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I'm working on this hobby project and I am trying to create a PCB for it. I want to power the project with 2 18650 batteries. Because the product needs to be used by other people as well, I would like to build in some protections. One of them is over-voltage, under-voltage, reversed polarity, and short circuit. Herefore I found a protection ic.

The rest of the PCB consists of an ESP32, WS2812 LEDS x 24, ICM-20948. The total current that can be drawn would be around 2.5 Amps or max 3 Amps.

For the protection IC, as seen in the datasheet, there need to be 2 MOSFET's. I have tried to search on the internet on how to choose which MOSFET is suitable for the protection IC and application. But I just can't figure out what is good or not. So now I have tried this with the following products.

Protection IC - R5460N208AA-TR-FE:
Datasheet
Product page

N-Channel MOSFET - CSD17484F4:
Datasheet
Product page

Battery Protection circuit Battery protection circuit

Battery Holder enter image description here

My choice of the current MOSFET's is based on this information:
How to Select a MOSFET – Battery Protection

The current design is also based on the typical design of the battery protection IC.

I'm not an expert on creating PCB's so there will be stuff that isn't correct if so I would like to know as well and if you have some tips or good instruction videos or posts I'm always in for that.

So now I have 2 questions.
1 - Did I choose the correct MOSFET's?
2 - Am I doing any good on the design?

For the full schematic look on my GitHub repository -> Taekwondo - PCB

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  • \$\begingroup\$ Those mosfet will be constantly closed so, the most important parameters are: the voltage and current rated of the mosfet, the Rdson should be has low as possible something around 6 to 10 mOhms, make sure the Vgs voltage of the mosfet is low enough compare to the gate voltage of the IC. Regarding to the design, follow the design recommendation of the datasheet. \$\endgroup\$ – Delphesk Feb 11 at 16:58
  • \$\begingroup\$ +1 for research, especially finding an application note addressing the issues. \$\endgroup\$ – Spehro Pefhany Feb 11 at 17:49
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For the circuit itself I don't know that part but these protectors are meant to be used in only one way, so you just have to keep the sample circuit and recalculate some component about your battery.

For the mosfet you simply needs two logic level n-channel power mosfets; drain rating and rdson depending on your current (they are not switching so you should optimize on rdson and not gate charge).

Your TI MOSFET is optimized for load switching so it could be good.

Good luck on soldering that, however :D I'd use a bigger part since you are really near to its maximum drain current (3A) so it will be quite hard to keep in temperature. Look at the fine print, you'll need a huge copper mass for that (a 70µm copper layer).

For a 3A load I'd suggest an optimized one in TO23 or maybe a TO223 if operating period at maximum current is long (just an indication, there are many different packages and many cooling strategies).

You need some experience to estimate the actual thermal resistance of your board but datasheet parameters are a good start.

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Datasheet "If the positive terminal and the negative terminal of the battery pack are short, during the delay time of short limit detector, large current flows through the FET. Select an appropriate FET with large enough current capacity to prevent the IC from burning damage."

"Output Delay for Short Detector vs. Temperature " (p24 18.) 400 us max.

This depends on battery ESR , short circuit protection, fuse rating of FETs and quality expectation of application, Tier (1,2,3)

Thus for short circuit safety, there should be some margin between short circuit explosion risk and protection IC to safely open current if a short occurred during charging.

    • Did I choose the correct MOSFET's?
  • Here you need to report Abs. Max Short circuit current of FETs and battery with assumptions. Assume Battery Isc > FET max.( will damage) enter image description here

It does not look like they will survive a short circuit for even 100us

2 - Am I doing any good on the design?

Do you feel lucky ? Consider what happens if battery bursts in flames if FET's fail short circuit? Show more attention to details in RdsOn, layout heatsink.

I would start by looking at battery ESR which determines short circuit current.
Then you can realize short circuit power dissipation depends on ratio of FET RdsOn *2 to battery ESR. e.g. 4.2V ^2/ESR=50m = 352 watts These FETs have RdsOn in the < 150 mOhm range for 4.2V. The better the battery (C rating), the lower its ESR and thus short circuit current.

You want the switch RdsOn (worst case) to be << 10% of load in this case dual FETs v battery ESR best case.

Then examine effects of temperature rise of copper pour on FETs suggested in Abs Max specs. and Watt's per sq. cm of copper area. See Abs. Max specs (1) above.

Keep up the good work and focus on temp. rise, electrical resistance (RdsOn) and thermal resistance Rja, Rjc, Battery ESR and short circuit protection for both FET's , Batteries and safety margins.

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