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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?)

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

Here is my schematics: enter image description here

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

Thanks

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    \$\begingroup\$ What David says - note that this means that Vgsth hould be usefully lower than the lowest possible battery voltage. DAvid's filter allows Vgsth value up to 3V. For a singke LiIon cell this is somewhat higher than safe. Vgsth of no more than 2V would be advised to allow some "headroom" amd worst cae values. || That leaves these \$\endgroup\$
    – Russell McMahon ♦
    Commented Sep 11, 2023 at 5:21
  • \$\begingroup\$ OK, so if I choose for exapmle this mosfet: digikey.co.nz/en/products/detail/infineon-technologies/…, is it applicable? if i understand correctly, i can draw max 1.5A from the battery? And what about the parameters of the Zener diode and resistor? Thanks \$\endgroup\$
    – iotdeveloper
    Commented Sep 11, 2023 at 9:30
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    \$\begingroup\$ That FET datsheet here is very very marginal at 500 mA and not good enough above that. See fig 7 page 5 which shows Vgs vs VDS and Id. || THIS FT is marvellous - See graph top left page 4. At say 1A and Vgs = 2.5V, Vds is about 50 mV (25 C and pulsed so maybe 50% higher in practice.). 8 pin pkg but easy to solder. || Zener not needed under about 15V input ... \$\endgroup\$
    – Russell McMahon ♦
    Commented Sep 11, 2023 at 11:04
  • \$\begingroup\$ R can be "anything". 10 K is fine. You could even just connect the gate to ground !!! :-) (R & zener are needed when Vin is close to Vgsmax or higher.) \$\endgroup\$
    – Russell McMahon ♦
    Commented Sep 11, 2023 at 11:06
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    \$\begingroup\$ @Davide Andre's list is good but some FETs have high Vgsth for a discharged LiIon cell. My above list was WRONG - I missed out some in error. I have corrected the link above AND this list contains 195 which are mostly good, including this lower cost one which is almost as good as the one you show above. || P Channel is placed in the positive rail and doesn't break ground continuity. || . N Channel is mirror image and in the negative rail. Either work OK. \$\endgroup\$
    – Russell McMahon ♦
    Commented Sep 11, 2023 at 11:40

2 Answers 2

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Regarding the zener diode and the resistor, you don't need them if you choose a mosfet with maximum gate-source voltage (Vgs max) higher than 4.2V.

Most of the mosfet suggested have Vgs-max above 8V, so you wouldn't need the zener and resistor.

The zener is there to prevent that an inpunt voltage (max voltage from your battery) higher than the max gate-source voltage damages the mosfet.

I don't know if there could be some eventual high voltage peak still to consider. Otherwise you have to choose the zener voltage below the Vgs max, to protect the mosfet.

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  • \$\begingroup\$ If Vgsmax is well below Vbattery_max you can connect the date to negative with no series resistor.A resistor adds a degree of safety against eg static input when the battery is disconnected. Rgate and gate capacitance (or maybe a very small added gate capacitor) can add some protection. \$\endgroup\$
    – Russell McMahon ♦
    Commented Sep 12, 2023 at 0:44
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Select a P-Channel MOSFET with a logic-level gate threshold voltage that can handle the peak discharge current. Here is a List. Filter for package type.

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    \$\begingroup\$ Vgsth of 3V is somewhat on the high side for an 18650 cell - Vbatmin can be 3V or even lower (although not advised). I'd suggest a Vgsth of no more than 2V, although 2.5V would usually work OK. This allows for typical vs worst case values and getting the FET well into saturation. \$\endgroup\$
    – Russell McMahon ♦
    Commented Sep 11, 2023 at 5:23
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    \$\begingroup\$ Davide - I subset your useful list to choose Vgsth <= 2V and Rdson of I think <= 20 milliohms. That gives 195 FETs here. \$\endgroup\$
    – Russell McMahon ♦
    Commented Sep 11, 2023 at 11:42
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    \$\begingroup\$ Many thanks - you taught me something useful. I've been using bit.ly to shorten URLs. It now will not handle Digikey long addresses - I suspect something changed. I was unaware of the Digikey url shortening facility. I wondered how you acieved it, looked at your link, and that led me to the solution. I wonder how many years I've been unaware of that?:-) :-( . || I tried goo.gl and it produced an incorrect target url, higher up the address chain! :-( \$\endgroup\$
    – Russell McMahon ♦
    Commented Sep 11, 2023 at 11:49
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    \$\begingroup\$ Your new link is broken. \$\endgroup\$
    – Russell McMahon ♦
    Commented Sep 12, 2023 at 0:27
  • \$\begingroup\$ Fixed. Thank you. \$\endgroup\$
    – Davide Andrea
    Commented Sep 12, 2023 at 12:23

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