Simple MOSFET Switch

Discussion in 'The Projects Forum' started by bricke, Nov 14, 2011.

  1. bricke

    Thread Starter New Member

    Mar 1, 2011
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    This should be very basic, but when turning on an N-Channel MOSFET to be used as a switch, what spec on the datasheet says the voltage required on the gate to turn the switch "ON" to allow current flow from Gate to Source. I thought it was the Vgs, but I'm seeing voltages of (min) 1V to (max) 3V and that sounds very low to me.

    Thanks in advance for the help.
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Vgs is the minimum threshold that in your datasheet could be between 1V and 3V. These are threshold tolerances. If you have a look at the Vgs parameter there will be a drain current indicated, which can only be a few hundred uA.

    The best is to have a look at the curve showing the relationship between Vgs/Id.

    If you have a question about a specific component please post it's datasheet or model #.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Vgs (threshold voltage) is the minimum voltage required for the MOSFET to just start conducting at a very low current level (specified in the data sheet). To turn in fully it on as a switch you need to use the gate-source voltage used to specify the "on" resistance value. This is typically 10V for standard MOSFETs or 5V for "logic level" type MOSFETS. But the data sheet will tell you exactly what voltage to use.

    You can always go higher than that voltage if you want (up to the Vgs max rating).
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    As far as going over the Rds(on) voltage(s) specified in the datasheet -
    Yes, you can do that; with some caveats:
    1) The gate acts electrically more or less like a capacitor. You charge it up to turn it on, and discharge it to turn it off. The more you charge it, the longer it will take to discharge it. This usually doesn't impact things like timing very much when you are operating at a low frequency; say below 5kHz. However, if you are switching the MOSFET on and off at medium to high frequencies, charging the gate to higher voltages will add a good bit of stress on the gate driver circuitry, along with the discharge time being somewhat longer than expected.

    2) Most MOSFETs I've seen have absolute maximum values for Vgs as ±20v; however I have seen ±15v, ±30v, and some lower maximum Vgs's as well. You must check the datasheet for the allowed range of Vgs, and never exceed those limits, or the MOSFET will be destroyed.
     
  5. bricke

    Thread Starter New Member

    Mar 1, 2011
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    Thank you all for the explanation. I was looking at the BSS138. I made a mistake on my original post. It should have said "voltage required on the gate to turn the switch "ON" to allow current flow from Drain to Source." My apologies.

    I'm looking to turn a MOSFET on using a voltage supply as low as 2.2V which is why I asked about this. I was thinking, from previous uses, that most MOSFETs needed near 10V at the gate to turn them on, I just couldn't remember what I was looking for on the datasheet.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    http://www.fairchildsemi.com/ds/BS/BSS138.pdf
    Take a look at Figure 1. When Vgs=2v, your Vds will climb rapidly if you get over roughly 100mA Id.

    Keep in mind that those numbers will change over temperature, and between individual MOSFETs. That plot is for a typical MOSFET at room temp (25°C)
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Don't bother with a regular MOSFET - look for a logic level. Even that won't be fully on at 2.2v but will work fine for low current. Look harder if your current is more than mA levels. Oops, I just read the datasheet and see that I haven't told you didn't know.
     
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