Mosfet V_GS question.

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  • Rob Northen
    Senior Member
    • Jan 2010
    • 137
    • Denmark

    #1

    Mosfet V_GS question.

    I've made a another little project, including a LCD. This time, I thought it could be interesting to control the back light with PWM. I'ts done with a PWM signal from a Atmega 328 MCU, which goes to a IRFZ46N mosfet. It is working like a charm.

    My question is, IRFZ46N V_GS (th) voltage is defined 2-4v. Its a power Mosfet. "Logic" Mosfet's usuallt has V_ GS (th) 1-2v.
    According to other forums, it should not be possible to drive a IRFZ46N from logic level, but it seems to work. The logic level is 5V.

    What do miss ?
  • Th3_uN1Qu3
    Believe in
    • Jul 2010
    • 6031
    • Romania

    #2
    Re: Mosfet V_GS question.

    5v on a 4v Vgs(th) mosfet is a bit on the edge, but your backlight presents a very low load, so it doesn't matter much. A few mW extra dissipated in the fet... Unless your device runs on batteries i wouldn't be concerned. When you get to switching high currents, that's where the level of the gate drive comes into play.
    Originally posted by PeteS in CA
    Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
    A working TV? How boring!

    Comment

    • Rob Northen
      Senior Member
      • Jan 2010
      • 137
      • Denmark

      #3
      Re: Mosfet V_GS question.

      No no, it runs from mains.
      What is the recommended voltage for Vgs(th) ?

      Comment

      • PeteS in CA
        Badcaps Legend
        • Aug 2005
        • 3578
        • USA, Unsure of Planet

        #4
        Re: Mosfet V_GS question.

        Not clear what is puzzling you. A standard MOSFET will turn on at ~4V, but at 5V G-S it may or may not be in saturation. A "Logic-Level" MOSFET will be in saturation at 5V G-S. If the gate drive signal from your MCU is 5V, a standard MOSFET will be on, and the Drain current in your circuit may be low enough that not being in saturation (if that is the case) isn't causing a problem. That your switch MOSFET is on the primary (mains) side of the circuit doesn't mean much, unless it gets ~300VDC on the Gate, in which case you'll have a brief, bright, noisy and bad-smelling event.
        PeteS in CA

        Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
        ****************************
        To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
        ****************************

        Comment

        • Rob Northen
          Senior Member
          • Jan 2010
          • 137
          • Denmark

          #5
          Re: Mosfet V_GS question.

          Originally posted by PeteS in CA
          Not clear what is puzzling you. A standard MOSFET will turn on at ~4V, but at 5V G-S it may or may not be in saturation. A "Logic-Level" MOSFET will be in saturation at 5V G-S. If the gate drive signal from your MCU is 5V, a standard MOSFET will be on, and the Drain current in your circuit may be low enough that not being in saturation (if that is the case) isn't causing a problem. That your switch MOSFET is on the primary (mains) side of the circuit doesn't mean much, unless it gets ~300VDC on the Gate, in which case you'll have a brief, bright, noisy and bad-smelling event.
          What is puzzeling is these line from the IRF46N datasheet:

          -------------------------------Min Max Units
          VGS(th) Gate Threshold Voltage 2.0 ––– 4.0 V

          I read it as max 4.0v. But I've been told that is not correct, and I read it wrong.

          Comment

          • budm
            Badcaps Legend
            • Feb 2010
            • 40746
            • USA

            #6
            Re: Mosfet V_GS question.

            You may want to read this to understand the Vgs and Vgs (th).Page 22 of Fairchild app notes:
            Threshold Voltage (VGS(th))
            This is the minimum gate bias which enables the formation of the channel between the source and
            the drain. The drain current increases in proportion to (VGS–VGS(th))2 in the saturation region.
            Attached Files
            Never stop learning
            Basic LCD TV and Monitor troubleshooting guides.
            http://www.badcaps.net/forum/showthr...956#post305956

            Voltage Regulator (LDO) testing:
            http://www.badcaps.net/forum/showthr...999#post300999

            Inverter testing using old CFL:
            http://www.badcaps.net/forum/showthr...er+testing+cfl

            Tear down pictures : Hit the ">" Show Albums and stories" on the left side
            http://s807.photobucket.com/user/budm/library/

            TV Factory reset codes listing:
            http://www.badcaps.net/forum/showthread.php?t=24809

            Comment

            • PeteS in CA
              Badcaps Legend
              • Aug 2005
              • 3578
              • USA, Unsure of Planet

              #7
              Re: Mosfet V_GS question.

              You may have spotted the point of confusion, bud. The V(GS) th specifies the range in which the part is guaranteed to start to turn on. The absolute maximum G-S voltage is 20V (Datasheet front page). Also, a MOSFET is not a perfect switch, either fully on or fully off. There is a voltage range between its turn-on threshold and the gate voltage at which it goes into saturation. In that range the MOSFET can be used as an amplifier rather than a switch.
              Last edited by PeteS in CA; 04-18-2012, 01:14 PM.
              PeteS in CA

              Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
              ****************************
              To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
              ****************************

              Comment

              • budm
                Badcaps Legend
                • Feb 2010
                • 40746
                • USA

                #8
                Re: Mosfet V_GS question.

                Agree, It confuses a lot of people when they see the MIN and Max numbers of Vgs(th) without looking at the Vgs spec.
                Never stop learning
                Basic LCD TV and Monitor troubleshooting guides.
                http://www.badcaps.net/forum/showthr...956#post305956

                Voltage Regulator (LDO) testing:
                http://www.badcaps.net/forum/showthr...999#post300999

                Inverter testing using old CFL:
                http://www.badcaps.net/forum/showthr...er+testing+cfl

                Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                http://s807.photobucket.com/user/budm/library/

                TV Factory reset codes listing:
                http://www.badcaps.net/forum/showthread.php?t=24809

                Comment

                • Rob Northen
                  Senior Member
                  • Jan 2010
                  • 137
                  • Denmark

                  #9
                  Re: Mosfet V_GS question.

                  Thanks to both of you, it made more clear. I did notice the Vgs +-20v, but VGSth
                  comfused me...

                  Comment

                  • tom66
                    EVs Rule
                    • Apr 2011
                    • 32560
                    • UK

                    #10
                    Re: Mosfet V_GS question.

                    In a robot competition, we used a power MOSFET to control a pump. The IO output gave 3.3V but this was fine. The MOSFET dropped about 0.5V at 500mA so this is about 1 ohm rds(on). Was just a prototype .
                    Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
                    For service manual, schematic, boardview (board view), datasheet, cad - use our search.

                    Comment

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