Reverse polarity protection circuit help

Thread Starter

ilium007

Joined Aug 24, 2013
132
Looking for some help selecting a zener diode for a reverse polarity protection circuit. I've never used this circuit before (usually a schottky diode) but now I am pushing up to 2A and a schottky diode would have to dissipate too much heat.

I need to protect a 5V LDO from reverse voltage, the regulator has a max input voltage of 18V, the P-Channel MOSFET Q1 (AO3401A http://www.aosmd.com/pdfs/datasheet/AO3401A.pdf) has a Vds of -30V which makes it perfect for my design but it has a +- 12V Vds so I guess I need a zener diode / resistor as I have drawn below.

Is the zener I have selected appropriate, MMSZ9V1T1G - https://datasheet.lcsc.com/lcsc/1809111622_ON-Semiconductor-MMSZ9V1T1G_C132795.pdf. The MOSFET has a 12V Vds so this diode should clamp this to 9.1V - yes?

Zener (Nom) (Vz): 9.1V
Max power: 500mW

The selection of the 100k resistor is just what I have seen used from searching other forums. I assume I need near zero current flowing here and just need the negative voltage to trigger the MOSFET.

How do I best select a zener diode and resistor for this circuit?

1630064556605.png
 
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Wolframore

Joined Jan 21, 2019
2,350
be careful of the body diode, unclear if source is from the Vin or connector side

what is the source voltage?

this design will not clamp gate voltage at 9V

unclear whether your design will work without this information
 

Thread Starter

ilium007

Joined Aug 24, 2013
132
be careful of the body diode, unclear if source is from the Vin or connector side

what is the source voltage?

this design will not clamp gate voltage at 9V

unclear whether your design will work without this information
Apologies - I can see why it was confusing. I had labelled Vin as the input voltage to the downstream LDO. Re-labelled now to make it clearer. I have fixed up schematic and updated original post.
 

Wolframore

Joined Jan 21, 2019
2,350
I can’t see your attachment, but since your source is 18V, make life easier for yourself and find a pmos that can accept +/-20V or better at the gate

if you’re married to the 3401, its possible to design a divider that will work in either polarity without a Zener, the issue with the Zener is that it acts like a normal diode in forward bias.

edit: 6-18v presents a problem… I would search for a new mosfet
 
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Thread Starter

ilium007

Joined Aug 24, 2013
132
I can’t see your attachment, but since your source is 18V, make life easier for yourself and find a pmos that can accept +/-20V or better at the gate

if you’re married to the 3401, its possible to design a divider that will work in either polarity without a Zener, the issue with the Zener is that it acts like a normal diode in forward bias.
No sure why you can't see the attachment. I've added it below again. I already have a tray full of AO3401A's so I want to use them and besides... I'm learning here.

When is the Zener ever forward biased? I don't understand this part of your response. This protection circuit seems to pop up all over the internet when I was researching.

1630066764771.png
 

Thread Starter

ilium007

Joined Aug 24, 2013
132
but the 6v input would pose an issue in correct polarity
Wouldn't the parasitic body diode put 5V (6V - 1V for diode Vf) on pin 2 of the MOSFET initially and with pin 1 of the MOSFET pulled low I would get approx -5V and turn on the MOSFET? This MOSFET will turn on with as little as -2.5V Vgs (logic level MOSFET)
 
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Wolframore

Joined Jan 21, 2019
2,350
hmm... you might be right, I never use a zener but it does appear to be correct. I stand corrected.

the zener never actives under 9.1V so you get ground and 6V - pretty cool trick, Q you will have to update your instructions.
 
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crutschow

Joined Mar 14, 2008
27,891
The only reason for the Zener is to protect the MOSFET from exceeding the gate-source maximum voltage rating when the input is positive.
If the applied positive voltage is less the the Vgs(max) rating then you don't need the Zener or resistor.

LTspice simulation below:
As you can see, it blocks the reverse input voltage (green trace) from the output (yellow trace).
The Zener limits the MOSFET gate-source voltage to <10V (red trace).
(I had to reduce the value of the R! gate resistor to ground, since the Zener model I had didn't work at the low bias current that a 100kΩ resistor provides.)

1630072889241.png
 

ScottWang

Joined Aug 23, 2012
7,070
If decrease 1.4V from the input voltage and it still could provides enough voltage for the output side then you could try to add a bridge rectifier put on the power input side.
 
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