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Discussion in 'Electronic Design' started by Jim Drew, Jan 3, 2006.

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  1. Jim Drew

    Jim Drew Guest

    I have a question about driving the gate on standard MOSFETS. I am very
    familiar with building h-bridge controllers using the typical 4 MOSFET
    h-bridge configuration, but my source voltage has always been a maximum
    of 12 volts. So, tieing the gate through a resistor to the positive
    rail for the P-channel fets (and driving them low with another
    transistor to enable the them) has always worked just fine. Now, I have
    an application where the voltage is an unknown between 7.4v and 65.0v.
    I know that the VGS max on most devices is +/-20v, so anything higher
    than this is going to kill the MOSFET. Does anyone have any idea on
    what should be used to drive the gate that will work within that voltage
    range? I am not aware of any gate drivers that will work that high. I
    was considering something as simple as zener diode/resistor voltage
    regulation to get the voltage to a usable level. For that matter, does
    the gate voltage really need to be any higher than about 10v?

    Thanks for any insight!
  2. Tim Williams

    Tim Williams Guest

    8V-ish zener from gate to source. Best results would be with a turn-off
    resistor parallelling the zener, and a constant current drive (open
    collector with emitter degeneration or switched current mirror). Resistor
    depends on how fast you want it and how much current you can spare. You'll
    need some resistance no matter what, since the gate capacitance isn't going
    to fall too quickly through a zener in the "off" region, especially with
    leakage current from the driver keeping it on.

  3. Fred Bartoli

    Fred Bartoli Guest

    How about HIP4080/4081? Plus you'll only need N mosfets.
  4. Jim Drew

    Jim Drew Guest

    Here is another delima....

    I also have to be able to reduce the input voltage to a level that is
    acceptable for a small 5v regulator (max input voltage for it is 20v).
    A PIC micro is used to control the high voltage motor via some sort of
    drive circuitry. Like I said before, I have perfectly working setup
    that works with a 12-14 volt input source. The catch now is being able
    to allow up to 65 volts. This also needs to be all SMT and as small as

    I think what I need really is a way to reduce the incoming voltage to be
    less than 20 volts, and I need probably no more than 100ma of current
    for everything.

    I am still looking at the idea of a zener diode/resistor voltage
    regulator. I believe I saw something with a transistor added somehow as
    well. Any thoughts?
  5. Joerg

    Joerg Guest

    Hello Jim,

    One simple method is driving them via little toroid transformers. You
    just have to make sure that they cover the frequency range you are
    dealing with.

    Regards, Joerg
  6. Jim Thompson

    Jim Thompson Guest

    I'll have to dig thru the 20 year old paper designs I did in that

    Transformers driven PWM, with DC restorers on the gate side.

    ...Jim Thompson
  7. Ken Smith

    Ken Smith Guest

    Consider this:

    / V++
    \ !
    ! !!-- NMOS
    ! ! !!--+--/\/\/--+--------- Load
    20V /-/ ! ! !
    ^ ! B !
    ! --------C E--------

    You may be able to do without the current limiting circuit. You can make
    it slow start by putting a capacitor across the zener.

    Beware that if you do both, you need a small resistor in series with the
    gate of the MOSFET. With no resistors near it, the MOSFET will look for a
    way to oscillate and fairly often find one.
  8. Ken Smith

    Ken Smith Guest

    Or, you can make it a bit more complex by adding a small power MOSFET like

    A S D !!------------
    --------- -----+------!!- BIG FET
    ( ! ! ! ! !!-
    ( ------- \ !
    ( ! / !
    ( ! \ Large !
    ( ! ! !

    When the A point pulses high, the body diode in the small MOSFET conducts
    and charges up the gate of the big guy. If the transformer voltage then
    droops, the capacitance of the FET will hold the gate voltage for quite a

    When the A point pulses low, the small FET turns on and the GATE of the
    big MOSFET gets discharged. If the transformer then droops, the gate
    voltage on the MOSFET settles back towards zero.

    The resistor is only needed if you disable the drive completely. Leakages
    could cause the big MOSFETs gate to drift up.
  9. Clark

    Clark Guest

    Under your search engine, look up Half bridge drivers, some will work pass
    100 vdc on the hi side, and two of them will form a full bridge driver. Some
    have a single input and some others have a hi and a low side drive input.
    They even have protection from turning a gate on if the other Fets gate is
    in the on position. Several kinds out there.
  10. Joerg

    Joerg Guest

    Hello Jim,
    Such archaeological excavation can take a while. Had to do it a few
    months ago to find a really old filter design disk. It turned out to be
    easy, it was a thin 5 1/4 floppy tucked into the back of my CAD binder.
    Duh! But that didn't occur to me until after several hours of digging. I
    had all but forgotten that some floppy disks were indeed 'floppy'.

