FET switch design for 24V

Discussion in 'General Electronics Chat' started by fsonnichsen, Oct 23, 2013.

  1. fsonnichsen

    Thread Starter New Member

    Jun 6, 2013
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    I have used a pair of FETs as shown in the attachment to switch loads across 12V with no problem. However I have a situation with a 24V system and while the same system works it is not safe.

    If you have a low impedance load at R, the Vgs for Q1 will effectively be around 24V. This supersedes the max Vgs for these devices (20V) and a lot of similar ones.


    I suppose I can seek a heavy FET with Vgs around 30V but perhaps I can design something better. I thought of dividing the voltage at the gate but I want to be sure to have a stiff pull up on the gate to admit full current.
    Anyone have a similar design that works within these specs?

    Thanks
    Fritz
     
  2. #12

    Expert

    Nov 30, 2010
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    How about a 10 volt zener from the output gate and the input drain. That way, the input transistor can not pull the following gate down more than 14 volts.
     
  3. fsonnichsen

    Thread Starter New Member

    Jun 6, 2013
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    Hmmm. That would clamp the gate on Q1 to 10V w.r.t its source (e.g. ground). Wouldn't that limit the current that Q1 can drive as compared to letting the gate reach the full 20V?
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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  5. SgtWookie

    Expert

    Jul 17, 2007
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    Consider using a high side driver with an N-channel MOSFET. A Micrel MIC5014 would be a suitable choice.
     
  6. #12

    Expert

    Nov 30, 2010
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    How about a (standard value) 4.3 volt zener diode? That would allow Q2 to pull the gate of Q1 down by 19.7 volts. What are you doing that the difference between 14 Vgs and 19.7 Vgs makes a significant difference? If it's that critical, use a bigger mosfet.

    Or do as Wookie said and reverse the polarity of a transistor.
     
  7. fsonnichsen

    Thread Starter New Member

    Jun 6, 2013
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    OK. All these suggestions make sense. Good information and two solutions.

    This was my first post on this site- I really appreciate your help-
    Thanks,
    Fritz
     
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