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pwm opinions

Discussion in 'General Electronics Discussion' started by sndscientist, Nov 5, 2015.

  1. sndscientist

    sndscientist

    47
    1
    Jul 10, 2013
    ok for a project i am currently working on i recently built a nice simple pwm circuit for controlling a rather hefty dc motor. the motor is rated at 18 amps at 130vdc. 1.5 rated hp. i am not going to be running it at 130 volts rather 36 to 48 volts (haven't decided yet). I have the PWM and snubber circuits already built to protect my output fets from the lightning storm the collapsing field will generate. also have a neon light across the motor for good measure to help the over 70v clamping. my question is what fets do you suppose i should use? my last 2 pwms i built i used the irf610's and the irf 540s. this one i started using the irfz48vs because of the pulse drain current of 290 amps where as the 640's have a pulsed drain current of 72 amps and the 540s at 110 amps. my concern rises mostly because the irf48v bank gets abnormally hot on a simple test using a smaller 3 amp 12 volt motor. much hotter than a single 640 with a tiny heat sink got on a 12 volt 30 amp motor.
     
  2. Alec_t

    Alec_t

    2,695
    712
    Jul 7, 2015
    Are you using gate resistors to suppress any HF oscillation?
     
  3. BobK

    BobK

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    1,662
    Jan 5, 2010
    Schematic please.

    Bob
     
  4. sndscientist

    sndscientist

    47
    1
    Jul 10, 2013
    Yea i am using gate resistors, a 1k feeding the gate and a 15 k biasing it to ground. As for the schematic i never drew one. The pwm circuit was an image i found on google a few years back. I built a test one and once i was sure it worked i built 4 and shelfed them.
     
  5. GPG

    GPG

    452
    66
    Sep 18, 2015
    That's going to slow the switching, resulting in higher dissipation.
    What is driving the gate?
     
  6. BobK

    BobK

    7,643
    1,662
    Jan 5, 2010
    Please draw a schematic for us. The MOSFET you are using should not even get warm at 3A if you are using it correctly.

    If you can't provide a schematic, at least assure us that the voltage on the gate is at least 10V higher than the source. I suspect t that you are using either insufficient gate voltage or you are using an N channel MOSFET as a high-side switch, with the drain connect to V+ and the source connected to the motor. Either of these would cause the MOSFET to heat up.

    The resistor in the gate circuit might also be the problem, but only if the switching frequency is high, like over 10KHz.

    Bob
     
  7. Minder

    Minder

    2,871
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    Apr 24, 2015
  8. sndscientist

    sndscientist

    47
    1
    Jul 10, 2013
    No prob ill draw one out later today. The gate is being driven via a 555 directly. Was going to use a pc817 opto but figured at the higher frequency it might give me issues. Redid my resistors this morning. Dropped the 1k's down to 470's and kept the 15k's and put a kb4uj across the motor as a snubber. So far this morning it seemed to do alot better.
     
  9. GPG

    GPG

    452
    66
    Sep 18, 2015
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