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Driving MOSFET gate from logic outputs

Discussion in 'Electronic Design' started by seanacais, Jun 1, 2007.

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  1. seanacais

    seanacais Guest

    Would a common base transistor configuration be a good way to drive
    the gate of an N-MOSFET from a logic output?

    The MOSFET is the high side of an H-Bridge sitting at about 40V. The
    gate will be drawn to 48V which is much too high for logic outputs.
    Since I want to run the bridge with high inputs it seems that a common
    base configuration would give large voltage amplification in phase
    with the inputs. The output of the logic gate would be fed into the
    emitter and the collector would hook to the MOSFET gate (and +48V
    through a current limiting resistor. I'm not positive but it seems as
    if a resistor between the collector and the gate would be a good idea
    as well.

    In reading about this configuration several references stated that
    it's normally used in RF circuits because of the large voltage
    amplification. The bridge is to be PWM driven at possible tens of KHz
    but nowhere near RF. Is there something I'm missing about this that
    would make it unsuitable for this application? Insights, suggestions
    and hints are all welcome!

    Thanks,

    Kevin
     
  2. The common base stage can be used to level shift and voltage
    amplify the logic swing, but it provides little drive
    current to quickly charge and discharge the gate
    capacitance. You will probably need to add a complementary
    emitter follower to provide this current capability.
     
  3. D from BC

    D from BC Guest

    I use mos driver IC's for low and high side switching.
    D from BC
     
  4. Consider one of these???
    http://www.st.com/stonline/products/literature/ds/7785/l6387.htm
    It drives the high and low side of half the H-bridge. Two logic
    inputs so you can control the dead time, etc. Bootstrap circuit
    allows use of N-chan mosfets. They have a whole family of devices
    that might be better for your particular application. I believe you
    could run the whole H-bridge from one driver if you wanted. They even
    have versions with the MOSFETs built in. Watch out for leakage on the
    bootstrap cap @ 100% duty cycles.


    -they call me frenchy.
     
  5. Marra

    Marra Guest

    You could use opto isolators to drive the gates of the FETs.
     
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