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parallel FETs

Discussion in 'Electronic Basics' started by V8TR4, Nov 16, 2004.

  1. V8TR4

    V8TR4 Guest

    Hello,

    I want to make a controller for a golf cart motor and am considering using
    paralleled MOSfets or HEXfets. I know that there can be problems with the
    individual xsistors not sharing the load equally. Would it help if I had a
    seperate line driving each one rather than hooking up to some common rail to
    activate them? Any info would be much appreciated.

    Thanks
     
  2. peterken

    peterken Guest

    nope, won't do...
    if using them digitally, you might parallel them as they are,
    or
    put a very small resistor in each source and actually feedback them which is
    the more complex way to go
     
  3. I'm sure you are already aware of "thermal runaway" in BJTs. But there have
    been some comments about using paralleled hexfets on sci.electronics.design, a
    few year ago, I believe. I gathered that safely paralleling power MOSFETs only
    works well when switched completely on/off, where V(GS) is substantially above
    the zero thermal coefficient point. Somewhere in the discussion about the
    problems of the temperature coefficient for Id, given a fixed Vg, the Hitachi
    2SJ160 and 2SK1056 family was mentioned as attractive because of their
    zero/negative coefficient in this regard.

    There may also be serious oscillations to worry about, particularly when
    operated linearly, as well, if memory serves. I'm no expert in this, at all. I
    haven't tried to analyze this problem; and never ever tried to do something like
    you are going to try -- hopefully someone who has can comment. But controlling
    large inductances at large currents with paralleled HEXFETs (or MOSFETs or BJTs)
    and where the actual motor load/acceleration can vary in spurts and jerks, I'm
    sure, will take some careful attention to get working well -- probably combining
    mechanical and electrical means.

    You might use google to look up some threads in sci.electronics.design and/or
    otherwise post this question there.

    Jon
     
  4. rayjking

    rayjking Guest

    Hi,
    Fets .
    The first requirement is to develop the gate drive pwm that has a rise time
    of at least 100ns and a range of zero to 12 volts.
    The fets should be of the same part number from the same mfg and connected
    close together as possible on a heat sink. The source leads as short as
    possible to the common connecting point. The most important requirement is
    the value of the series gate resister for each fet. If you are using To-220s
    use 33 ohm resisters. If you are using To-247s use 15 ohm resisters. The
    above is if the number paralled is below 10. If you are paralleling more
    increase the above resister value by 20% for each additional 10. The gate
    drive return should connect as close as possible to the source of the fets
    to the center point of the common connection. Also a large filter cap should
    be placed as close to the fets and the free wheeling ( fast recovery diode
    or schottky diode ) path. The motor can be located at length away but it is
    best to keep the switching transistors near the battery.
    Bipolars.
    No one has been able to parallel bipolars successfully without careful
    matching. They can be purchased in modules that work great but they use
    large die and are very close together.

    I hope this is of some help.

    Ray
     
  5. Neil Preston

    Neil Preston Guest

    Have you considered using the FETs as on/off switches and controlling their
    duty cycle to control the speed of the motor? Much more efficient. Fewer
    heat problems, among other things. If driven hard enough, load sharing
    should be a minor issue. Switching frequencies of a few hundred hertz should
    be adequate.
     
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