Hi, I have attached a circuit for peverse voltage and over voltage protection that include a P-FET and a zener diode. I have seen it in many power supply circuits but I think there is a voltage drop which the FET is ON between drain and source, right ? If the voltage drop is 1 V across the FET wheh FET is ON and the load will have smaller voltage. How to compensate this voltage drop ? Is there any other circuit that provide same voltage at the load ? How about just using one shotkey diode in reverse bias and no FET ?

 

The voltage drop will be I x Rds(on), which will probably be lower than 0.15V which is the lowest you are likely to get with a Schottky diode.
A schottky diode with several amps of current through it may drop as much as 0.7V.
If the voltage drop across the FET is 1V, then you need a bigger FET (with lower Rds(on))

 

How about just using PMEG4050EP - Schottky-diode, 5A, 40V, SOD-128, NXP as shown in the circuit in reverse bias which will have no drop. Can this be used as reverse voltage protection and also over voltage protection ?

 

Sure, if you want to start a fire when the input is a lithium battery, or destroy a power supply that is connected in reverse.

 

I have searched and it look like that circuit in the first post is reverse voltage protection but there is also a zener diode which will protect from over voltage, right ? or should there be another circuit for over voltage protection ? I will search for over voltage circuit.

All these are discrete components and occupy space in PCB that means more components in BOM and more soldering. Do we have any smart IC that can do both jobs, reverse voltage protection as well as over voltage protection ? Let's assume the power budget is 2 A @ 5 V for a typical custom microcontroller based PCB.

 

Why would you expect anyone to put 40V+ on a board requiring 5V? Don’t they get what they deserve if they do that? Use a zener diode plus fuse for the simplest circuit. The fuse gives additional protection against a short on the board if placed in the V+ input line with a zener following to ground.