PWM of DC brished motor. HV spikes on MOSFET drain

Discussion in 'General Electronics Chat' started by oidium45, Dec 15, 2012.

  1. oidium45

    Thread Starter Active Member

    Apr 24, 2010
    130
    8
    I will try to post a schematic but I figured this was a common issue...

    Issue #1
    I have a 20kHz square wave feeding an IRFP260. With a motor load (approx 12V 2.6A) placed on the drain of the mosfet i get a 2.8MHz ringing both on the gate and drain. http://www.flickr.com/photos/51714097@N05/8254864113/in/photostream
    I do not get this ringing without a load on the drain. I tried using an IRF510 and the issue dissipated but the IRF510 shortly died. I guessing this was caused by issue #2. I am thinking that the ringing was due to the internal capacitance of the IRFP260? If so, what would be the best way to resolve the issue?

    Issue #2
    When running the motor, under all percentages of duty cycle, I get a high voltage spike on the drain of the mosfet.
    http://www.flickr.com/photos/51714097@N05/8219240612/in/photostream
    Originally the spike was around 400V. I added an ultrafast diode across the motor leads (HFA15TB60) which reduced the spike to around 200V.


    It was originally suggested that I add an RC filter across the motor leads to absorb both the ringing the spike using the following formula.

    Z=2(pi)Fr(L) for the resistor
    C=1/2(pi)Fr(R) for the capacitor

    Is this correct?
     
    Last edited: Dec 15, 2012
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Are you using a series gate resistor ? If not that might be part of the issue.
     
  3. oidium45

    Thread Starter Active Member

    Apr 24, 2010
    130
    8
    Yes, i have am using a 10K resistor from gate to source. I have tried a few different values with no success. I will post a schematic. I have to add in all of the safety stuff before posting. transformers, fuses, etc..
     
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,015
    1,531
    The resistor from gate to source is a pull down resistor, to make sure the gate completely drains when shut off. The gate resistor goes from the source of the square wave signal to the the gate. A value of 4.7ohm to around 25ohm is the normal used.

    Also have the signal source as close to the mosfet gate as possible.
     
  5. oidium45

    Thread Starter Active Member

    Apr 24, 2010
    130
    8
    Oh, ok, I misunderstood your question. Yes, I have a resistor between the gate driver ic and the gate of the mosfet.
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    Can you post a diagram ?
     
  7. oidium45

    Thread Starter Active Member

    Apr 24, 2010
    130
    8
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