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Using a MOSFET instead of a BJT

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Discussion in 'General Electronics Discussion' started by Cirkit, Dec 12, 2016.

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  1. Dec 12, 2016 #1
    Cirkit

    Cirkit

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    Oct 28, 2015
    I am currently redesigning a circuit and standardising on components as much as possible.

    Part of the circuit is an Op-Amp configuration to generate PWM pulses to drive a motor. I thought that substituting a BJT transistor on the output of an LMC6762 Op-Amp with a PMF370 would be a better choice. Are they any implications with doing this? I have placed a 47 Ohm resistor between the output of the Op-Amp and the the MOSFET and a 100k Ohm resistor from Gate to Source.
     
  2. Dec 12, 2016 #2
    Harald Kapp

    Harald Kapp Moderator Moderator

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    There is no general rule when replacing a BJT by a MOSFET. The application has t o be taken into account.

    In your case it is fairly common to use MOSFETs to PWM-drive a motor. Make sure VDS and IDS match the requirements from your motor. Show us the respective part of your circuit diagram so we can check for any issues there might be. Also note that an RDS of 440mΩ is not the best you can get.
     
  3. Dec 12, 2016 #3
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It seems a reasonable option, however with such a low Vgs(th) the turn off time may be longer than you expect.

    What frequency is this operating at, and what is the peak current demanded by the load?
     
    Last edited: Dec 12, 2016
  4. Dec 12, 2016 #4
    Cirkit

    Cirkit

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    Oct 28, 2015
    I have attached a circuit diagram of the output configuration. The inverting input is fed from an oscillator (~10Hz) and the non-inverting from another comparator Op-Amp circuit.

    The MOSFET will be driving a 3V motor with a maximum current of 100mA. There is a back EMF diode across the motor.
     

    Attached Files:

    Last edited: Dec 12, 2016
  5. Dec 12, 2016 #5
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That's really low speed. I wouldn't be too worried about the speed of switching. Also the current is well within the capability of the device.
     
    Cirkit likes this.
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