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Selecting a MOSFET

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

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  1. jgauthier


    Mar 22, 2013

    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


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



    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

    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:
    Tha fios agaibh likes this.
  5. jgauthier


    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

    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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