Driving N Channel Mosfets from MicroControllers

Discussion in 'General Electronics Chat' started by Maketronic, Jun 22, 2011.

  1. Maketronic

    Thread Starter Active Member

    Mar 21, 2009
    49
    0
    Hi All

    I am building a PWM dc motor speed control for a 12v electric trolling motor for a colleague and was hoping to get some advice.

    I have a number of logic level mosfets IRLZ34N mosfets that I plan to drive in parallel (5 of) from a PIC 12F683 Micro

    I plan to drive these direct as they are logic level mosfets however I anticipate also doing some boards with higher current regular n mosfets (due to there ease of availability).

    Can I use a micro to drive a 74 series buffer. logic 5v with the buffer supplied by 12v to drive a mosfet or a number of mosfets? I understand at 5v many mosfets would not switch on fully?

    Also how does one size diodes for flyback or back emf from a pwm motor drive?

    Many Thanks

    Bruce
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    This is the flea market forum. You needed to post this in the electronics chat forum. I suspect a moderator will be along shortly and move it.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
    1,605
    Hello!

    A logic level MOSFET is doing pretty good at 5V, that's what its designed for. It gets a little better with more voltage (don't we all?) but quite acceptable with 5V.

    I am not familiar with any 74 series parts that buffer 5V to 12V. I'm not saying they don't exist, just I don't know.

    A flyback diode only comes into play when the motor turns off and the diode needs to carry the motor current till it dies down. So the safest size is a diode able to carry the full motor current, but much less would probably be acceptable. A 1N4001 is usually fine and even overkill.
     
  4. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    Each MOSFET you use has a certain gate capacitance which must be overcome to turn that MOSFET "on". The longer it takes to overcome this capacitance, the longer your MOSFET(S) will be in the linear region creating heat, lots of heat. For high speed switching, a lot of current (like an amp or more) will be required to overcome this capacitance. 5 MOSFETS will be 5 times worse.

    The spec sheet will list "gate charge" (Q) in pcb (picocoulombs), knowing that value and plugging in a desired value for time, use I = Q/T to determine how much current you need. There are truckloads of dedicated MOSFET drivers available that will make life with MOSFET's better.

    I forgot- A Fairchild 1N4001 can eat as much as 30 amps for a very short while (8.33ms). I don't know how much current those 100/$ Radio Shack unbranded units are good for.
     
  5. Maketronic

    Thread Starter Active Member

    Mar 21, 2009
    49
    0
    Hi All

    Thanks for your help thus far

    Appologies for the post in the wrong category, it was an inadvertant mistake on my part.

    Regards

    Bruce
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    A common mistake and no big deal, the moderators are used to moving theads I suspect.

    Drivers are usually required to make MOSFETs switch. LEDs, 555s, Flashers, and Light Chasers Chapter 10 and 12 show some examples how this might be done. There are also plenty of chips designed to do the job.
     
Loading...