MOSFET - Is Vgs(th) turn on voltage?

Discussion in 'General Electronics Chat' started by MrJojo, Aug 26, 2014.

  1. MrJojo

    Thread Starter Member

    Jan 23, 2013
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    Hello all,

    I haven't looked at MOSFET's in a long time, and I'm a bit rusty. I wanted to make sure that I can "turn on" the MOSFET.

    I am looking at a IXFA8N50P3 - http://ixapps.ixys.com/DataSheet/DS100455A(IXFA-FP8N50P3).pdf. From my quick review (reading the spec sheet, checking AAC references, and Google searching) I know that Vgs is the MOSFET turn on voltage and the for the MOSFET to turn on, Vgs>Vth.

    The spec sheet stated Vgs(th) to be 3-5V. From what I've gathered, the Vgs(th) is the voltage required to turn on the MOSFET. I am a bit confused by this and I have 2 interpretations.
    1) a Vgs between 3-5V will turn on the MOSFET.
    2) Vgs must be greater than 5V, since that is the minimum voltage required to turn on the MOSFET.

    Can someone point me in the right direction?

    Thanks,
    Matt
     
  2. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    It depends on what you are doing with it. Vgth is the gate threshold voltage, or, the voltage that the channel substrate becomes inverted.

    If you are wanting to use it as a switch, you'll need over Vgth, in order to ensure high conductivity through the channel.

    Running the MOSFET near Vgth means it has significantly higher resistance than it would otherwise, which means a hotter FET, depending on Id.
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
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    Vgs between 3-5V will turn some parts on, some not. Guaranteed for every part at 5V.
    Dont forget that Vgth means that at that voltage the mosfet will let pass some specified current, like 500µA, so not much current. Look at Id vs Vgs curves to get better idea what you actually need.
     
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  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Vth is the voltage at which the mosfet starts to turn on. If you want it fully on, Vgs must be greater.

    Check Figure 1 of that datasheet. Notice at the same Ids, the Vds decreases with increasing Vgs (to a point). In other words, Rds decreases with increasing Vgs.

    You want to operate a mosfet in the ohmic region, not where the plot of Ids v. Vds becomes horizontal.

    John

    Edit: Here's a link that explains the datasheet: http://www.microsemi.com/micnotes/APT0403.pdf
    Edit2: Unfortunately, that link doesn't go directly to the tutorial. I hate it when that happens. You need to search on "APT0403" to get to the article.
     
    Last edited: Aug 26, 2014
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  5. crutschow

    Expert

    Mar 14, 2008
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    If you want to fully turn on the MOSFET as a switch then the voltage needs to be significantly higher than Vgs(th). Look in the transistor specification table for the ON resistance to see what Vgs they use for that measurement. For a normal MOSFET it's typically 10V, for a logic-level MOSFET it's typically 3V or 5V.
     
  6. MrJojo

    Thread Starter Member

    Jan 23, 2013
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    0
    John,
    Thank you for the link, it helped a lot. I was making my best guess at most of the symbols and meanings in the spec sheet.

    I know crutschow said MOSFETs should be typically run 1t 10, but after looking at Figure 1 from the spec sheet, I can get away with anything higher than 7V. It'd be pushing it a bit on my end, so chances are I'm going to use either a 9V or 12V Vgs simply because those are common voltages.

    Hypothetical question. If I set Vgs=4, then that ~50% of the MOSFET will turn on. So then that corresponds to a very low current through the MOSFET, correct? I know I want Vgs>Vgs(th), I'm just curious. That would make sense with that kubeek said.

    Thanks all for your help,
    Matt
     
  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    The figures in the datasheet will allow you to estimate the effective drain-source resistance (Rds) at that Vgs. I wouldn't say low Vgs corresponds to a very low current. It corresponds to a higher Rds. If you increase the drain-source voltage, you can get almost any current you want according to Ohm's Law. At the same time, however, the high current and high voltage drop (Vds) will cause a lot of heat (I^2R). That will cause the device to burn up.

    On the other hand, you might say that a low Vgs will be okay if the device is used for small currents and low Vds.

    John
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,016
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    Note that most of the graphs in the data sheets are for "typical" characteristics. For reliable turn on of an arbitrary MOSFET you should use the worst case values. For switching, that's the Vgs values they use when the specify the ON resistance.
     
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