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Help neeed regarding MOSFET H-bridge..

Discussion in 'Electronic Design' started by Arun Prakash, Feb 10, 2007.

  1. Arun Prakash

    Arun Prakash Guest

    * I use a Mosfet H bridge to control the speed of a DC motor.

    * I use a PC's parallel port to do this.The output is 4.5 V

    * I use a optocoupler for isolation.The parallel port drives
    optocoupler.

    * The transistor in the optocoupler is given to the gate of MOSFET
    IRF730.The Vcc of transistor is 5.5V

    * All are N channel MOSFETs.

    The problem is while gating the mosfets.Only one mosfet of the High
    side bridge arm is ON,the low side isnt conducting.

    One mosfet is getting heated so much the other remaining cool.No gate
    driver ICs are used.No feedback diodes are used.Is the problem anyway
    related to gate grounding resistance?...

    Please help me in this case......Iam totally confused and
    frustrated....
     
  2. Indeed. Some proper study, and/or Google exploration is in
    order for you. A few points. An H-bridge has a set of four
    switches arraigned to provide bidirectional drive to a motor
    from a unipolar power supply. You have added speed control,
    in addition to direction, which means you'll need PWM, or
    pulse-width-modulation. This modulation should be applied
    precisely and quickly, which means you should not attempt
    to create the PWM waveform with software in a PC and deliver
    it via the parallel port. You'll need a PWM-generating IC,
    or at least a dedicated uP to create the modulation signal.

    Next, you cannot drive your MOSFET gates with 5.5 volts.
    The low-side switches may function OK, but only IF you are
    using logic-level MOSFETs. The IRF730 is not such a beast.
    Also, you'll want to have sufficient gate-current drive to
    charge the MOSFET's high gate capacitance.

    What's more, the high-side MOSFETs require a level-shifting
    gate drive with a flying-drive capability. That's because
    the gate voltage needs to be 10 to 15V above the MOSFET's
    source, which itself is changing as the switch turns on
    and pulls up the motor winding. We usually use a special
    IC, with a flying capacitor, to provide this function.

    Also, since you are switching an inductive load, you'll
    need to have snubbers to protect the MOSFETs.

    So there's a start, at least five things to learn about:
    PWM, MOSFET gate-voltage drive, MOSFET gate-current drive
    requirements, MOSFET high-side switching, and snubbers.
    A sixth item will be combining all this into an H-bridge.

    Good luck in your studies. Come back and talk to us later.
     
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