Battery Protection IC layout - Page 1
Forum section: electronics / projects

author: Kuusou
author_rank: Regular Contributor
timestamp: November 07, 2020, 04:22:46 am
content: I'm trying to incorporate a battery protection IC circuit into my board for a single cell lipo. I ordered several lipos off of aliexpress for cheap, and they're suppose to have a BMS built in. They do have a board but after testing they don't appear to work, and even if they did they claim they don't have overdischarge protection kick in until 2.75v or below, which seems far too low for a lipo. Although after doing some research I know this is dependent on battery chemistry, I'm not able to find any datahseets for these lipos. So to be safe I've picked the FS312F-Ghttps://lcsc.com/product-detail/Battery-Protection-ICs_FS312F-G_C82736.html/?href=jlc-SMT. It has over discharge protection at 2.9 volts.My first question is will the FS8205Ahttps://lcsc.com/product-detail/MOSFET_Fortune-Semicon-FS8205A_C16052.html/?href=jlc-SMTbe a suitable MOSFET for M1 & M2?My second issue is that I'm confused by the typical application circuit.How am I suppose to incorporate this into my board. I'm guessing that the battery symbol beside R1 and C1 is where I'd have the battery pads on the board, and then the BATT- would go straight to ground and the BATT+ line would be my power trace that goes to all the components?Sorry if this is a noobish question I'm just a little confused and was hoping someone could clear this up.

author: kjr18
author_rank: Regular Contributor
timestamp: November 08, 2020, 11:06:39 am
content: Yes, replace this battery symbol next to R1 and C1 with your typical battery contact, BATT- will be your circuit ground ( FS312F-G gnd pin is connected only to negative battery terminal) BATT+ will be your battery voltage.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: November 08, 2020, 01:30:21 pm
content: The FS8205A is commonly used for this application, so it should be fine.

author: Clear as mud
author_rank: Regular Contributor
timestamp: November 09, 2020, 05:55:19 pm
content: For the MOSFETs, it really depends on the amount of current you want to draw, and the gate drive voltage. Look at the data sheet and try to estimate the worst-case ON resistance at whatever the gate voltage will be, then Power = I2R, and then do a thermal calculation to see if the MOSFET operation temperature will be acceptable.

author: Kuusou
author_rank: Regular Contributor
timestamp: November 12, 2020, 08:28:57 am
content: The voltages would be within 2.9 to 4.2 volts and the current will be a max of 2amps.I reviewed the FS8205Ahttps://lcsc.com/product-detail/MOSFET_Fortune-Semicon-FS8205A_C16052.html/?href=jlc-SMTdatasheet and did the thermal calculations, It should be well within limits as far as a heat dissipation. My only confusion is the Vgs to turn on the gate. It has it listed on page 4 that the min value is 0.5v and the max is 1, however on page 3 under the absolute maximum ratings section it has it listed as +-12v. Although the datasheet says this mosfet is for Li-ion battery management applications...I don't know lipos to be 0.5 to 1 volt so I'm rather confused by this. Would voltages between 2.9 to 4.2 be too high for this mosfet?Below is my schematic and layout, I was hoping someone could give me some feedback on it, I want to make sure that it would work as far as the layout goes and that there aren't any major errors with it that I'm making.50 50 spinner

author: drvtech
author_rank: Regular Contributor
timestamp: November 12, 2020, 11:44:12 am
content: Vgsth of 0.5 to 1.0V is telling you what you NEED to turn the MOSFET on. In other words you'll never need more than a volt to make it work and it will likely work quite happily at 0.5V. The figure of 12V in the absolute maximum part of the datasheet is just telling you how high it CAN go before you will damage the device. With a circuit running off a single cell you'll never approach this regardless of what the circuit looks like.

author: Wolfram
author_rank: Frequent Contributor
timestamp: November 12, 2020, 12:37:43 pm
content: Quote from: drvtech on November 12, 2020, 11:44:12 amVgs_th is the gate voltage where it will pass some specified current, 250 uA in this case. For this device, that point is specified to fall between 0.5 and 1 V, with the drain connected to the gate. To turn the device fully on takes more voltage than this. How much extra voltage you need depends on the amount of current the MOSFET needs to pass. Fig 1. in the datasheet shows that the device is already close to its specified Rdson at 2V Vgs, and there is a small improvement when going to 2.5 V, with little improvement beyond that. Note that this plot gives typical values at room temperature, not worst case, so it can pay to add some margin.Vgsmax is specified to be +- 12 V, this is the upper limit. So to turn the device off, Vgs should be between -12 V and +0.5 V, and to turn it on it should be between +2.5 V and +12 V, with + 2V also being acceptable if you can live with slightly higher conduction losses.

author: Clear as mud
author_rank: Regular Contributor
timestamp: November 14, 2020, 11:54:51 am
content: The layout looks good. It's good that you got the bypass capacitor right next to the IC, and I like the thick traces. You used fill zones with a curved corner radius? Someone had warned me recently that if you use zones without thermal relief on surface-mount components, it can lead to tombstoning, but I think these components will be safe from that, since everything is pretty well symmetrical. It's not really a problem for one-offs or very small production runs, anyway, because you can always re-work an SMT component that doesn't get mounted properly at first.

author: Kuusou
author_rank: Regular Contributor
timestamp: November 20, 2020, 11:25:11 pm
content: Quote from: Clear as mud on November 14, 2020, 11:54:51 amThanks for the feedback, yes that's what I did but this is the first I'm hearing about the tombstoning issue! I may have to go back and add thermal relief just to be safe. Ultimately this board is going to be produced in batches of 30 so I'd prefer to avoid that issue completely, thanks for the heads up.