+ Post New Thread
Page 1 of 2 1 2 LastLast
Results 1 to 20 of 22
  1. #1
    Advanced Member level 4
    Points: 12,674, Level: 27
    Achievements:
    Created Blog entry 7 years registered
    thannara123's Avatar
    Join Date
    Jan 2010
    Location
    God's Own country India, India
    Posts
    1,448
    Helped
    118 / 118
    Points
    12,674
    Level
    27
    Blog Entries
    2

    Require a Desgin of A MOSFET based Switch , for study purpose .

    I am stauding the MOSFET caharcterstics ,
    May I get a Desgin for a MOSFET based Switch for ON and OFF .
    with least resistance and maximum current .
    Where I start ?
    I can choose any MOSFET so please help me?
    I Respect You

  2. #2
    Super Moderator
    Points: 81,122, Level: 69
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    16,453
    Helped
    3732 / 3732
    Points
    81,122
    Level
    69

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    Hi,

    MOSFETS have the internal body diode, that may not be wanted with switches.
    Other FET don't have the diode.

    Did you do an internet search for "FETs as switch" circuit?
    Many articles with explanations and calculations are available. Study them.
    Did you read any MOSFET datasheet? Resistance and current is given in every datasheet.

    Generally you may use any (MOS)FET .... but nobody can tell you which one to choose as long as you don't give any specification.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


    1 members found this post helpful.

  3. #3
    Super Moderator
    Points: 264,250, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,176
    Helped
    14042 / 14042
    Points
    264,250
    Level
    100

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    "MOSFET based switch" might refer to a solid state relays circuit rather than a bare MOSFET. In any case, you should specify voltage and current rating, in case of a relays circuit also control voltage, isolation and speed rating.


    1 members found this post helpful.

  4. #4
    Advanced Member level 4
    Points: 12,674, Level: 27
    Achievements:
    Created Blog entry 7 years registered
    thannara123's Avatar
    Join Date
    Jan 2010
    Location
    God's Own country India, India
    Posts
    1,448
    Helped
    118 / 118
    Points
    12,674
    Level
    27
    Blog Entries
    2

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    Consider this only for learning purpose .
    I am Choosing N Channel MOSFET 90NF03L https://www.st.com/resource/en/datasheet/stp90nf03l.pdf
    Click image for larger version. 

Name:	1.PNG 
Views:	4 
Size:	69.0 KB 
ID:	157289Click image for larger version. 

Name:	2.PNG 
Views:	6 
Size:	94.2 KB 
ID:	157290

    As per datasheet ,

    Countinues Current @ 100degree The Current Id is 65 A
    Maximum VDS 30 volt

    My load has 20 watt (LED Bulb) so the aproximate Current 1.7 Amp and VDD as 12 Volt .
    So I takes as follows

    Vdd = 12 volt
    Id = 2Amp

    Switch has two Position ie ,Cutoff and Saturation

    So Cutoff = Vgs<Vth ---- = 0 volt ----> MOSFET OFF.

    At switch Close Position ---> saturation

    Vds>Vgs-Vth

    So I takes Vgs as 10Volt .


    is it correct way .or pleas lead me correct way
    What is the other considrations ?
    is require surge protection circuits ,cappacitance ,Gate resistance ?
    I Respect You



  5. #5
    Super Moderator
    Points: 81,122, Level: 69
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    16,453
    Helped
    3732 / 3732
    Points
    81,122
    Level
    69

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    Hi,

    At first please clarify the expected function:

    In your first post you wrote "MOSFET based Switch for ON and OFF".
    Thus my first idea was a circuit like a bilateral analog switch fir signals...
    But now it rather seems like a "low side power switch"

    To your specifications:
    A Mosfet needs to withstand:
    * the maximum applied voltage V_DS --> in your case the supply voltage (plus some headroom)
    * the maximum drain current --> in your case the load current (plus some headroom)
    * the temperature = maximum ambient temperature + temperature rise caused by I_drain_RMS ^2 × R_ds_on

    And yes, V_gs should be close to zero to switch OFF the Mosfet
    And should be much higher thanV_gs_th but lower than V_gs_max to switch ON the Mosfet.

