseconds to kiloyears.
    They estimate time of death by how much a body has cooled down.

    If a device has a thermal tau of, say 1 second, it will jump about
    1/10th of the way to its steady-state temp if you pulse it for 0.1
    second.

    This is excellent, and free:

    http://web.mit.edu/lienhard/www/downloadform1.html

    John
     
  19. John  Larkin

    John Larkin Guest

    Cool. Shows an 0805 being good for about 1.2 watts at 20 milliseconds.
    Sounds conservative to me. But the 1206 data looks strange.

    John
     
  20. John  Larkin

    John Larkin Guest

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