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Slew Rate Booster

Discussion in 'Electronic Design' started by Haude Daniel, Mar 28, 2007.

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  1. Haude Daniel

    Haude Daniel Guest

    For the umpteenth time I'm thinking about a way of replacing horrenduously
    expensive commercial piezo motor drives that we use with our STMs with something
    cheap and simple.

    The motors are driven in a slip-stick fashion using a waveform like the one
    shown here:

    (To reverse the direction of motion the polarity needs to be inverted).

    The S-shaped part of the curve isn't terrible critical. What is
    critical is the steep slope between the slow parts. With a 20n capacitive load,
    this is almost 1A of current at 44V/µs

    The obvious approach to this would be to follow a low-voltage waveform generator
    with some big-ass HV amplifier. I've played around a bit with stuff along the
    lines of Fig. 3.75 in AoE but never got it anywhere near the needed performance.
    Besides, the thing needs to be short-circuit proof in both polarities. The split
    supply is a requirement because I need ~400V swing but a maximum voltage of
    ~200V across the piezos.

    So I thought if it weren't possible to exploit the fact that we know exactly
    when the steep rise is supposed to happen, and to add a "yanker stage" to the
    output of a slow and cheap HV amplifier like this:

    +250V supply----+--V.Reg.------+---+ +220V
    | | |
    | D 1uF
    | +-G |
    | | S GND
    | | |
    +------+ +----+---+ | |
    |wavefm| | | | |
    |gen. +---------+ HV amp +---R1-|---+---R2----+
    +---+--+ | | 1k | | 100 |
    | +----+---+ | | |
    | | -+ | CLoad
    trig _____|_ Gate__/ | 1n..20n
    | | | Drv. \ D |
    +--L.Shift--+ | ---G GND
    | S
    | |
    -250V supply-----+--V.Reg.------+--1uF--GND

    The slow amp needs to be isolated from the yanker in some way, otherwise the
    yanker would probably upset the feedback loop. For this I added R1.
    The idea is that once the rising slope comes along, the top MOSFET essentially
    shorts the output against some power rail whose voltage is adjusted to the peak
    of the amplified waveform. The FET is kept on for some time until the slow amp
    has caught up and then turned off, so that the HV amp can take over for the slow
    part. A mismatch between the yanker supply voltage and the amp peak output of a
    few volts wouldn't matter.

    There could be intrinsic short-circuit protection by foldback-limiting the
    yanker supplies to the few mA of average current that this thing actually needs.
    In case of a short circuit the voltage of the 1uF storage caps would collapse,
    and the HV amp would protect itself.

    Before I start thinking about actually implementing this beast I'd like to hear
    if anybody thinks that there's anything fundamentally wrong with it, and if
    there are better ways to solve the problem.

    And an adjustable amplitude would be great as well... nah, let's leave it.

  2. MooseFET

    MooseFET Guest

    Things like this have been done in the past and work fairly well so
    long as you keep them from crashing into the rails.

    In the usual form of this, the circuit works as an inverting

    The slow amplifier has to be made such that it doesn't attempt to
    clamp its inverting input to ground. It also needs to have a high
    open loop output impedance.

    The fast boosting section tends to make the output do what the input
    commands it to do, so the error signal on the slow amplifier's
    inverting input is greatly reduced.

    If you are careful about preventing oscillations, you can use a couple
    of comparitors on the signal at the inverting node to develop the
    trigger signal for the booster. In your case, I think you could
    simply trip oneshots and a bit of interlock stuff from the
  3. John  Larkin

    John Larkin Guest

    That looks OK. The gate drive could be some dirt-cheap ASDL

    But how about this:

    | s
    | |
    | |
    | |
    in------ wimpy hv opamp------------+---R--+-------+------out
    | |
    | |
    | |
    | s

    so the fets help when the current gets big enough to drop a few volts
    across R. Some sort of compound feedback would probably be needed, but
    that shouldn't be hard. As you suggest, current-limit the hv rails to
    protect the fets. This does provide your "adjustable amplitude."

    For more interesting dynamics, R could be a true current limiter, like
    some depletion-mode Supertex fets.

    The "wimpy hv opamp" could of course be based on my optocoupler trick,
    so the whole thing becomes about a dozen cheap parts.

  4. Haude Daniel

    Haude Daniel Guest

    Looks good.
    You mean, some feedback around the HV amp only, and some more around the
    whole thing?
    Yeah, but the wimpy amp now (as opposed to my original idea) needs to
    have the full 40V/Âs slew rate.