    Mind: any inductance will cause voltage spikes during switch/OFF. You need to take care that this is less than V_DS_max .... in any case and even for nanoseconds.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  6. #6
    Advanced Member level 4
    Points: 12,674, Level: 27
    Achievements:
    Created Blog entry 7 years registered
    thannara123's Avatar
    Join Date
    Jan 2010
    Location
    God's Own country India, India
    Posts
    1,448
    Helped
    118 / 118
    Points
    12,674
    Level
    27
    Blog Entries
    2

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    Quote Originally Posted by KlausST View Post
    Hi,

    At first please clarify the expected function:

    In your first post you wrote "MOSFET based Switch for ON and OFF".
    Thus my first idea was a circuit like a bilateral analog switch fir signals...
    But now it rather seems like a "low side power switch"

    To your specifications:
    A Mosfet needs to withstand:
    * the maximum applied voltage V_DS --> in your case the supply voltage (plus some headroom)
    * the maximum drain current --> in your case the load current (plus some headroom)
    * the temperature = maximum ambient temperature + temperature rise caused by I_drain_RMS ^2 × R_ds_on

    And yes, V_gs should be close to zero to switch OFF the Mosfet
    And should be much higher thanV_gs_th but lower than V_gs_max to switch ON the Mosfet.

    Mind: any inductance will cause voltage spikes during switch/OFF. You need to take care that this is less than V_DS_max .... in any case and even for nanoseconds.

    Klaus
    Then
    is correct My aproximation?
    I Respect You



    •   AltAdvertisement

        
       

  7. #7
    Advanced Member level 5
    Points: 54,096, Level: 56
    Achievements:
    7 years registered
    Audioguru's Avatar
    Join Date
    Jan 2008
    Location
    Toronto area of Canada
    Posts
    8,815
    Helped
    2068 / 2068
    Points
    54,096
    Level
    56

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    Don't look at Vgs(th) because that is the gate-source voltage when the Mosfet is almost turned off and it is barely turned on. Since you want a switch then you should look at the leakage current when the Vgs is 0V and look at the Vgs voltage at a certain maximum on resistance. The Mosfet you selected is fully turned on when Vgs is 10V and it is turned on pretty well when Vgs is 5V.

    For your fairly small current of only 1.6A then using that Mosfet rated at 90A is probably throwing away a lot of money. Use a smaller, less expensive Mosfet which will still have a rating much higher than you need.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  8. #8
    Super Moderator
    Points: 264,250, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,176
    Helped
    14042 / 14042
    Points
    264,250
    Level
    100

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    Switch has two Position ie ,Cutoff and Saturation

    So Cutoff = Vgs<Vth ---- = 0 volt ----> MOSFET OFF.

    At switch Close Position ---> saturation

    Vds>Vgs-Vth

    So I takes Vgs as 10Volt .
    Vgs = 10V is good (low Rdson, low switch voltage drop), but the "on" operation point is not saturation, it's ohmic mode.



  9. #9
    Advanced Member level 4
    Points: 12,674, Level: 27
    Achievements:
    Created Blog entry 7 years registered
    thannara123's Avatar
    Join Date
    Jan 2010
    Location
    God's Own country India, India
    Posts
    1,448
    Helped
    118 / 118
    Points
    12,674
    Level
    27
    Blog Entries
    2

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    If Vgs is 11volt and Vds is 10 volt it saying that Ohmic region .
    Click image for larger version. 

Name:	Triode region.PNG 
Views:	5 
Size:	16.4 KB 
ID:	157349Click image for larger version. 

Name:	saturationregion.PNG 
Views:	4 
Size:	18.2 KB 
ID:	157350Click image for larger version. 

Name:	Capture.PNG 
Views:	8 
Size:	15.0 KB 
ID:	157351

    As per the ohmic and saturation region the plots shows no diffrents why so .

