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Pulse Amplifier

Discussion in 'Electronic Basics' started by URPU36, Jun 12, 2004.

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  1. URPU36

    URPU36 Guest

    I have an Allen Automotive Distributor Machine that was built in 1958. The
    distributors then were all points. I need a circuit that will take the input
    from a modern electronic distributor(small magnet/coil generated signal, or
    Hall switch signal) and convert it to a square wave signal like points would
    do. There were pulse amplifiers available way back when, but are hard to find
    and very expensive. I would imagine the circuit would be quite basic. Thank
    You, Paul
  2. Bob Masta

    Bob Masta Guest

    Err, the output from points was anything but a square wave. Huge
    voltage spike (over 100V as I recall) and a lot of ringing. After
    all, the points were opening a big inductive circuit with a capacitor
    (err, make that "condenser") across them.

    The question is what your machine is actually doing. If it has a
    dummy ignition circuit in it, it may have its own coil and be
    expecting a contact-closure with a condenser across it, to
    generate a spark in the old-fashioned way. This might be
    tricky to emulate exactly, but you might start with an ordinary
    junction power transistor to replace the points. Get the
    drive for that with a simple low-voltage amp on the output
    of your distributor pickup. This is pretty much what the
    early "transistor ignition" circuits did, just used the points
    to fire the transistor. They didn't have MOSFETS then,
    but one might work. The reverse spikes from the coil
    kick-back will require a reversed diode across the
    transistor to keep it out of breakdown.

    Hope this helps!

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  3. Bob Masta wrote:
    A diode across the transistor will clamp reverse voltage, not excess
    normal voltage. If you put a diode across the coil ot prevent the 100
    volts or so normal spike, the coil won't produce a spark. The
    transistorized points I have seen just use a high voltage capable
    transistor so it can survive the high voltage the coil produces.
  4. I was going to point you toward a web page that showed how to do this,
    but found lots of crap. For instance, this site has so many errors
    that we could have a course based on only what it gets wrong:
    Here is an application note that describes the difficulties:
    Here is the data sheet for a chip that ties a hall sensor to a
    transistor ignition that gives you a good idea what all is involved:
  5. Bob Masta

    Bob Masta Guest

    John, it's the reverse voltage I was talking about. I'd think
    I'd want the transistor to be rated for several hundred volts.
    I was particularly thinking of the diode for use with a MOSFET,
    since their reverse breakdowns are pathetically low. Don't know
    if they would take well to a bolt of reverse current in breakdown.
    (Anyone know about this?) I think junction transistors may be
    less of a problem. (Can't seem to find any reverse breakdown
    data at the moment.)

    Best regards....

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  6. Mosfets have an internal diode junction between source and drain that
    may or may not have a particularly fast reverse recovery when the
    drain voltage again is in the correct direction.

    Junction transistors have a reverse current gain of beta that allows
    them to conduct somewhat in the backwards direction. The real danger
    with a junction transistor is that they will not have enough reverse
    beta to carry the reverse current before the emitter to base junction
    gets reverse biased to its breakdown voltage which is only 5 or 6
    volts. If that happens, the transistor will steadily loose gain.
    Another problem with reversed collector voltage is that the forward
    biased base to collector junction will tie this reverse voltage into
    the circuit driving the transistor, and may cause damage or
    misoperation of that circuit. But that depends on what that circuit
    consists of. If the driving circuit strongly clamps the base to the
    battery rail during the off time, the collector to base junction will
    perform the reverse diode function.
  7. John Popelish wrote...
    I think what Bob is talking about is the voltage rating of the
    MOSFET before avalanche breakdown, which is one thing limiting
    how far an inductor can flyback. You can get low-cost MOSFETs
    up to 1200V, so that's not much of a limitation. As far as an
    avalanche is concerned, FETs can handle much more than BJTs,
    with their second-breakdown, and SOA safe-operating-area limits.
    For those concerned, there are several easy ways to prevent
    breakdown of the switching transistor, whether FET or BJT.

    Another attractive choice these days is high-voltage IGBTs.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  8. Bob Masta

    Bob Masta Guest

    Actually, it was the reverse conduction that I was talking
    about. I have been thinking about how to use MOSFETS
    for high-power AC control, and this is the problem. It's
    easy to find high-voltage, high-current devices, but they
    don't block the opposite polarity at all. I was hoping to
    take advantage of low MOSFET losses and easy l
    oad-sharing, but it seems you need a rectifier in series
    with the device... so there goes all your power savings
    into the diode drop!

    In the case of the OPs problem, I was concerned that
    the transistor might not take kindly to the reverse spike
    from the coil transient, hence the reverse shunt diode


    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  9. Bob Masta wrote...
    Correct, all vertical MOSFETs (often called DMOS, VMOS, etc)
    have an intrinsic diode in parallel with the FET.
    If you wish to use FETs, the solution to this is to use two
    N-channel parts back-to-back (connect their sources together
    and their gates together). FETs happily conduct current in
    both directions, so when you apply gate voltage, the FET that
    has reverse current will be on, shunting its intrinsic diode
    with a low Ron, so you won't suffer any diode drop.

    .. --------+-|<|-+-+-+-|>|-+------- +/-400V and 2A rms max
    .. | | | | | (w/o any heatsinks)
    .. IRF740A ', ,-,' | ',-, ,'
    .. (2) | V |s | s| V | the diodes shown across each
    .. ----- | ----- FET's drain-source leads are
    .. PVI5033R ---, | ,--- intrinsic to all vertical FETs
    .. ------, | | |
    .. gate |-----+--|--'
    .. drive |--------' isolated gate-drive voltage
    .. ------'

    Excuse the poor ASCII drawing. One thing that's evident from
    the drawing is that for AC-line switching, the gate drive will
    have to be floating. That's the problem faced in a solid-state
    relay, or SSR, that uses FETs, and this is solved by using a
    stack of photodiodes driven by an IR LED. It's not fast (200us,
    etc) but it works well and gives 2500V of optical isolation.

