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Bootstrap diode

Discussion in 'Electronic Design' started by Jon Slaughter, Sep 30, 2009.

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  1. Can someone explain how it works and/or give a schematic/circuit? I just
    know they are used in some high-side mosfet drivers for creating the
    high-side gate voltage but can't seem to find out any details.

    Seems to imply that such a configuration cannot work unless the low side is
    used often? Probably just as often as the high side?
  3. Joerg

    Joerg Guest

    Pretty much, no exercise -> no voltage. How often depends on the size of
    the cap and the hunger of your high side circuitry. Bootstraps are meant
    for complementary circuits where the whole chebang runs at some
    reasonable frequency and both high- and low-side are switched.
  4. Rich Grise

    Rich Grise Guest

    Yes - ideally, it'd be symmetrical, to avoid current imbalances and stuff.

    I kinda redrew the circuit in my head, and the "bootstrap" portion is just
    a bent half-wave voltage doubler. :)


  5. I guess then I can't use this for a full bridge since the low side drain and
    high side source must connected directly. I guess with a motor inbetween the
    cap will not be charged properly?
  6. legg

    legg Guest

    The bootstrap cap is normally referenced to the high-side source. As
    long as the source is pulled low, either by the low side fet or the
    load, the bootstrap cap will charge.

    In ~self-oscillating full-bridge situations, where the drive signals
    depend on the output state, care must be taken to ensure that the
    bottom fet does in fact turn on at least once before drive to the
    upper fet is required.

    This may complicate start-up and limiting situations.

  7. But with a motor the cap will not be pulled to ground or even close.
    My confusion is actually how it accomplishes. In the pdf link it has a
    diagram of a basic driver. It shows a level shifter and some logic then an
    omp amp.

    My guess it that it has some logic to level shift the signal to whatever
    floating hi and low it uses. The op amp is for drive capabilities. The op
    amp is floating as the cap supplies power to it. Essentially as Rich as
    mentioned it is similar to a voltage doubler. The cap is charged on the low
    side then "lifted" to the high side giving effectively 2Vcc from ground but
    puts Vcc across the op amp to power it and drive the high side gate.

    If a load is between the high side source and low side drain then the cap
    will not be charged up properly.

    I imagine I could create some additional circuitry to charge the cap up on
    the low side. Basically disconnect the cap and connect it to ground during
    the low side. This is so that the cap can be charged during the load side
    but is separate from the load. Of course this creates more problems than
    it's worth.
  8. legg

    legg Guest

    If the way you use it prevents it's normal function, you are
    misapplying the part. I think this is one of the reasons why app notes
    are published.
    In a full bridge there are four switches. The low side switches in
    each case are responsible for refreshing the bootstrap cap specific to
    the high-side switch on it's own side of the bridge. The load is
    applied between the switch pair junctions on each side of the bridge.

    Your reference app note describes a half-bridge driver; one side of a
    bridge is used.

    An asymmetrical bridge replaces two diagonally situated switches of
    the full bridge with rectifiers - it is capable of producing load
    current in only one direction.
    A 'leak' resistor between the source and the negative rail could serve
    this function, prior to the first high-side gate drive signal. Once
    any motor load current is established, its own flyback energy will
    ensure that the source is pulled low in the freewheeling interval.

  9. Joerg

    Joerg Guest

    It can be, depends on the load. If the low side FET conducts then its
    drain pulls towards zero. Then the load tugs on the source of the high
    side FET and pulls that to ground.

    Of course if the load is sluggish this falls apart.

    In that case I'd use a real motor driver or something wit a separate
    charge pump.
  10. Jamie

    Jamie Guest

    For what its worth..

    I see the circuit as a simple method of supplying the HS line, which
    is the Vee of the driver of the high side, a "common" point as long as
    the charge in the cap sustains..
    When the low side output is on, this will then provide the "COMMON"
    point for the HS line and also the cap how ever, the high side isn't on
    at this time so the only thing that takes place is the recharging of the

    The diode is there only to prevent back drain of this cap and supply
    voltage for charging.

    For larger caps with slower switching times, an external larger
    diode should be used to relieve the strain from the internal one, which
    could actually short it. This is something you do not want because it
    will then most likely take out the high side output due to insufficient
    drive at the gate which will put it into linear state.

    These types of drivers are only good for PWM system or some kind of
    pulsed system.. Most of the time, at idle, you keep the low side on to
    keep the cap charged. I guess the ideal thing would be to run it into an
    inductive load or have a inductor in series and have the circuit
    generate a very narrow PWM to maintain the cap. With the inductor in
    line, you shouldn't see much DC at the other end.

    If you were to combine two of these for a full bridge, you'd keep both
    low side outputs on at idle. Using the load isn't going to help much
    here since it may be reactive or a some what higher level of R>

    That's my take of it for what its worth!..
  11. I just see it as using the cap for temporary power to the op amp. It charges
    the cap on the low side(obvious when the low side mosfet is shorted) and
    supplies power to the op amp on when the low side is off.

