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I have used the following circuit to for the purpose of reverse voltage protection. But, I am a little bit confuse about the working of the zener diode in combination of the 100k resistor in this circuit.

MOSFET Circuit with Zener Diode

So, what is the need of the zener diode in combination of the resistor in this circuit? How they are exactly working here?

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So, what is the need of the zener diode in combination of the resistor in this circuit? How they are exactly working here?

The maximum gate source voltage for the FDN306P MOSFET is +/- 8 volts hence, on a 12 volt supply, if you didn't use a zener diode and the gate resistor you would be exceeding the limit specified in the data sheet.

enter image description here

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  • \$\begingroup\$ But,in this circuit by turning on this power supply, i have measured the Gate and source voltage wrt GND. The source voltage is found to be 11.96 V and gate voltage is found to be 8.4. \$\endgroup\$ – Aakash Dey Dec 27 '18 at 14:08
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    \$\begingroup\$ Yes, and the difference between those voltages is 3.56 volts and pretty much what you'd get from a 5.6 volt zener operating on such light currents. Zeners are by no means perfect devices and don't magically switch from an open impedance to a short impedance at their zener voltage. \$\endgroup\$ – Andy aka Dec 27 '18 at 14:10
  • \$\begingroup\$ Now, removing only the Zener Diode (not the resistor), I found the same voltage as above. So, is there any problem in the circuit? \$\endgroup\$ – Aakash Dey Dec 27 '18 at 14:17
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    \$\begingroup\$ Without the zener, the gate source voltage would be excessive and potentially damage that junction. Once damaged it may naturally limit the voltage to a value lower than 12 volts. \$\endgroup\$ – Andy aka Dec 27 '18 at 14:30
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    \$\begingroup\$ The protector also needs protection. The zener is the protector (of the mosfet gate source junction) and the resistor protects the 5.6 volt zener receiving a gaziliion amps from the 12 volt supply. The resistor can certainly be lower in value and still protect the zener of course. \$\endgroup\$ – Andy aka Dec 27 '18 at 14:46
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As I understand it, the point of the zener diode is to prevent the \$V_{\rm GS}\$ voltage from getting too high, although I don’t think many MOSFET would not support a \$V_{\rm GS}\$ of 12V.

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  • \$\begingroup\$ But,in this circuit by turning on this power supply, i have measured the Gate and source voltage wrt GND. The source voltage is found to be 11.96 V and gate voltage is found to be 8.4. Now, removing only the Zener Diode (not the resistor), I found the same voltage as above. So, is there any problem in the circuit? Again , when I vary the input voltage, the Gate and Source voltage changes in a linear way in both the cases. So, is this the ant property of MosFET or any other reason? \$\endgroup\$ – Aakash Dey Dec 27 '18 at 14:14
  • \$\begingroup\$ So,with the change in supply voltage, if the change in the gate voltage is controlling by the zener diode by varying its resistance, it will be fine. If my this point is right, then thanks for help and the guidance. \$\endgroup\$ – Aakash Dey Dec 27 '18 at 16:28
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Your first and most major problem is that the FDN306P is totally inappropriate for this application.
Its datasheet shows it is a -12V Absolute Maximum rated device. If you apply reverse voltage to this in your application it is already at its Absolute Maximum V(DS) rating. You should be selecting a device with a V(DS) rating of at least 15-20V.
The FDN304 with a -20V rating would be a much more appropriate device to use.

It would be worth noting that by using an FDN352 with a greater V(GS) rating of 25V you could eliminate the resistor and Zener in your application. Given it's the same price as an FDN306 you save money on your application.

This would simplify your circuit to:

schematic

simulate this circuit – Schematic created using CircuitLab

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