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    MOSFET linear and saturation region operation

    Hi,

    N channel MOSFET Saturation operation condition is Vds > (Vgs - Vth).
    But in saturation region, MOSFET will act like closed switch. hence Vds is approximately 0V(Rds*I). Examble VGS = 10V and VTH is 2V, and if switch is closed means VDS= 0V, but above formula is not satisfied.

    Anyone kindly clarify this ?

    And How to drive the N channel MOSFET to act as variable resistor ?
    I saw the condition Vds < (Vgs -Vth), but how to calculate Vgs for required resistor ?

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    Re: MOSFET linear and saturation region operation

    i searched on "international rectifier fet gate charge" (google)

    look at


    International rectifier power mosfet basics
    figure 13
    Power MOSFET Basics - Infineon Technologies
    https://www.google.com/url?sa=t&rct=...VIhaoOEA610FJ-

    International rectifier application note AN-944
    figure 5
    Use Gate Charge to Design the Gate Drive Circuit for Power MOSFETs ...
    https://www.google.com/url?sa=t&rct=...fZe6deoAi2juEK

    those figures show the behavior of fet as a function of gate charge
    there are clearly indicated regions - off, turning on, linear, and on (saturated)

    to make the fet act like a variable resistor, apply a feedback circuit to control the gate based on the desired ???
    sometimes it helps to add a resistor across the drain source to be the maximum resistance
    then, as the fet moves through the linear regions, the resistance drops
    the saturated fet gives the lower limit of the resistance


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    Re: MOSFET linear and saturation region operation

    MOSFET behaves like a resistor in linear region with resistance = 1/(un*cox*(W/L)*(Vgs - Vth)) for small values of Vds.



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    Re: MOSFET linear and saturation region operation

    Quote Originally Posted by BoopathiS View Post
    Hi,

    N channel MOSFET Saturation operation condition is Vds > (Vgs - Vth).
    But in saturation region, MOSFET will act like closed switch. hence Vds is approximately 0V(Rds*I). Examble VGS = 10V and VTH is 2V, and if switch is closed means VDS= 0V, but above formula is not satisfied.

    Anyone kindly clarify this ?

    And How to drive the N channel MOSFET to act as variable resistor ?
    I saw the condition Vds < (Vgs -Vth), but how to calculate Vgs for required resistor ?
    MOSFET acts like a switch and it is opened when vgs < vth but closed when vgs > vth.
    From simple MOSFET equation, ignoring body effect and etc.
    When Vds < vgs - vth it acts like a variable resistor. Current allowed to pass through will be directly proportional to Vds.
    But when Vds > vgs - vth, it will be saturated and the current allowed to pass through will be saturated, no matter how high is the Vds.



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    Re: MOSFET linear and saturation region operation

    Hi,

    In saturation region, with the constraint you provided, namely: Vg > Vth and Vds > Vds - Vth, the MOSFET is an amplifier.

    This is a major difference between the MOSFET and the BJT.
    Last edited by Akanimo; 15th July 2019 at 18:32.
    -------------
    --Akanimo.


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    Re: MOSFET linear and saturation region operation

    Quote Originally Posted by BoopathiS View Post
    N channel MOSFET Saturation operation condition is Vds > (Vgs - Vth).
    But in saturation region, MOSFET will act like closed switch. hence Vds is approximately 0V(Rds*I). Examble VGS = 10V and VTH is 2V, and if switch is closed means VDS= 0V, but above formula is not satisfied.

    Anyone kindly clarify this ?
    Perhaps this diagram from Wikipedia will help clarify:

    Click image for larger version. 

Name:	Capture.PNG 
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Size:	30.1 KB 
ID:	154431

    And How to drive the N channel MOSFET to act as variable resistor ?
    I saw the condition Vds < (Vgs -Vth), but how to calculate Vgs for required resistor ?
    There's no easy way to calculate the Vds required for a required resistor value since there's a wide variation in the Vgs threshold voltage between MOSFETs, even those of the same part number.
    One way is to measure the transistor you have to determine the resistance versus Vgs curve.
    Zapper
    Curmudgeon Elektroniker


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    Re: MOSFET linear and saturation region operation

    One question I have when someone understands this stuff is why?

    A mosftet goes from 'as off as it gets' -> partly on -> 'more on' -> 'mostly on' -> 'as on as it gets' as Vgs increases. But the Vgs threshold is so widely specified you can't count on anything in the linear region. So the exact relationship and the equations governing it are typically irrelevant. I've designed a lot of linear circuits and I've certainly never referenced any.


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    Re: MOSFET linear and saturation region operation

    The MOSFET acts as a zero-voltage drop device (ideally) IF it is connected to a load. If the MOSFET is by itself, it will of course have a voltage at the drain.



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    Re: MOSFET linear and saturation region operation

    Quote Originally Posted by asdf44 View Post
    One question I have when someone understands this stuff is why?

    A mosftet goes from 'as off as it gets' -> partly on -> 'more on' -> 'mostly on' -> 'as on as it gets' as Vgs increases. But the Vgs threshold is so widely specified you can't count on anything in the linear region. So the exact relationship and the equations governing it are typically irrelevant. I've designed a lot of linear circuits and I've certainly never referenced any.
    Because that describes how the MOSFET responds to the gate-source voltage in the linear region.
    Just because you've never used that info doesn't mean no-one does.

    Besides, the info in the "linear region" is typically used when the MOSFET is used as a switch or variable resistance.
    For linear (i.e. AC) amplifiers the MOSFET typically operates in the saturated region.
    (It can be confusing because that's more or less opposite of the BJT definitions for the two operating regions).
    Zapper
    Curmudgeon Elektroniker



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    Re: MOSFET linear and saturation region operation

    Quote Originally Posted by crutschow View Post
    Because that describes how the MOSFET responds to the gate-source voltage in the linear region.
    Just because you've never used that info doesn't mean no-one does.

    Besides, the info in the "linear region" is typically used when the MOSFET is used as a switch or variable resistance.
    For linear (i.e. AC) amplifiers the MOSFET typically operates in the saturated region.
    (It can be confusing because that's more or less opposite of the BJT definitions for the two operating regions).
    Eh my point is that once you wrap it with feedback the exact equations become irrelevant.

    And if you don't wrap it in feedback part variations mean you better not care about the exact relationship either.

    Both cases are well served by understanding the shapes of the curves but don't necessitate plugging numbers into the equations.



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    Re: MOSFET linear and saturation region operation

    Quote Originally Posted by asdf44 View Post
    Eh my point is that once you wrap it with feedback the exact equations become irrelevant.

    And if you don't wrap it in feedback part variations mean you better not care about the exact relationship either.

    Both cases are well served by understanding the shapes of the curves but don't necessitate plugging numbers into the equations.
    Okay.
    But that's generally true for any parts in a feedback loop, not just MOSFETs.
    It you don't need the equations, don't use them.
    But they are necessary for a fundamental understanding of the devices.
    Zapper
    Curmudgeon Elektroniker



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    Re: MOSFET linear and saturation region operation

    I have designed power outputs with MOSFET's where the short circuit protection was implemented using the Ids/Vgs diagram in the datasheet.
    By not switching on the MOSFET to reach the minimum Rds, it will act as a current limiter.
    To allow for temperature and individual variations, we adjusted Vgs so the maximum current in the MOSFET would nominally be 3 times the maximum load current.
    When a short circuit occurs, the voltage will increase a lot over the MOSFET and that is very easy to detect, and Vgs can be switched off early enough to keep the MOSFET within the SOA (Safe Operating Area).



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