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Why are SSR so slow?

Discussion in 'Electronic Design' started by Joerg, Aug 30, 2009.

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  1. Joerg

    Joerg Guest

    Hey guys,

    Got a situation where a solid-state relay would be great, provided it
    can avoid the build of a circuit board. Anyhow, why are these things so
    friggin' slow, msec range? Example:

    Ok, I do understand that it takes time for the photons to muscle around
    the gate of a big FET. But why can't they internally rob and store a wee
    bit of energy to make the following transitions zippier?

    I have to do a 100Hz PWM at a couple of amps and with this sort or
    "performance" SSR relays don't cut it, plus would probably go phut ...
  2. Hammy

    Hammy Guest

    I did some experimenting with the vo1263

    And the LH1262C.

    I found if you give them a jolt near there max peak rated current it
    will improve the on time. You then have to bring the current source
    back down to a reasonable continuous level to maintain the desired Vgs
    over temperature range. I wound up using a pulsed current source then
    switch it out and use a temperature controlled current source.

    Depending on the duty cycle you could do something along those
    lines,minus the temperature controlled CS.The appnote below has other
    suggestions for improveing rise/fall times.

    Your SSR has integrated FETS the ones I used were for driving external
    FETs but it should work the same.

    It all wound up being to much of a pain in the ass, so I just built an
    isolated supply using a cheap murata pulse XT to power a PIC10F and
    discrete zero crossing detector and a floating FET driver.

    Here is an application note I found useful from Vishay.
  3. Joerg

    Joerg Guest

    That's a teeny one, I need several amps at up to 40V.

    That one I don't know. National's site didn't find it either.

    The jolt would be a problem in my case. What I want to try is replace a
    small board with a LabJack and PWM some sort of electronic switch with
    it. If I have to provide separate opto-drivers plus maybe a buffer and
    whatnot I might as well design the whole circuit as usual.

    The duty cycle would need to vary between 50% on to 95% on.

    Nice app note, although they should get into the habit of running a
    spell-checker before release :)

    All those methods they present are great but why not do something more
    elaborate? For example, run a little charge pump in there that builds up
    a voltage in a small external ceramic cap that gets hooked up via a 5th
    contact and input-ground. It would build up charge during the first
    on-phase so that all the following transitions would be fast. Should not
    be a big deal inside a chip.
  4. Joerg

    Joerg Guest

    I had hoped that by now someone would have come out with a device that
    has a charge pump, so that at least after one or very few high phases
    some more oomph would be available. But ...

    Actually not, this one can cost $10-$20 for the SSR function. But if I
    need to supply some regulated 15V or so at tens of mA I might as well go
    discrete like I used to. Simple 4N-whatever, then a MOSFET driver, done.
    I wanted to avoid the extra power supply. Guess that ain't in the cards.
  5. Jon Kirwan

    Jon Kirwan Guest

    On the first page, right side column, I read "...the illustration in
    Figure 1 should speak for itself. One can see a mechanical relay with
    a current handling capacity of 25 A and a power MOSFET with a current
    handling capacity of 30 A side by side."

    I can find Figure 1, just fine. On page 2. But I certainly don't
    "see" a mechanical relay and a power MOSFET side by side, there.

    On page 2 they say, "The detector or output side of the VO1263 can be
    thought of schematically as a matrix of series photodiodes similar to
    what is ilustrated in Figure 1."

    Now that I do see in Figure 1.

    I wonder where the first "Figure 1" went to.


    P.S. I see some nice "product placement" of LeCroy on the scope
  6. Jon Kirwan

    Jon Kirwan Guest

    Um. Figure 16 doesn't have a Q6. You are talking about Figure 17.

    And in Figure 17, I would have drawn it this way (including what then
    appears to be a question to me):
    That gets across things very quickly. There are two voltage lines
    with separate current limits set via Q3/R3 and Q6/R4 and the
    collectors are wire-OR'd to the common gate line of Q2/Q4. What then
    appears to be a question is that Vdd must be on the same side of
    ground as Vcc -- Q4 is NMOS and Q6 is NPN, just like Q2 and Q3,
    respectively. I had imagined from their use of "bipolar" and the
    presentation of Vdd 'below ground' on their diagram, that Vdd would be
    on the other side of ground.

  7. Jon Kirwan

    Jon Kirwan Guest

    The whole thing looked screwy, so I'm not surprised.
    Well, I hope so. Otherwise, I'd need to improve my drafting skills.
    I must have missed that. Where?
    I'm still wondering about Vdd, though. Being a mere hobbyist with
    sorely limited experience, I can't say for sure. But it sure looks
    like they have just used cut and paste and got it all wrongly said.

  8. Hammy

    Hammy Guest

    Well yes it's not the most intuitively structured pdf. It does provide
    some useful information. Compared to inductor datsheets it's a wealth
    of information.

