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Selecting a MOSFET

Discussion in 'Power Electronics' started by jgauthier, Sep 29, 2021.

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  1. jgauthier

    jgauthier

    35
    1
    Mar 22, 2013
    Hello,

    I am having some trouble selecting a MOSFET, and maybe I need more than one.
    The scenarios is a I have an MCPU that will be switching a GPIO at 3.3V. I originally wanted to switch a MOSFET to actuate a solenoid. The solenoid may be 12V, or it may be 24V. It's not completely decided yet, but I am trying to find a MOSFET capable (at 12V is needs to have and Id=5A, at 24V Id=2.5A). The issue I am running into is that many MOSFETs with a Vgs(th) <= 3.3V also have a much higher Vgs, sometimes 10V, or even upwards of 20V and 30V. My understanding of Vgs is that closer you get to Vgs the more "on" the MOSFET is. But I can only provide 3.3V.

    I'm wondering if this is a scenario where I can find one MOSFET to switch 12V to have enough across Vgs on a secondary MOSFET so the MOSFET can be fully activated and the solenoid fully actuated.

    Advice welcomed. Thanks!
     
  2. Bluejets

    Bluejets

    5,037
    1,052
    Oct 5, 2014
    Drive your mosfet gate via a small properly biased transistor from the micro 3v3 output.

    fet2.jpg
     
  3. PETERDECO

    PETERDECO

    185
    45
    Dec 19, 2019
    We use IRLZ44N logic level MOSFET for our 3V circuits. A direct replacement is STP55NF06L.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,702
    2,717
    Nov 17, 2011
    Are you maybe looking at VGSmax? That is the max, Gate-Source voltage the MOSFET can tolerate without being destroyed. This voltage is not a parameter to be used for designing normal operating conditions.
    When you look at e.g. the datasheet of the IRLZ44N as proposed by @PETERDECO , you see that this MOSFET is good for almost IDS = 30 A at VGS = 3.3V:
    upload_2021-9-30_9-55-34.png
     
    Tha fios agaibh likes this.
  5. jgauthier

    jgauthier

    35
    1
    Mar 22, 2013
    Thanks everyone. I believe my issues was an understanding that as Vgs was increased so was ID. And while that is true, I need to look at the characteristics graphs to determine a MOSFET will actually fit my needs. I appreciate it, everyone.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    11,702
    2,717
    Nov 17, 2011
    That is true, as you can see from the graphs, but not to more than IDS = (Vsupply-VDS)/Rload.
    Once VDSis saturated (minimum value reached), increasing VGS will no longer increase IDS.
     
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