need help in increasing PWM ouput current

Discussion in 'The Projects Forum' started by steveth, Aug 6, 2006.

  1. steveth

    Thread Starter New Member

    Aug 6, 2006
    2
    0
    I am in the process of building a jet engine and for the fuel control I am using convertible top motor/pump. I have tested the current draw to be a start-up of about 20 amps and running at about 14 amps. The mosfet irfz44 smokes almost instantly when I try to implement this as the motor speed control for the fuel injection. I would like to know how I can beef up the output of the pwm so it can handle the full range needed to control this motor. I am wondering if a different mosfet or any other transistor type would work. Or, putting more mosfets in a parallel circuit would work. If there is maybe a better pwm design that I could use that would make this task simpler and that there is already a circuit board layout completed that I could get a hold of. If needed, I can post the current schematic of the one I have built.

    Thanks for any replies,
    Steve
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Greetings steve,

    If you can post the schematic that would be very useful in helping us to help you. You may need to check your company policy in case there are any proprietary issues connected with sharing such information. You may be able to crop the schematic so that only the circuitry you are interested in improving is visible.

    hgmjr
     
  3. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    The MOSFET you are currently using is rated far higher than the 20A maximum you required. It shouldn't overheat if the circuit is designed and built properly.

    Your schematic doesn't include the components values, can you tell me the Vcc, R1 and R8 values that you are using? Also, the part numbers for D1, D2, D4 and the switching frequency? These affect the switching behaviour of the MOSFET.

    Looking at your current circuit diagram, without the component values, I guess the most critical component is R1, as it controls the turn-off time of the MOSFET. Too high a value and you are having long off tails which would overheat the MOSFET, too small and you would divert the gate drive to ground and slow the on switching time down which would have similar effect.

    I assume you have adequate cooling for the MOSFET? Sufficiently rated heatsink, thermal compound and all that stuff?
     
  4. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    Edit: double post.
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Enam,

    I agree with n9352527 that among the critical aspects of the circuit you have posted are the switching speed of the MOSFET and the operating frequency of your PWM. Since the MOSFET is rated for more than the current you are switching, the fact that it is getting hot would ordinarily indicate that it is spending too much time in the linear region between the on state and the off state.

    We will need component values for your circuit before we can get to the heart of the problem.

    hgmjr
     
Loading...