    What is the wrong with me ?
    I Respect You



  10. #10
    Advanced Member level 4
    Points: 12,674, Level: 27
    Achievements:
    Created Blog entry 7 years registered
    thannara123's Avatar
    Join Date
    Jan 2010
    Location
    God's Own country India, India
    Posts
    1,448
    Helped
    118 / 118
    Points
    12,674
    Level
    27
    Blog Entries
    2

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    And Comonly what type of protections are required for a MOSFET.
    I Respect You



  11. #11
    Super Moderator
    Points: 264,250, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,176
    Helped
    14042 / 14042
    Points
    264,250
    Level
    100

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    As per the ohmic and saturation region the plots shows no diffrents why so .

    What is the wrong with me?
    V1 is the supply voltage, not Vds. Vds in on-state is in the mV range, both for V1 =10 and V1=12V.


    1 members found this post helpful.

  12. #12
    Advanced Member level 4
    Points: 12,674, Level: 27
    Achievements:
    Created Blog entry 7 years registered
    thannara123's Avatar
    Join Date
    Jan 2010
    Location
    God's Own country India, India
    Posts
    1,448
    Helped
    118 / 118
    Points
    12,674
    Level
    27
    Blog Entries
    2

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    Quote Originally Posted by FvM View Post
    V1 is the supply voltage, not Vds. Vds in on-state is in the mV range, both for V1 =10 and V1=12V.
    confused a little
    Mesuare the Vds when it ON it gives 12mv and 10.5mv for V1 =12v and V1 = 10 respectively .
    I Respect You



  13. #13
    Super Moderator
    Points: 264,250, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,176
    Helped
    14042 / 14042
    Points
    264,250
    Level
    100

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    Measure the Vds when it ON it gives 12mv and 10.5mv for V1 =12v and V1 = 10 respectively.
    Should be considered ohmic range, isn't it.

    The supply voltage is dropped at R1.



  14. #14
    Advanced Member level 4
    Points: 12,674, Level: 27
    Achievements:
    Created Blog entry 7 years registered
    thannara123's Avatar
    Join Date
    Jan 2010
    Location
    God's Own country India, India
    Posts
    1,448
    Helped
    118 / 118
    Points
    12,674
    Level
    27
    Blog Entries
    2

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    Click image for larger version. 

Name:	e.PNG 
Views:	4 
Size:	35.7 KB 
ID:	157366
    How can i attain in ohmic / Triode region ?
    As per the data sheet The Vds should be in the range of 2.5volt for the ohmic region.
    and Vgs is 5volt .

    In the above circuit ,
    V2 is the Vgs volt at the ON time isnt it ? it have 11 volt ..
    And is the VDD volt will be the Vds volt (which will be appere the Vds after load drop) ?

    I want a desgin as follows ?may get with calculation

    i need control a bulb 12 volt ,2 Amp. which is controlled by the Vgs of the above MOSFET .
    I Respect You



  15. #15
    Advanced Member level 5
    Points: 54,096, Level: 56
    Achievements:
    7 years registered
    Audioguru's Avatar
    Join Date
    Jan 2008
    Location
    Toronto area of Canada
    Posts
    8,815
    Helped
    2068 / 2068
    Points
    54,096
    Level
    56

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    The graphs on a datasheet are usually for a Mosfet or transistor with "typical" spec's. Yours might have different minimum or maximum spec's. The text in a datasheet shows minimum and maximum spec's.
    You will need to design a circuit with an adjustment for matching the spec's of the Mosfet you use. Sometimes adding negative feedback can reduce the range of spec's.

    You said that you want to "control" a 12V/2A bulb. To turn it on and off? The Mosfet datasheet says the Mosfet is off when Vgs is zero and it is turned on well when Vgs is 10V.



