N-ch Mosfet characteristics

Discussion in 'General Electronics Chat' started by Col16, Feb 5, 2015.

  1. Col16

    Thread Starter New Member

    May 2, 2013
    3
    0
    Hi there

    I am looking for some assistance in understanding the Vds waveform I measure when switching off an N-ch enhancement mode power mosfet. The transistor is simply switching off a filament lamp that draws about 5A from the 12Vdc supply. The waveform attached has Vgs on channel 1 and Vds on channel 2.

    I am thinking that the spike and discharge curve is due to some parasitic components of the transistor. The transistor is being driven very quickly with a 4A driver with no gate resistor.

    Has anyone ever seen a similar voltage at the output that can shed some light on the cause?

    Any help is much appreciated.

    Thanks
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    There appears to be some inductance in the circuit.
    Post a schematic and picture of the circuit if you can.
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,138
    1,787
    Don't wound filaments look like tiny inductors?

    Twinkle, twinkle little star,
    V is equal L(di/dt)

    OK so it doesn't rhyme.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    Filaments would have a tiny inductance but I doubt it's sufficient to give the voltage spike he is seeing. ;)
     
  5. tindel

    Active Member

    Sep 16, 2012
    568
    193
    Yep - inductive kick from your current turning off so quickly. try to put some resistance on the gate to limit the turn on / turn off current.
     
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