mosfet gate oscillation problem

Discussion in 'The Projects Forum' started by mobydick, Mar 19, 2012.

  1. mobydick

    Thread Starter New Member

    Nov 19, 2010
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    hello
    i am trying to build half bridg using p and n mosfets
    when p type mosfet turn on oscillation appeare at the gate
    and when i disconnect drains of the p and n mosfet it disappears
    any help pleas
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    The gate-source of a Mosfet is a capacitor. The wire feeding the gate is a series inductor. The capacitance and inductance oscillate or ring.
    That is why most Mosfets have a low value resistor (10 ohms to 100 ohms) at the gate pin in series with it to kill the inductance.
     
  3. mobydick

    Thread Starter New Member

    Nov 19, 2010
    27
    1
    but there is 500ohms resistor on the gate during the on time
     
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  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    but that is reistance in parallel, not in series with the gate.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Do you have power supply decoupling caps on +30V?
    Do you have a load on the drains that is not shown?
     
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  6. mobydick

    Thread Starter New Member

    Nov 19, 2010
    27
    1
    during off time resistor is in parallel , but i have no problem at off time of the p mosfet
    at on time of the p mosfet the resistor is in series (right !!)

    no i didnot
    after adding 470 n oscillatin decreased (new screen shot attached ,note NG ch1 is 1v/div while PG ch2 is 5 v/div)
    no i donot . will that make diffrance?
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    It will probably make a difference. Why don't you try it?
     
  8. mobydick

    Thread Starter New Member

    Nov 19, 2010
    27
    1
    when i put 50 ohms from drain to GND no change
    and when i put 50 ohms from drain to +30 it changes( picture attached)
    what is the reason for that oscillation?
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Can you show us a picture of your hardware, i.e. breadboard or PC board?
     
  10. mobydick

    Thread Starter New Member

    Nov 19, 2010
    27
    1
    hello all
    sorry for late respond (exams :mad:)
    any way , i did some simulations and put random inductors every where . i found the exact same noise happens when i put an inductor between source of the mosfet and ground . bad nois also from source to vcc inductor .
    so i redesigned the circut with huge vcc and ground tracks and problem disappeared :)
    thanks
     
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