+ Post New Thread
Page 2 of 2 FirstFirst 1 2
Results 21 to 34 of 34
  1. #21
    Full Member level 5
    Points: 2,801, Level: 12
    Achievements:
    7 years registered

    Join Date
    Aug 2009
    Location
    India
    Posts
    289
    Helped
    3 / 3
    Points
    2,801
    Level
    12

    Re: Over Voltage Protection at 28V

    Or should it be anode to ground and cathode to gate

    GND ---|>---- Gate


    I am now using D18 at 27V so the cutoff is at approx 27.6 V


    D19 will be for 12V approx

    and will still keep voltage divider for gate and will maintain Vgs at less than 10V


    Any suggestions with this configuration??


    I have been told to consider maximum input voltage at 30V and have the circuit

    so the input zener current will be

    Considering 30V

    The zener current is about (3-0.7V) / 1kΩ + 3v / 470Ω = 2.3mA + 6.4mA = 9mA (Approx).
    The zener power dissipated would then be 27V * 9mA = 243mW

    I am selecting a zener with power dessipation of 500mW, 80R impadence and 50nA reverse current



  2. #22
    Advanced Member level 5
    Points: 17,156, Level: 31
    crutschow's Avatar
    Join Date
    Feb 2012
    Location
    L.A. USA Zulu -8
    Posts
    3,379
    Helped
    788 / 788
    Points
    17,156
    Level
    31

    Re: Over Voltage Protection at 28V

    Quote Originally Posted by tiwari.sachin View Post
    I have used R93 so as to control Vg and keep Vgs around -10V in the input range of 21.6 to 26.4. R93 and R92 act as the voltage divider to control Vg
    Why?
    There's no reason to do that.
    The zener (when correctly connected) will limit the gate-source voltage to -10V.
    Quote Originally Posted by tiwari.sachin View Post
    When you say zever is backwards. Do you mean that I should connect anode to gate and cathode to source

    gate ----|>----- Source

    How does this work. I am confused on the working concept of D17
    A zener works in the reverse direction (cathode positive with respect to the anode).
    In the forward direction it looks like a normal diode and will start conducting at about 0.6V.
    Thus to clamp the gate-source voltage to -10V you need to to connect the zener cathode to the most positive voltage, which is the MOSFET source terminal.
    Zapper
    Curmudgeon Elektroniker



  3. #23
    Full Member level 5
    Points: 2,801, Level: 12
    Achievements:
    7 years registered

    Join Date
    Aug 2009
    Location
    India
    Posts
    289
    Helped
    3 / 3
    Points
    2,801
    Level
    12

    Re: Over Voltage Protection at 28V

    Click image for larger version. 

Name:	11.jpg 
Views:	5 
Size:	78.6 KB 
ID:	134224

    If I use D17 as shown in sch. Would it not limit the source voltage flow as I am using a 10V Zener?? I want input to the MOSFET upto 28V



    •   Alt6th December 2016, 07:06

      advertising

        
       

  4. #24
    Advanced Member level 5
    Points: 17,156, Level: 31
    crutschow's Avatar
    Join Date
    Feb 2012
    Location
    L.A. USA Zulu -8
    Posts
    3,379
    Helped
    788 / 788
    Points
    17,156
    Level
    31

    Re: Over Voltage Protection at 28V

    Quote Originally Posted by tiwari.sachin View Post
    Click image for larger version. 

Name:	11.jpg 
Views:	5 
Size:	78.6 KB 
ID:	134224

    If I use D17 as shown in sch. Would it not limit the source voltage flow as I am using a 10V Zener?? I want input to the MOSFET upto 28V
    Voltage doesn't "flow", only current does that.: wink:
    With D17 connected as shown and transistor Q3 is ON, the MOSFET gate voltage will be very near 28V and Vgs will be essentially 0V (turning the MOSFET OFF).
    Alternately when Q3 is OFF, Vsg will be equal to the Zener voltage (10V) because of the current through the Zener and R92 (fully turning ON the MOSFET).
    That's exactly the two states you want.
    Zapper
    Curmudgeon Elektroniker



    •   Alt6th December 2016, 09:34

      advertising

        
       

  5. #25
    Full Member level 5
    Points: 2,801, Level: 12
    Achievements:
    7 years registered

    Join Date
    Aug 2009
    Location
    India
    Posts
    289
    Helped
    3 / 3
    Points
    2,801
    Level
    12

    Re: Over Voltage Protection at 28V

    get your point... and ofcourse current flows ;)



  6. #26
    Full Member level 5
    Points: 2,801, Level: 12
    Achievements:
    7 years registered

    Join Date
    Aug 2009
    Location
    India
    Posts
    289
    Helped
    3 / 3
    Points
    2,801
    Level
    12

    Re: Over Voltage Protection at 28V

    I have used the following circuit

    Click image for larger version. 

