Reverse voltage and over voltage protection

[engr_joni_ee]

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 ?

 

[crutschow]

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 ?

That circuit does not provide overvoltage protection, only blocks reverse voltage.

Since the MOSFET is turned on when the correct polarity voltage is applied, its drop is equal to the MOSFET's on-resistance times the load current.
Thus you select the MOSFET with an on-resistance that keeps that voltage drop low, which can be well below that of a Schottky diode.
That's why the MOSFET circuit is used instead of a diode.

 

[engr_joni_ee]

How about just using PMEG4050EP - Schottky-diode, 5A, 40V, SOD-128, NXP as shown in the circuit.

 

[KlausST]

Hi,

Is there any other circuit that provide same voltage at the load ?

No. Even a wide piece of wire will have some voltage drop.
Ohmic law is still valid.

You post some (partly unrealistic) assumptions. Why don´t you do the math first? or use a simulation software.

--- Updated Sep 9, 2023 ---

Hi,

How about just using PMEG4050EP - Schottky-diode, 5A, 40V, SOD-128, NXP as shown in the circuit.

Before you are concerned about voltage drop - now you add a series resistor? --> counter productive.

Klaus

 

[engr_joni_ee]

Remove the resistor in series, just a Schottky diode in reverse bias which will have no drop. Can this be used as reverse voltage protection and also over voltage protection ?

 

[betwixt]

It will work as reverse protection but consider that it does it by (almost) shorting out the incoming supply. For example, if the power source can deliver 2A and you connect it in reverse, the full 2A will flow from it. The wise thing to do if using a diode that way is to add a fuse on the supply side.

It offers no over-voltage protection but you could consider using a Zener diode with suitable voltage and power rating.

Brian.

 

[KlausST]

Hi,

you´re not a newbie in electronics.
1) a Schottky as overvoltage protection? What is your idea? How do you think this works?
2) in case you use a zener ... then you need the series resistor to "drop the overvoltage". Without the resistor - no drop.

Is it asking too much
* to do a little research on your own.
* and to elaborate your questions?

Again: what about a simulation.

****
What´s wrong with the first circuit? And please provide useful values like:
* nominal voltage range (load)
* maximum input voltage
* maximum allowed voltage drop
* maximum load current
* and so on..

Klaus

 

[engr_joni_ee]

According to reply #2 "That circuit does not provide overvoltage protection, only blocks reverse voltage."

@KlausST, I don't see anything wrong with the circuit in the first post but just was thinking if we can reduce the number of components in PCB because these components are discrete and if we add an other circuit for over voltage protection, assuming the circuit in the first post is only reverse voltage protection then the circuit will be even bigger, more components, more soldering etc.

Apart for size of the circuit I will consider simulation in LT Spice that would be interesting learning.

Do we have smart ICs that can do both jobs, reverse voltage protection and also over voltage protection. Let's assume the power budget is 2 A @ 5 V for a custom development board.

 

[KlausST]

Hi,

Do we have smart ICs that can do both jobs, reverse voltage protection and also over voltage protection.

If I had to answer this question, I had to go to my distributor's internet site and use the selection tool.

Klaus

 

[crutschow]

As betwixt noted, a Zener diode with a series fuse, will provide overvoltage and reverse voltage protection.
To minimize the reverse voltage generated by the Zener, you could add a Schottky diode in parallel.

If you want to avoid a fuse, then you will need a more complex circuit.

 

[Easy peasy]

Remember the fet channel is in parallel with the internal diode - so less than 0.7V drop straight away

We use this ckt for rev input protection - works well - with the right P fet you can get as low as 50mV drop.

--- Updated Sep 10, 2023 ---

For reverse protection - a poly fuse - properly rated and an inverse diode across the supply will work well

the poly fuse goes upstream of the diode, make sure the diode is rated for the surge current.

 

[KlausST]

Hi,

with the right P fet you can get as low as 50mV drop.

@OP:
You say 2A , now if you want these 50mV, then just use what we call Ohm's law:
R = V/ I = 50mV / 2A = 25mOhms.
--> now use a Mosfet selection tool for P-Mosfet with R_ds_on of 25mOhms @ 5V V_GS.

Klaus

 

[engr_joni_ee]

Thank you very much for your replies and comments. I have seen the following videos to learn more about over voltage protection and reverse voltage protection circuits. The concept is clear to me. Thanks again.

How to design NPN BJT based Overvoltage protection? Overvoltage protection using NPN BJT.

How to design PNP BJT based Overvoltage protection? Overvoltage protection using PNP BJT.

Different types of Reverse Voltage Protection types | What is the need? Reverse polarity Protection