    Actually I have the schematic of a commercial unit before me.
    They do use some optocoupler scheme for the positive drive,
    but I haven't fully understood it. They have the gate of a
    BUP37 IGBT riding at 14V on top of the output voltage, and a
    Darlington NPN between the IGBT's emitter and the output whose
    base is controlled by the OC.

  5. Fred Bloggs

    Fred Bloggs Guest

    If you can't adapt the AoE Fig 3.75 ckt to fit your needs, then you
    don't know what you're doing. Take your question to SEB...
  6. Haude Daniel

    Haude Daniel Guest

    If it is that simple to adapt the circuit to do >40V/µsinto
    20nF with bipolar short-circuit protection, I'll happily expect
    your suggestions to appear on sci.electronics.basic.

    Thanks for your help.

  7. Jim Thompson

    Jim Thompson Guest

    Not to mention that Fig. 3.75 doesn't really address, even with
    modifications, what Daniel specifically needs to do.

    Daniel doesn't simply need a power booster.

    It might be more accurately called a "reset".

    ...Jim Thompson
  8. John  Larkin

    John Larkin Guest

    Something like that. The local fb would be fast, and the global fb a
    bit slower, especially if the load is capacitive.
    Then ignore my suggestion! If tha driver amp isn't already this fast,
    the follower can't help, and you will have to go to the pulsed
    booster. That has interesting dynamics.
    Sounds strange. Post to abse?

  9. John  Larkin

    John Larkin Guest

    Fred, of course, doesn't want to help. He wants to demonstrate that
    we're stupid and he's smart. So far, it's not working.

  10. Fred Bloggs

    Fred Bloggs Guest

    The main thing he needs is voltage gain, and that 40v/us seems
    suspiciously close to a standard 35v/us interference level. It is best
    to just adapt the AoE circuit with suitable protection and current capacity.
  11. John Larkin

    John Larkin Guest

    But it's more fun to design stuff.

  12. Haude Daniel

    Haude Daniel Guest

    An edited portion of the ckt can be found here:

    Note the line labeled "boost". This goes to a logic output in the
    wave generator section to help the high side driver to get its
    ass in gear on a rising slope. The negative side doesn't seem to
    need this.

    I omitted the output current monitor / overcurrent shutoff portion.
    And I can't make up my mind if the circuit is ingenious or stupid.
    All I know is that the device has been working flawlessly for some
    15 years now and has taken lots of abuse.

  13. Haude Daniel

    Haude Daniel Guest

    Actually I'm beginning to like this circut. I've figured out hoe the
    negative side works:

    When U3's output is at or above 0V, R21 feeds 4.5 mA through D11 and D13,
    causing -490 V to appear at Q5's gate which is close to the -488 V at
    Q4's gate, so Q4 and Q5 are off.

    Whan U3 goes negative, its output starts robbing about 1.1mA/V from this
    current, which makes Q5's gate move towards the negative rail with 4.2 V
    per volt change on U3's output. I need to look up the subthreshold
    characteristics of the IGBT, but at any rate this turns the Q5 and Q4
    cascode-like combo, letting the output go negative. The R13/R28 divider
    provide a little negative feedback to Q3's base, probably for stability.

    In the positive leg, Q1 and Q2 form a cascode driven by U1, riding on
    top of the output voltage. D4 and D5 provide fast turn-off of Q2 on a
    falling slope, and they make sure that Q2 can never turn on when Q4/Q5
    are on.

    At first I wondered why they would use IGBTs instead of MOSFETs. But it
    turns out that 1200V MOSFETs are rare (and may be even rarer back in
    1990), but a big, cheap, wildly overrated IGBT like the FGA15N120 is easy
    to get.

    I think I'm gonna prototype this. But first I need to come up with some
    good SPICE models for optocouplers and IGBTs. Is Mr subthreshold, Win
    Hill, reading this?

  14. Guest

    Actually, fig 3.75 can be used to address the issue. Until
    last year we always made our own stick-slip piezo stepping
    drivers for our STMs, basically using the MOSFET totem-pole
    drive scheme shown in fig 3.75.

    The problem with using a "reset" approach is it assumes
    you want to take the piezo all the way to the rail, which
    you may not want (although some commercial units do this).
    We wanted ours to be fully programmable.

    What's needed is a high enough current capability and a fast
    enough slew rate. Fig 3.75 can easily operate at even 1A or
    above currents, because the pulldown and pullup devices are
    power MOSFETs. The Institute's AMP-10 circuit is an enhanced
    version of fig 3.75 with bipolar capability. An early version
    of the AMP-10A that I made in 1991 had a 40V/us slew rate,
    the AMP-10A-4 had 60V/us, and some later ones were faster yet,
    delivering high currents into the modern lower-voltage higher-
    capacitance piezo actuators.
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