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Fast but small optocouplers?

Discussion in 'Electronic Design' started by Joerg, Sep 1, 2012.

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  1. Joerg

    Joerg Guest

    Hello Folks,

    So far I've never had size constraints and my staples were logic devices
    such as the HCNW137 series. Now I need one that ideally doesn't require
    a logic supply on the output, rise/fall times well under 1usec even when
    not fed more than 1mA into the LED, as small as possible. No more than
    about 0.150" wide and 0.250" long. Isolation around 2kV RMS is fine.

    This would be at the size pain threshold but as usual is way too slow:

    I went through scores of them and all in the several usec. Any
    sub-microsecond ones?

    Unfortunately the bases aren't pipe out on these so not speed tricks :-(

    What I want to do is ferry a fast PWM across a barrier, 100kHz range,
    needs to be somewhat accurate. I could do the transformer plus NRZ thing
    but that gets old and not enough space.
  2. Joerg

    Joerg Guest

    The Baker clamp is nice but all the tiny ones I found had only four
    pins, so no access to the base :-(
  3. Joerg

    Joerg Guest

    Well, I don't really want to build my own optocoupler :)
    I can, and thought about that because I've used this on medical gear.
    But even really little transformers are big when they come with
    isolation ratings. Plus I'd need more stuff to restore a clean DC, just
    a rectifier wouldn't be accurate enough. We are really space-constrained
    on this one.

    It's essentially hi-Z inputs (<100nA bias). I have to feed a voltage
    into those, between zero and about a volt, and it has to cross the
    barrier. I could also use a DAC but that's another part. Cost is not so
    important but real estate is.

    The signal itself is steady for a millisecond at a time but the change
    to another value must happen in tens of usec, that's why the PWM has to
    run fast. I can goose it a bit though, by having the controller step to
    full scale or zero for a while. But then the firmware guys will throw
    things at me because I'll keep piling on the work for them. This whole
    PWM is already a "By the way, we'd also need ..." job for them.
  4. hamilton

    hamilton Guest

    On 9/1/2012 11:15 AM, Joerg wrote:

    Googleing found this:

    Rise time
    VCC = 10 V, IC = 10 mA, RL = 100 Ω tr 3.5 μs
    VCC = 10 V, IF = 16 mA, RL = 180 Ω tr 1 μs
    Fall time
    VCC = 10 V, IC = 10 mA, RL = 100 Ω tf 14.5 μs
    VCC = 10 V, IF = 16 mA, RL = 180 Ω tf 20.5 μs
    Turn-on time
    VCC = 10 V, IC = 10 mA, RL = 100 Ω ton 4.5 μs
    VCC = 10 V, IF = 16 mA, RL = 180 Ω ton 1.5 μs
    Turn-off time
    VCC = 10 V, IC = 10 mA, RL = 100 Ω toff 29 μs
    VCC = 10 V, IF = 16 mA, RL = 180 Ω toff 53.5 μs
  5. hamilton

    hamilton Guest

    Never mind !!!
  6. Joerg

    Joerg Guest

    With photodiode ones I am doomed as well, then I need too many parts on
    the receiving end to turn the PWM into clean DC. No available space.
  7. Joerg

    Joerg Guest


    RL of 180ohms and IF 16mA is pretty unrealistic in real life, that
    sounds like written by marketing EEs. Plus it's still too slow.
  8. Jamie

    Jamie Guest

    Even a baker clamp as simple as it maybe can also be a challenge to
    implement if you're trying for a clean switch.

    The last one I did use high speed S diodes with low cap and a
    -0.5 supply for the emitter to get it down close to 0 volts and the
    use of RC lead and lags to remove parasitic on the corners.

  9. Joerg

    Joerg Guest

    No need for a one-shot, a comparator will do (NRZ scheme). But
    unfortunately transformers with isolation ratings are large.
  10. Joerg

    Joerg Guest

    Good idea. But unfortunately the usual small ones are still well above
    Well, you can goose a PWM if needed. Like the kick-down switch in some
    automatic transmission cars.

    But they also need to have a life :)

    We already do, and it's all full. Like our drill sergeant said "The day
    has 24 hours, and if that ain't enough you've still got the night!"

    They have to be retrofitted into a given space, no luck there.
  11. Joerg

    Joerg Guest

    Well, there is hope:
  12. Joerg

    Joerg Guest

    The usual 2kV RMS. It's not that I can't make a small one, done it many
    times. The issue is that on most such projects it has to come with "the

    Meantime I've found this from Sharp, requires a voltage and no
    uncommitted emitter but might do the trick here:
  13. Joerg

    Joerg Guest

    I'd appreciate a hint if they still have them. But this Sharp part
    should do the job. Best of all, it's only 80c a pop and half a bazillion
    in stock which is always a good sign.

    The PWM filter is done as well but the smallest 1mH inductor I can get
    is still 0.100" by 0.100". Big. Strange, one problems solved and then
    there's always the next one. Like that li'l yellow light on the dash of
    my car. But luckily it was just an evap alert caused by a slightly loose
    gas cap and I was able to reset it via OBD-II. The shop would surely
    have charged $50 plus. The OBD-II interface that just arrived was $12
    including shipping :)
  14. Joerg

    Joerg Guest

    They even have kids :)

    The one a couple miles from here has a horse ranch.
  15. Joerg

    Joerg Guest

    I've done that as well when there was no other way. NRZ on the other
    side, or DC clamper. But ... then you are on the hook to get it
    certified and that's a ton of paperwork. BTDT, a lot.
  16. Joerg

    Joerg Guest

    Good point. Other than Avago, Japanese companies have always been
    impressive when it comes to fast yet energy-efficient opto parts.
  17. Syd Rumpo

    Syd Rumpo Guest

    Loads of different types, eg

    But not opto...
  18. Joerg

    Joerg Guest

  19. Joerg

    Joerg Guest

    Yup, typo. The test circuit in figure 1 would not be able to generate a
    signal at all with 1.9ohms.

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