    If you want to make an optocoupler SSR with your own choice of
    FETs, you can use IR's PVI5033R isolated 10V 5uA gate-driver,
    which includes a special fast shut-off circuit (see figure 4).
    DigiKey (and Newark) has these in stock at $5.75 ($5.45) each.

    For example, using a PVI5033R and two IRF740A MOSFETs you can
    make a +/- 400V, 1-ohm switch that will turn on within 12 ms
    and off in about 0.5 ms, using 5mA drive to the PVI5033's LED.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  10. Bob Masta

    Bob Masta Guest


    Thanks for the detailed reply. I wasn't aware that the
    FET would work in reverse... good thing to know about!

    My application (heating a low-ohm resistive load from
    the mains) requires an average current that could run
    to 40A. I'd be using zero-crossing control of the
    duty cycle to get this, but the peaks would still be 80A
    or more. Can I safely parallel devices to get the
    total current handling, and expect them to share
    nicely? Any other tips about applying your dual-N-FET
    method at these high currents?


    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  11. Bob Masta wrote...
    You can use larger FETs and parallel them, too, but you'd
    be better off with opto-coupler SSRs that employ triacs
    rather than MOSFETs. Such high-current solid-state relays
    are sometimes inexpensively found on eBay, e.g.,
    You may need to mount them on heat sinks, like this:

    - Win

    (email: use hill_at_rowland-dot-org for now)
  12. Bob Masta

    Bob Masta Guest

    Win, are you implying that I can parallel 4 of these
    25-amp SSRs to get 100A handling? I didn't
    think there were any good ways to parallel triacs
    because they don't share current like MOSFETS
    do. Also, I need to zero-cross duty-cycle modulate the
    load, so the devices need to be fairly fast. I need
    proportional control, not the sort of bang-bang
    control on (say) conventional ovens. Are SSRs
    fast enough for proportional control?

    One advantage of parallel MOSFETS is that
    they are way easier to heat-sink than one monster
    device. Especially since the "monsters" nowadays
    come in little power-tab packages... I have some
    40A triacs in TO-218 cases, and suspect
    it would be quite difficult to heat sink them enough
    to justify their ratings.

    Thanks for your advice...

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  13. Bob Masta wrote...
    Not triacs, and if you re-read what I wrote you'll see
    I said you can parallel switched MOSFETs. But that I
    recommended triacs instead.
    Certainly they can be, you have to read the datasheet.
    I think we can't give you futher advice unless you can
    give us much more detail about your load. E.g., a 400V
    MOSFET may have little problem handling high currents
    for a few tens of microseconds, but if it's a 40A peak
    pulse lasting for most of an ac half-cycle , etc., that's
    likely going to be too much for ordinary FETs, even if
    they are in parallel. If you're seeking ease of use,
    easier gate drive, etc., use IGBTs instead of FETs.

    You can get nice IGBT power blocks on eBay.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  14. Bob Masta

    Bob Masta Guest

    On 18 Jun 2004 05:22:52 -0700, Winfield Hill

    Win, thanks very much for your advice.
    The application is driving silicon carbide heating elements
    for a small electric kiln. The elements (2 in series) have
    a nominal resistance of 2.36 ohms at temperature, increasing
    to maybe twice that as they age (months to years of use).
    SiC doesn't like the surges of on-off thermostat-type control
    (as used in home ovens and most pottery type electric kilns),
    but is happier with proportional control.

    I have a monster tapped transformer rated to 25 amps
    for driving these now, but would like to replace manual tap
    adjustment with computerized proportional control. And
    I'm thinking it might be nice to run off the mains directly,
    to avoid the current limits of the transformer. (The elements
    are good for 40A.)
    WOW... 600V at 200A!!! This definitely looks like it could handle
    anything I could throw at it. But I'm still curious about heat
    dissipation. I'm assuming that just about any device is going to
    have a volt or two of drop at 25A. Even the MOSFETs would
    seem to have this on the reverse phase where the body diode
    is conducting. Do these IGBTs have subtantially lower drops, or do
    they need fancy cooling schemes?

    Thanks again for your help.

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  15. Rich Grise

    Rich Grise Guest

    If you have zero-crossing duty cycle control, why not
    use triacs? You can get them to practically arbitrary

    I notice a 40A 800V unit has a Vtm of 1.55; hmm - that's
    almost 50 watts - you might need a fan.

    Have Fun!
  16. Bob Masta

    Bob Masta Guest

    Rich, this is exactly the issue: You can get triacs with
    monster specs, but how to get the heat out of the
    tiny packages? That's why I was thinking about
    ways to have multiple devices, simply to spread out the
    heat. And as far as I know, only MOSFETs can be
    paralleled and share load current happily without
    hogging and thermal runaway.

    Thanks for the thought!

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  17. Bob Masta wrote...
    The datasheet says these 600V IGBTs drop less than 1.5V at 40A,
    or 60W max. The IGBT's thermal resistance spec is 0.14C/W,
    which means 60W dissipation will increase its junction temp by
    less than 10C above the insulated (!) thermal mounting plate.

    Compare this to the FDH44N50, a high-performance 500V FET. It
    has R_on = 0.25 ohms max (warmed up) for a huge 10V drop at 40A.
    You'd have to parallel six FETs to match Toshiba's MG200 IGBT.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  18. Rich Grise

    Rich Grise Guest

    I'm honored to bow to Mr. Hill's experience and stuff here. :)

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