  16. #16
    Advanced Member level 4
    Points: 12,674, Level: 27
    Achievements:
    Created Blog entry 7 years registered
    thannara123's Avatar
    Join Date
    Jan 2010
    Location
    God's Own country India, India
    Posts
    1,448
    Helped
    118 / 118
    Points
    12,674
    Level
    27
    Blog Entries
    2

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    [QUOTE=You said that you want to "control" a 12V/2A bulb. To turn it on and off? The Mosfet datasheet says the Mosfet is off when Vgs is zero and it is turned on well when Vgs is 10V.[/QUOTE]

    Thanks ,
    But then Is the MOSFET works on cutoff and Ohmic region ?
    or cutoff and stauration region?
    I Respect You



  17. #17
    Super Moderator
    Points: 264,250, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,176
    Helped
    14042 / 14042
    Points
    264,250
    Level
    100

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    You are still missing a basic understanding of MOSFET operation.

    To get a better idea, you can use the post #9 simulation circuit and perform a dc sweep as shown below. Determine for which Id and Vds range the transistor is in cutoff, saturation and ohmic mode. You'll find that a transistor switch is only temporarily in saturation, you want to avoid this operation due to high power dissipation.

    Click image for larger version. 

Name:	dcsweep.PNG 
Views:	8 
Size:	82.9 KB 
ID:	157373


    1 members found this post helpful.

  18. #18
    Advanced Member level 5
    Points: 54,096, Level: 56
    Achievements:
    7 years registered
    Audioguru's Avatar
    Join Date
    Jan 2008
    Location
    Toronto area of Canada
    Posts
    8,815
    Helped
    2068 / 2068
    Points
    54,096
    Level
    56

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    You are confused about Mosfet operation because you are using English terms for an ordinary transistor.
    An ordinary transistor is linear when it is partially turned on. It is saturated when it is turned so that it has a low Vce.

    But a Mosfet is said to be "saturated" when it is partially turned on and it is "linear" or "ohmic" when it is turned on so that it has a low Vds.



    •   AltAdvertisement

        
       

  19. #19
    Advanced Member level 4
    Points: 12,674, Level: 27
    Achievements:
    Created Blog entry 7 years registered
    thannara123's Avatar
    Join Date
    Jan 2010
    Location
    God's Own country India, India
    Posts
    1,448
    Helped
    118 / 118
    Points
    12,674
    Level
    27
    Blog Entries
    2

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    Quote Originally Posted by Audioguru View Post
    You are confused about Mosfet operation because you are using English terms for an ordinary transistor.
    An ordinary transistor is linear when it is partially turned on. It is saturated when it is turned so that it has a low Vce.

    But a Mosfet is said to be "saturated" when it is partially turned on and it is "linear" or "ohmic" when it is turned on so that it has a low Vds.
    Hai sir i don't have confusion as you said . I know ohmic or linear or triode region is the low resistance mode of the MOSFET operation. Where the condition also known Vds <Vgs-Vth.

    I am confused with how attain the ohmic region.and it's calculations
    I have to test the simulation as per FvM.
    I Respect You



  20. #20
    Advanced Member level 5
    Points: 54,096, Level: 56
    Achievements:
    7 years registered
    Audioguru's Avatar
    Join Date
    Jan 2008
    Location
    Toronto area of Canada
    Posts
    8,815
    Helped
    2068 / 2068
    Points
    54,096
    Level
    56

    Re: Require a Desgin of A MOSFET based Switch , for study purpose .

    The spec's for every Mosfet show its maximum on-resistance when Vgs is 10V. Some Mosfets are called "logic level" and they show the maximum on resistance when Vgs is 4.5V or 5V. There is nothing to calculate unless you test the on resistance of each Mosfet you are using (each one is different even if they have the same part number). You might find (not calculate) a Mosfet that turns on well with a lower Vgs. Like a transistor, a Mosfet has a range of spec's. Some are minimum and some are maximum.

    A simulation uses a device with a "typical" on resistance and a "typical" Vgs. But the spec's for the Mosfet you use might be different from a "typical" Mosfet.


    1 members found this post helpful.

--[[ ]]--