    What I did was drive one channel with a 100mA pulse for about 0.5mS or
    so then had the current source take over to maintain 11V gate to
    source. I also used the second channel to drive a small NTS4001 or
    2n700x FET to snap off the primary fets in uS time. In the pdf they
    use a passive approach for discharging the FET Cin, or a JFET.

    I did this all a while ago so my times and currents probably aren't
    100% accurate.

    There were a few things I didn't like about them though they require
    about 8 to 15mA CC to maintain 11Vgs (Very temperature dependent) and
    if you want snappy performance for even moderately sized mosfets
    (40nC) (snappy performance for an SSR is on times below 1mS) the drive
    cct can get a bit complex and require to many parts. This is why I
    just wound up building a small isolated supply it was cheaper and used
    only 2 to 3mA.
  9. Mike

    Mike Guest

    Do you have any examples of waveforms posted on your web site?

    I needed a fast optocoupler a while back and analyzed all the different
    configurations in SPICE. Your totem pole configuration was one of the
    slowest. I suspect it was due to the top element acting as a weak current
    source instead of an emitter follower as you might conclude from looking at
    the schematic.

    If you have any speed comparisons vs conventional hookups, this would be an
    ideal time to show them.

    The circuit is so simple it shouldn't take more than a few minutes to set
    up and photograph. Since you are the inventor, we should expect the results
    to be the best possible with that configuration.
  10. Joerg

    Joerg Guest

    Normally between 20V and 40V, must be able to go to about 60V without

    Oh yeah, I can steal almost all the current I want. Well, as long as
    it's less than half an amp or so.
  11. Tim Williams

    Tim Williams Guest

  12. Joerg

    Joerg Guest

    Sure, I can do that as well or I should give back my degree :)

    But: I was trying to avoid a circuit board since I already have a device
    that can drive the signal, at 3.3V and a few mA. It would be nice to be
    able to just lash up a SSR, sans other electronics. But I guess not :-(
  13. Joerg

    Joerg Guest

    Why? I love'em.
    True, but: In most power applications (think switch gear closet) you
    don't need to switch in 100nsec. But you do need tens of usec. A SSR
    that yawns for a millisecond inside the linear range is fairly useless
    to me because it won't be fault-tolerant. If a control signal rattles
    for a while and it switches back and forth you'll hear a loud bang and a

    It would (usually) be no problem to pipe the control voltage also to the
    cabinet. But SSRs don't have a 5th pin for that. Beats me why not.
  14. Joerg

    Joerg Guest

    Yep, I've switched one at 40kHz and needed a mere 20mW. All you need is
    a little energy for the swing, and only for the positive going transitions.
  15. Joerg

    Joerg Guest

    Cost? LAN transformers are dirt cheap and you can get a four-pack for a
    song. Even with common mode chokes in there. The rest, well, that's what
    engineers are for :)


    Hey, you still use a dot matrix printer? I thought they were almost extinct.
  16. Tim Williams

    Tim Williams Guest

    Deep Friar: a very philosophical monk.
    I've got one with a similar aspect ratio,
    although it doesn't use nearly so many transistors.

    Too bad it's been blowing transistors. Something about temp compensation.
    Once it passes about 40C it's downhill, even with a small fan. Weird thing
    is, it's got a Vbe multiplier in there, it should be perfect. Do 2N3904s
    have less tempco than darlingtons or something?

  17. Joerg

    Joerg Guest

    Ok, yeah, at your typical quantities you can't be picky about pricing.
    At least not for custom. But if you do high volume and have places like
    Xfmrs in China wind the cores, different thing. The topper was a
    single-winding toroid for a switcher I designed. Found a COTS version
    for 60c or so. Client inquired anyhow and lo and behold they lined up a
    place in Taiwan that made them for 35c. Tested, bagged, shipped, no
    complaints in years.
    Your sump pump has its own room? Maybe it even has a gold-plated
    discharge pipe :)

    I am amazed you can still get ribbons for them. We find it increasingly
    hard to buy cassette tapes for our church.
  18. Tim Williams

    Tim Williams Guest

    Obviously, it's not falling at the same rate as the darlingtons, which is
    what Phil observed four posts ago.

    The problem is overcompensating it so it's thermally stable. Maybe a CCS
    pulling a constant voltage into one of the Vbe divider resistors would work.
    That would increase gain required for a given Vbe offset = more tempco for
    the same voltage.


  19. Another reason that wasn't mention was the price.
    They are too expensive anyway. As relay replacement
    their advantage is the infinite number of on-off
    cycles. And most are made as zero crossing switch
    When we're talking about power, don't forget the
    rather hefty voltage across them, in the order of
    1.5V at least. There is nothing like 10mOhm or so.

  20. For rather modest values of "infinite" in many cases.

    Best regards,
    Spehro Pefhany
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