    On many H-bridges that commutate regularly you can get away without DC
    restore. Sometimes I was even able to do it sans transformer, with just
    a coupling capacitor.

    How's the spider bit coming? Did it heal a little?

    Regards, Joerg
  11. Jim Thompson

    Jim Thompson Guest

    Turned out to be a cyst that burst.

    We're just home from the surgi-center, lighter by ~$3K :-(

    The joys of self-employment and $5K deductible ;-)

    But I'm still ahead by ~$25K since I made that decision to risk a high

    ...Jim Thompson
  12. Joerg

    Joerg Guest

    Hello Jim,
    And they probably slapped you with full fare, like $40 per bandage where
    the HMO pays only their negotiated rate of $7 or so. But you belong to
    the part of society that plans for a rainy day and doesn't blow it all
    on fancy cars.

    Well, your wife can now say that you spent $3k on her around Christmas
    time ;-)

    At least we can now deduct the premiums. What's still not fair is that
    we must pay the 15.3% SE tax before deducting and employees don't. IOW
    we pay 15.3% more than they.

    I am thinking about that as well. Right now we pay a little over $500/mo
    for two at Kaiser. Deductibles kick in mostly for hospital stays,
    $200/day or so but we are never sick. When I checked high-deductible
    plans they often aren't that much cheaper.

    Regards, Joerg
  13. Jim Thompson

    Jim Thompson Guest

    You're NOT incorporated ??? Look up "Key Employees" ;-)
    I was paying $900/month for $500 deductible.

    Went to $500/month for $5K deductible... for the both of us.

    I'm now just shy of 66, so I'm on Medicare... $261/month premium right
    now for the wife only. Next August the wife will be 65 and high cost
    will be over ;-)

    "we are never sick" either... first claim in 6 years.

    ...Jim Thompson
  14. Joerg

    Joerg Guest

    Hello Jim,
    We live in a blue state. What can I say? Like to keep it simple, no more
    bureaucratic layers.

    Wow. I never saw a plan for us that would drop the premium by 40%. At
    least not for health care.

    Then comes that drug plan with the mandatory donut hole. We do a lot of
    ministry and visitation in nursing homes. It's sad, many of those people
    had plenty of savings because of hard work and a frugal or at least
    prudent lifestyle. All it takes is a serious fall, stroke, whatever and
    you'll lose all that in a jiffy. $5k a month is nothing there and you
    still get jam packed into a small room with three others.

    Same here. Except for rather stupid reasons like poison oak exposure.
    When we moved here I did not know what that was. So .... $45 of copay
    later I knew that I should have gotten out of those clothes right away
    and not finish up the brush clearing.

    Regards, Joerg
  15. Jim Thompson

    Jim Thompson Guest

    [I was going to say something, but I promised :-]
    Hospital and surgical only, no doctor visits.
    Between the two of us we only have ~$200/month, so I think Part D is
    likely to be useless.
    I did that in poison ivy ONCE... blisters everywhere, including
    everywhere I scratched :-( Potassium Permanganate gauze soaks for a

    ...Jim Thompson
  16. Joerg

    Joerg Guest

    Hello Jim,

    Yes, I read something to that effect. That you and Win wouldn't throw
    snowballs at each other anymore. Or at least not with rocks in them.

    But seriously, it's getting nasty here at times. I mean, when the owner
    of a perfectly healthy surfboard manufacturer throws in the towel that
    should say something to the local politicians. Unfortunately it didn't.
    I'd be fine with that but the premiums are still so high. Probably
    because it's Kahlifohniah.
    Don't know, depends on which meds you need. I have seen some really
    cheap plans. Even Walmart out here offered one.

    Well, I waited so long that the doc said the rash migrated to the inside
    of my skin. Anyway, for the next season of poison oak removal he
    recommended "Tecnu soap" to wash down with right after the work and that
    stuff really does the trick for us.

    Regards, Joerg
  17. Jim Thompson

    Jim Thompson Guest

    Rocks? Rocks? When I was a kid we used coal ;-)

    (Tallmansville, WV, is just south of Philippi, WV, where my
    grandparents (on my mother's side) had a HUGE farm with their own seam
    of coal.)
    Good point... I ought to see if my business membership at Sam's Club
    has any "benefits" ;-)

    ...Jim Thompson
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