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  1. #1
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    Over Voltage Protection at 28V

    I am using the following circuit

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    The load will draw a max of about 10 amps at 24V

    I want a voltage cut off when the input voltage crosses 28V DC or the fuse resistance will increase if the current crosses 10A. I am using PTC

    I need to decide on part values.

    The SCR I want to use is S602TSRP
    TL431 - Voltage reference

    Transistor and other components need to be decided

    Is the concept I am trying correct for given ratings

    •   Alt28th November 2016, 08:59

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    Re: Over Voltage Protection at 28V

    SCR crow-bar circuits have been used since long and are surely effective. The threshold would be more precise if R205 is connected to 24V node.

    You should check that the PTC fuse can stand a full short without damage.



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    Re: Over Voltage Protection at 28V

    Hi,

    The SCR will be low impedance as long as current flows.

    And a PTC fuse will never be that high impedance that an SCR releases.
    So once it was triggered it will stay in "failure" mode.

    Klaus



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    Re: Over Voltage Protection at 28V

    So once it was triggered it will stay in "failure" mode.
    Yes it requires power cycling, think this is intentional.



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    Re: Over Voltage Protection at 28V

    I have changed the sch

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    I am using S602TSRP - SCR, TL431CDR and transistor BC856

    I havenot used TL431 earlier, I found that the reference has to be set from 2.5 to 30V and the SCR should trigger when the supply reaches 28V

    I am not clear on how the calculations for resistors are done. Its a voltage divider at TL431 but for what voltage should i design this so that TL431 operates when the inout is 28V.

    If I am considering 2.5V as reference

    If I use R7 as 10K and R9 as 980R. The out voltage when input is 28V will be 2.5V. Will this activa



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    Re: Over Voltage Protection at 28V

    Quote Originally Posted by tiwari.sachin View Post
    ................
    If I use R7 as 10K and R9 as 980R. The out voltage when input is 28V will be 2.5V. Will this activa
    Yes, the voltage at the Tl431 control pin will reach 2.5V when the output is 28V with those resistor values, which will then cause the cathode (top) pin to start conducting current above that point, turning on Q1 and triggering the SCR.
    Zapper
    Curmudgeon Elektroniker



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    Re: Over Voltage Protection at 28V

    I am thinking of using a P MOSFET to cut off the supply when 28 V is reached.

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    This option should also work right? Seems ok

    I ofcourse need to do my calculations yet. Choosing MOSFER with Vgs -4V and Id = 13 or 14A



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    Re: Over Voltage Protection at 28V

    Quote Originally Posted by tiwari.sachin View Post
    I am thinking of using a P MOSFET to cut off the supply when 28 V is reached.
    ............................
    This option should also work right? Seems ok

    I ofcourse need to do my calculations yet. Choosing MOSFER with Vgs -4V and Id = 13 or 14A
    You want a MOSFET with a low ON resistance (no more than a few milliohms) to minimize any voltage drop and I²R power dissipation during normal operation.
    The Vgs(th) is generally not critical in this application.
    Zapper
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    Re: Over Voltage Protection at 28V

    I am thinking of using Si4401DDY which has following specs

    0.015 at VGS = - 10 V when Id = 16.1 Amps
    0.022 at VGS = - 4.5 V when Id =13.3 Amps

    Max Vds = -40V

    Vgs threshold is -1.2 to -2.5 (Max)

    Max Vgs is 20V (This is a concern)

    I am connecting 24V to source. This voltage maximum can go to 30V (Failure case)

    gate is controlled via transistor to GND (When on gate voltage will be 0)

    In this scnerio (Vgs = Vg -Vs = 0 - 24V which is greater than 20V given in datasheet

    How can I control gate voltage so that it is less than 20V (Max Vgs) maybe somewhere around 10-12V (More acceptable value)



    •   Alt2nd December 2016, 08:08

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    Re: Over Voltage Protection at 28V

    How can I control gate voltage so that it is less than 20V (Max Vgs) maybe somewhere around 10-12V (More acceptable value)
    Zener diodes have been invented for similar purposes...

    You should add a few volts hysteresis to your circuit to enforce clear on or off state of the switch and avoid fast switching bursts that might damage MOSFET and load. A well calculated resistor between Q1 collector and TL431 control input will do.



