P-MOSFET selection for Reverse Polarity protection for 18650 li-ion battery circuit?

Thread Starter

iotdeveloper

Joined Sep 11, 2023
8
Hello,
I'd like to make a reverse polarity protection circuit for 18650 3,7V Li-ion battery (like here: Can I use a p channel mosfet as reverse polarity protection but still charge a Li-Ion battery?)

Here is my schematics:
1694425594829.png

I only know, that the device will draw 500 mA from the li-ion battery (maximally 1 A, often it draws low currents <50 mA). The voltage of the battery is in 3.3-4.2 V range.

But I'm not sure which specific type of P-MOSFET is the most suitable (for example)? I also need to select proper type of Zener diode and resistance value of resistor to use for this circuit?

Thanks
 

crutschow

Joined Mar 14, 2008
33,346
Is the reverse voltage being applied on the right side of the circuit.

That circuit won't work for your purpose, since any applied negative voltage on the right will simply conduct through the MOSFET substrate diode.
 

Thread Starter

iotdeveloper

Joined Sep 11, 2023
8
For better clarification - I need to protect the components connected to the terminal block behind the battery. So the flow is from left to right.

Here is the whole schematics:

liion2.PNG
 

crutschow

Joined Mar 14, 2008
33,346
Still unclear as to what you what you want to protect.
Obviously that circuit won't protect any load connected to the terminal block on the right.

What does "behind the battery" mean?

Please show the whole schematic with what "components" are connected to the terminal block, and without any wires running off the page, as you show above.
 

Thread Starter

iotdeveloper

Joined Sep 11, 2023
8
I can't show the whole schematic. It's secret. To the terminal block there is connected ESP32 and sensors like BMP280.

Why should not this circuit protect the load connected to the terminal block against reverse polarity? Everywhere is this circuit with P-Mosfet described.
 

crutschow

Joined Mar 14, 2008
33,346
Why should not this circuit protect the load connected to the terminal block against reverse polarity?
I'm confused as to where this "reverse polarity" is coming from.(?)
It will protect against reverse polarity if the battery is reversed, but it certainly can't protect against reverse polarity caused by a reverse connection to the terminal block.
How could it know/detect that the load is connected in reverse?
 
Last edited:

Thread Starter

iotdeveloper

Joined Sep 11, 2023
8
Yes, that's what I need. It should protect against reverse polarity if the battery is reversed, nothing else.

So please, i need to find applicable P-MOSFET and parameters of other conponents.
 

crutschow

Joined Mar 14, 2008
33,346
I only know, that the device will draw 500 mA from the li-ion battery (maximally 1 A, often it draws low currents <50 mA). The voltage of the battery is in 3.3-4.2 V range.

But I'm not sure which specific type of P-MOSFET is the most suitable (for example)? I also need to select proper type of Zener diode and resistance value of resistor to use for this circuit?
Yes, that's what I need. It should protect against reverse polarity if the battery is reversed, nothing else.
Okay.
For voltages below the Absolute Maximum Vgs of the MOSFET (typically 20V) you don't need the Zener or the resistor, as their purpose is to protect against Vgs overvoltage.
You can connect the MOSFET gate directly to ground.

The MOSFET needs to be a logic-level type (max Vgs threshold voltage ≤2V) so it fully turns on with the 3.3V battery voltage.
It's on-resistance should be ≤100mΩ to minimum voltage drop and power dissipation.
Otherwise there are no other critical parameters.
 

crutschow

Joined Mar 14, 2008
33,346
Hello, I would like a SMD part, better something smaller (not as large as DPAK). I'm going to order the PCBs at JLCPCB, even with SMT assembly. So I need to choose parts from their list: https://jlcpcb.com/parts
Sorry, but searching for a part on that website is more grunge work than I care to do.
So it will be a learning experience for you. ;)
Just search for ones with equal to or better parameters than what I listed in post #8.

If you have any questions about that, just ask here, and I will try to answer.
 

RyanLu

Joined Sep 12, 2023
4
Okay.
For voltages below the Absolute Maximum Vgs of the MOSFET (typically 20V) you don't need the Zener or the resistor, as their purpose is to protect against Vgs overvoltage.
You can connect the MOSFET gate directly to ground.

The MOSFET needs to be a logic-level type (max Vgs threshold voltage ≤2V) so it fully turns on with the 3.3V battery voltage.
It's on-resistance should be ≤100mΩ to minimum voltage drop and power dissipation.
Otherwise there are no other critical parameters.
Dual P-CH MOSFET can solve your problem...
 

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