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  1. #21
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    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



    •   Alt5th December 2016, 15:06

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  2. #22
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    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
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    Re: Over Voltage Protection at 28V

    11.jpg

    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



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    Re: Over Voltage Protection at 28V

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

Name:	11.jpg 
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    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
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    Re: Over Voltage Protection at 28V

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



    •   Alt6th December 2016, 11:51

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  6. #26
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    Re: Over Voltage Protection at 28V

    I have used the following circuit

    1.png

    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



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    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.



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    Re: Over Voltage Protection at 28V

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



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    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.
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    •   Alt10th January 2017, 15:37

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    Re: Over Voltage Protection at 28V

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



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    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



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