    •   Alt2nd December 2016, 08:40

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    Re: Over Voltage Protection at 28V

    I have added a resistor in the following sch

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    At say at MIN INPUT : 22V, gate will have 14.966 and Vgs = 14.966-22V = -7.034 and MOSFET is ON
    At INPUT:24V, gate will have 16.327 and Vgs = 16.327-24V = -7.67 and MOSFET is ON
    At say at MAX INPUT : 28V (or 27.9), gate will have 19.048 and Vgs = 19.048-28V = -8.95 and MOSFET is ON

    At INPUT is 28V (+ few mV maybe) or more, the control pin on TL431 will be at 2.5V (Vref of TL431), PNP will turn ON, Voltage at gate will go to 0 and Vgs = 28V-28V and MOSFET will be off

    Why and how are we talking about connecting extra zener?? what voltage??



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    Re: Over Voltage Protection at 28V

    Using a voltage divider to reduce the gate voltage is an option, a zener diode has the advantage of giving constant Vgs for optimal MOSFET operation independent of supply voltage.

    I keep my point that a hysteresis would be preferred.



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    Re: Over Voltage Protection at 28V

    Quote Originally Posted by tiwari.sachin View Post
    I am thinking of using Si4401DDY which has following specs

    0.015 at VGS = - 10 V when Id = 16.1 Amps
    0.022 at VGS = - 4.5 V when Id =13.3 Amps

    Max Vds = -40V
    .................
    That device will dissipate 1.5W@10A so it may need a small heatsink since the case thermal resistance to air is a maximum of 50C/W.
    Zapper
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    Re: Over Voltage Protection at 28V

    Alternately if you use a MOSFET with less than 10mΩ ON resistance you shouldn't need a heatsink.
    Zapper
    Curmudgeon Elektroniker



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    Re: Over Voltage Protection at 28V

    And if you anyway have a MOSFET in your power path, consider replacing the fuse with active current sensing and cut the power using the MOSFET. A MOSFET is way faster than a fuse.



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    Re: Over Voltage Protection at 28V

    Quote Originally Posted by ArticCynda View Post
    And if you anyway have a MOSFET in your power path, consider replacing the fuse with active current sensing and cut the power using the MOSFET. A MOSFET is way faster than a fuse.
    Depends upon the MOSFET and the maximum short circuit current available, as to whether the MOSFET will open before the fuse.
    Zapper
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    Re: Over Voltage Protection at 28V

    I have changed the sch a bit. Removed TL431 and replaced with a NPN and a zener of 28V

    Attachment 134134

    FVM: I dint understand how zener should be used. D17 that I have connected is it right? and how would that be help ful


    Crutschow: I shall have values such that Vgs is about -10V (plus minus as the input voltage can vary). The MOSFET Rds is 8mOhm and -10V

    Considering this, the power would be around 1.25W at 12Amps

    Max Vds is 30V and since Vds can go upto 28 (although this is rare), I am more towards choosing MOSFET with max Vds around 40V but cannot due to cost issues

    Is 30V good enough??

    I think the Power dessipation with about 1.5 sq cms PCB heat sink on all 6 layers should do the trick

    - - - Updated - - -

    How can I calculate power dessipation of the 28V zener by the reverse current when the input crosses 28V so that the diode doesnt fail (as someone can just plug in 30V and leave). In that case MOSFET can go off but the zener shouldnt get damaged and shouldbe within its thermal limits

    - - - Updated - - -

    I think MMSZ5255B-7-F (28V Zener) should do the work

    Below is its spec

    Click image for larger version. 

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    - - - Updated - - -

    Simulations are not giving correct results. Can I even use a 28V zener like that??

    I kept this video as reference

    https://www.youtube.com/watch?v=XjVHNaPZEwE



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    Re: Over Voltage Protection at 28V

    Click image for larger version. 

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    Updated SCH



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    Re: Over Voltage Protection at 28V

    Quote Originally Posted by tiwari.sachin View Post
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    Updated SCH
    At 30V the D18 zener dissipation would be 28V times its current.
    The zener current is about (2V-0.7V) / 1kΩ + 2v / 470Ω = 1.3mA + 4.26mA = 5.6mA.
    The zener power dissipated would then be 28V * 5.6mA = 157mW.

    D17 is in backwards.

    When connected correctly it's dissipation would be ((24V-10V) / 10kΩ) * 10V = 14mW.

    Yes, you can use a Zener like that. But note that the threshold is 28.7V due to the 0.7V drop of Q4's base-emitter.

    Eliminate R93. It serves no useful purpose.

    How does the simulation not work?
    Zapper
    Curmudgeon Elektroniker



    •   Alt5th December 2016, 09:09

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    Re: Over Voltage Protection at 28V

    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

    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



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