n-channel MOSFET selection

Discussion in 'The Projects Forum' started by BruceMellen, Jun 30, 2011.

  1. BruceMellen

    Thread Starter New Member

    Jun 25, 2011
    Background: As a newbie, I’ve readily got 5V and 12V power available and want to drive a PC case fan (which could be as large as 2.2A Locked Rotor Amperage) at very very close to 12V when the (I’m thinking 25-100kHz)PWM driver input is at 100%,. After hearing I need to likely generate power at 17V to drive the MOSFET, I’ve been looking for several possibilities…?

    Should I be looking at a Vds=12V n-channel MOSFET, or something in a 20V or more for a safety margin or other reasons? My reading never seems to come out and say one or the other.

    And when a Logic Level MOSFET datasheet lists a Vgs of ±8, ±12, …,(seemingly never greater than the Vds), AND a resistance for Vgs of 4.5V, what are the limitations/voltage protections needed, other than knowing the resistance/package thermal limitations? What Vgs input voltage range would be appropriate for a Logic Level MOSFET listed as, say Vds=20V, Vgs=±12, to turn it on hard?

    So do I need to apply the bias of 17 as one person suggested, so that the internal resistance is minimized, or will close to 5V or 12V work on a logic level MOSFET? Is there an advantage to driving the MOSFET with logic at more than 5V, or 12V?

    …I’m trying to learn and related links to helpful reading for beginners, along with more direct answers/explanations, are very welcome.
  2. #12


    Nov 30, 2010
    Vds must always be higher than the supply voltage plus inductive reactance from the fan when you turn it off. In short, higher is better. No problem using a 100 volt mosfet on a 12 volt fan.

    There are "logic level" mosfets that claim to be "on" with +5 Vgs. +12 Vgs will work with just about any mosfet and not hurt them. edit: (as long as the drain is at a higher voltage than the gate) The goal is to slam them hard on, quickly.

    The datasheet is your best friend for this job. I'd check it for you, but you didn't provide any part numbers.
    Last edited: Jun 30, 2011
  3. BruceMellen

    Thread Starter New Member

    Jun 25, 2011
    Btw, what does a Gate Source Voltage Vgs of ±8 really mean at the top of most datasheets, then?

    So, if I take, for example, a SI3460DDV-T1-GE3 Mosfet and subject it to Vds=12, and Vgs=very slightly less than 12V as a PWM at 25kHz (due to the as-yet-to-be-named driver), it will be hard-on during the duty cycle? Will I be able to get to within 11.95V Vout?

    Will I also be well within its thermal limitations if the resistive/inductive fan load is a 2.2A LRA and/or run relatively cool with 1A continuous running load at 100%PWM? Or do I need to go to a more expensive MOSFET further down the list below, which has a lower Rds? (sorry about the table columns not lining up)

    And are these packages readily hand-soldered (not flow), as I’m not readily finding a cheap prototype adapter for 6-tsop packages, like I do, cheap, for SOIC. And who are the solid manufacturers?

    If you can tell me about how you are making this assessment, I will learn much more about the implications associated with these datasheets and my evolving design…

    Part cost Logic/std Rds max @Vgs=4.5 V A W Pkg Vgs Mfr.
    $0.43 logic 0.0230 20 7.9 2.7 6-TSOP 8 Vishay
    $0.52 logic 0.0180 20 8.6 2.0 6-TSOP 12 Alpha&Omega
    $0.62 logic 0.0155 30 13.0 3.1 8-SOIC 20 Alpha&Omega
    $0.73 logic 0.0146 30 9.9 2.5 SO-8 12 Intl. Rectifier
    $0.76 logic 0.0135 30 13.4 3.7 8-SOIC 12 Alpha&Omega
    $0.79 logic 0.0108 30 15.0 3.1 8-SOIC 20 Alpha&Omega
    $0.95 std 0.0080 30 24.0 6.0 8-SOICN 20 Vishay
    $0.95 logic 0.0100 30 16.0 2.5 8-SOP 20 DIODES, INC