    I have a similar one, the MAX5062(I think). It says it can be used for motor
    control but all the circuits show it as a half bridge. I've tried using it
    once and could run a small motor but trying to run a large one caused the
    IC's to burn up. Not sure why the motor size had anything to do with the
    IC's burning up... maybe inductive kickback somehow screwed it up?
  12. if you use a resistive load I don't see how this can happen except at the
    moment of switching. If the low side switch is on and the high side is off
    then the cap will charge up through the resistor which means it is not
    grounded... but if the cap is small enough it might approach ground. The
    higher the load the better as it will approach ground more. But then it also
    depends on the switching times and requires a dead time.

    I could use a small low side mosfet or maybe even a bjt that alternates with
    the high side. This allows the cap to charge every time the high side is
    used. Then just use the drivers for high side and use something else for a
    low side. This might cause some problems with inductive loads. Also if the
    low-side small mosfet is ever turned on without the motor's low side then it
    will take all the current and burn up rather quickly. (this shouldn't happen
    if everything is consistent though)

    They seem to be hard to find compared to these "half-bridges".
  13. Joerg

    Joerg Guest

    Well, of course you can't leave both switches on for too long. Then you
    are better off with a real motor driver.
    Not really:

    But I don't know what it exactly is that you want to do with it. This is
    an example for high power, there's cheaper ones for less amps.
  14. Ok? For bi-directional application you use two asymmetrical half-bridges...
    I have done that with the MAX5062 but with a much larger motor it causes
    them to burn up. The MAX5062 seems to be designed similar to the app note.
    In the datasheet for the MAX5062 it says it can be used for Motor control. I
    can't think of any other way than to use it asymetrically. It uses a boot
    cap and high side sensing and shows circuits similar to other half bridge
    drivers. it does not show one for a motor though.

    It works fine on a small 12V DC motor(<1A) but the chips burn up very
    quickly on a 15A 12V DC motor. I have no clue why this happens. I get bi-dir
    and speed control(I use a pot and a pic to determine the speed/dir then
    setup the pwm to feed the max's) just fine with small motor. I get dead IC's
    when connectin the large one. (even with added diode supression across the

    This was about a year ago and I got tired of going wasting so many chips I
    just gave up.

    This might cause some problems with the gate drive since the HS source is no
    longer floating. One would need to match the resistance to the capacitance
    so that it can be charged fast enough yet not pull the source down to ground
    to much reducing the HS gate drive.
  15. was just looking at

    which shows what I did but my load was a motor. I used two of them
    configured "asymmetrically" as to get bi-dir. I think I even tried
    disconnecting one side with similar results.
  16. But these have built in mosfets ;/ I've seen a few of these. I need more
    than 10A ;/ Why can't they just have pins for external mosfets? This one has
    the 2 low side gate drivers but has built in high side. I've thought about
    trying to parallel such devices because they have everything else I could
    want. But I think paralleling is probably a bad idea. If only I could use my
    own high side drivers then I'd have all my problems solved ;/

    Maybe if I can find those in 20A+ from 15-30V then I could use them too. Of
    course I would still like to use my own outboard mosfets or at least have
    the option.

    They do have

    Which I used two MAX5062's to get the same effect but the drivers burned up
    on a large motor(everything worked fine for a small <1A one). But if a
    charge pump is different than the boot strap then maybe thats the reason.
    Even though the MAX5062's datasheet say they can be used for motor control.
  17. Thanks though, I've seen some of hte "pre-drivers" which is what I'm
    basically looking for. Again I've done this with the MAX5062's and can't see
    why such a device would burn up just because I'm using a larger motor. The
    external mosfets are the only ones that should care about the side of the
    motor and they were just fine(even after the drivers burned up).

    I'm going to try some of those full-bridge pre-drivers though and see what I
    can get.
  18. Joerg

    Joerg Guest

    I never use Maxim so I can't say. One scenario for a burn-up is stray
    spikes finding their way into the chip. Traditionally such spikes will
    have 10 times the gusto when you go from a 1A motor to a 10A motor :)
  19. boB

    boB Guest

    In addition to this, if there was current in the motor inductance when
    the top FET turned OFF, the the corresponding bottom FET doesn't even
    have to come on in order for the boost cap to charge..... There will be
    a path to ground through the bottom FET's intrinsic, or additional,
    Drain-Source diode.

  20. Jamie

    Jamie Guest

    Since the HS line is most likely connected directly to the load, having
    inductive collapse would simply put a (-), not common, potential on
    the HS line which will increase the overall voltage supplied to the
    driver inside the chip. (Vee and HS). to solve that, put a heavy clamp
    like a
    zener/transorb across the Vcc and HS line. This should absorb the over
    voltage being generated.

    The other problem could be that you are not properly latching the
    low side on in time to clamp this energy. If you are attempting to
    do so, maybe you need a small cap to handle the clamp while the low
    side fet is clamping.
    If you are free running the circuit at times, then you are in trouble.
    Induction will kill your driver chip on the high side if you don't have
    some kind of clamp there.

    In most digital drives, especially AC inverters, you have the option
    of using a DB resistor which ties to the DC bus and goes through a
    switch that clamps the bus while the motor is not under load, other wise,
    you'll get over voltage on the DC bus due to the motor regenerating
    energy that adds to the bus through the protective circuits in the bridge..

    Something to ponder on.
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