Name:	1.png 
Views:	9 
Size:	24.9 KB 
ID:	135191

    Seems to work fine but if i keep input at... say 30V.... the R6 heats up quite a lot... PMOS would be shutdon but momentarily as R6 burns.

    Where am I going wrong.

    - - - Updated - - -

    I am using all resistors of 0603 packages



    •   Alt10th January 2017, 13:43

      advertising

        
       

  7. #27
    Super Moderator
    Points: 233,633, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    40,361
    Helped
    12331 / 12331
    Points
    233,633
    Level
    100

    Re: Over Voltage Protection at 28V

    R6 has unreasonably low value.

    And of course you might need larger resistor packages than 0603 from time to time.



  8. #28
    Full Member level 5
    Points: 2,801, Level: 12
    Achievements:
    7 years registered

    Join Date
    Aug 2009
    Location
    India
    Posts
    289
    Helped
    3 / 3
    Points
    2,801
    Level
    12

    Re: Over Voltage Protection at 28V

    I did try with 10K for R6 and still I had issues with R4 at 47K



  9. #29
    Advanced Member level 5
    Points: 12,435, Level: 26
    schmitt trigger's Avatar
    Join Date
    Apr 2013
    Location
    C4E4DEEEDCE6ECD2
    Posts
    2,103
    Helped
    686 / 686
    Points
    12,435
    Level
    26

    Re: Over Voltage Protection at 28V

    Quote Originally Posted by tiwari.sachin View Post
    How does this work. I am confused on the working concept of D17
    That is to prevent exceeding the maximum (breakdown) gate to source voltage.
    Some Mosfets have a Vgsmax of 20 volts, but the vast majority are only 15 volts.
    In either case a gate-source voltage close to 24 volts would destroy the Mosfet. So you clamp it to a lower voltage.

    Good engineering practice dictates that you take the worst case scenario (15 volts) and give yourself some margin. Thus the 10 volt zener, although a 12 volt one would also work.
    My batteries are recharged by "Helpful Post" ratings.
    If you feel that I've helped you, please indicate it as a Helpful Post



  10. #30
    Full Member level 5
    Points: 2,801, Level: 12
    Achievements:
    7 years registered

    Join Date
    Aug 2009
    Location
    India
    Posts
    289
    Helped
    3 / 3
    Points
    2,801
    Level
    12

    Re: Over Voltage Protection at 28V

    If I load D17, the MOSFET never turns ON. I am using a 12V zener.



  11. #31
    Super Moderator
    Points: 47,554, Level: 53
    Awards:
    Most Frequent Poster

    Join Date
    Apr 2014
    Posts
    9,721
    Helped
    2344 / 2344
    Points
    47,554
    Level
    53

    Re: Over Voltage Protection at 28V

    Hi,

    If I load D17, the MOSFET never turns ON. I am using a 12V zener.
    Most probably you have connected it the wrong way round.

    Klaus



    •   Alt16th January 2017, 13:28

      advertising

        
       

  12. #32
    Full Member level 5
    Points: 2,801, Level: 12
    Achievements:
    7 years registered

    Join Date
    Aug 2009
    Location
    India
    Posts
    289
    Helped
    3 / 3
    Points
    2,801
    Level
    12

    Re: Over Voltage Protection at 28V

    I am sure I have connected it the right way. Exactly the way its shown in schematic



  13. #33
    Super Moderator
    Points: 47,554, Level: 53
    Awards:
    Most Frequent Poster

    Join Date
    Apr 2014
    Posts
    9,721
    Helped
    2344 / 2344
    Points
    47,554
    Level
    53

    Re: Over Voltage Protection at 28V

    Hi,

    Then show us your exact schematic how and where you connected the zener.
    And confirm, that everything works when zener is not installed.

    Klaus



  14. #34
    Full Member level 5
    Points: 2,801, Level: 12
    Achievements:
    7 years registered

    Join Date
    Aug 2009
    Location
    India
    Posts
    289
    Helped
    3 / 3
    Points
    2,801
    Level
    12

    Re: Over Voltage Protection at 28V

    Consider following sch configuration. its a PMOS

    Click image for larger version. 

Name:	MOSFET_CNTRL.jpg 
Views:	1 
Size:	59.9 KB 
ID:	135496

    - - - Updated - - -

    [QUOTE=tiwari.sachin;1555278]I have used the following circuit

    Click image for larger version. 

Name:	1.png 
Views:	9 
Size:	24.9 KB 
ID:	135191

    This schematic also shows how zener is connected



--[[ ]]--