Need help with battery polarity protection circuit - Page 1
Forum section: electronics / beginners

author: zaphod5
author_rank: Contributor
timestamp: April 21, 2021, 04:01:13 am
content: Hi eevbloggers, I have a battery (2x AA, approx 2.6V - 3V) protection circuit that uses an N-mosfet (AO3402) that works well. Now that only one battery is being used, the mosfet gate voltage (0.8V - 1.5V) is not enough to turn on the AO3402. I'm a noob with this part selection and unsure when browsing Mouser. The circuit draws a maximum of 80ma, and spends most of its time asleep. Any suggestions for a replacement part? Thanks in advance for any help.

author: Algoma
author_rank: Frequent Contributor
timestamp: April 21, 2021, 04:30:58 am
content: There exists dedicated protection ICs, generally a MOSFET with a bit of monitoring built in that can shut down a circuit if things are outside of configured safety levels.Over Voltage, under voltage, reverse voltage, over current.. etc. Some are as simple as a 3 pin and an external resistor or two.

author: zaphod5
author_rank: Contributor
timestamp: April 21, 2021, 04:35:01 am
content: Thanks for your reply. I was hoping to get by with a simple mosfet, as I'm doing now. Only polarity protection is needed.

author: Algoma
author_rank: Frequent Contributor
timestamp: April 21, 2021, 04:44:57 am
content: To establish, the existing part is 30v rated at 4A in a SOT23-3L package. Data sheet shows it takes minimum of 2.5v to fully turn on..What would be the minimum voltage the rest of the circuit shuts down, so the mostfet isn't starting to turn off before everything else is.

author: zaphod5
author_rank: Contributor
timestamp: April 21, 2021, 05:01:13 am
content: The minimum battery voltage is 0.8V, typical is 1.5V. The battery feeds a DC-DC boost converter where the rest of the circuit operates at 3V. Previously I just used the two batteries, no boost converter needed.

author: Algoma
author_rank: Frequent Contributor
timestamp: April 21, 2021, 05:08:36 am
content: That package footprint is for large body surface mount MOSFETs. The current protection rated for 30V @ 4A (120w) is a massive overkill for protecting a quarter watt circuit., a more suited part would be much smaller footprint with far smaller pad sizes.

author: zaphod5
author_rank: Contributor
timestamp: April 21, 2021, 05:19:00 am
content: I'm open to suggestions.

author: Algoma
author_rank: Frequent Contributor
timestamp: April 21, 2021, 05:44:53 am
content: Search for n-channel MOSFETs with a nice low gate drive of 0.9v seems to be the lowest I could find on Digikey.I'm on my cellphone so trying to do detailed part lookups is.. awkward.MCH3484-TL-W From ON Semiconductor as an example seems a good match for your existing part's specs with a nice low gate drive that will work under the heaviest of loads, but it's on resistance will waste battery power.Even smaller, like the following is even better suited.RYM002N05T2CLI'm not sure what mouser has in stock. A cellphone is a terrible way to search.PerhapsRYC002N05T316

author: zaphod5
author_rank: Contributor
timestamp: April 21, 2021, 06:00:40 am
content: Thanks, the RYM002N05T2CL seems like a good fit.

author: Algoma
author_rank: Frequent Contributor
timestamp: April 21, 2021, 06:12:30 am
content: It's VERY tiny little part.. like an easily lost with a slip of the tweezers, but when working with maximizing battery life, smaller size components can be useful.RYC002N05T316 may work as well.

author: zaphod5
author_rank: Contributor
timestamp: April 21, 2021, 06:27:00 am
content: I've been soldering 0604's for this work, any smaller and my eyes/dexterity have trouble. A few tweezer slips already! Thanks again, I'll be ordering some RYM002N05T2CL as I see they are in stock.

author: Algoma
author_rank: Frequent Contributor
timestamp: April 21, 2021, 06:32:16 am
content: Actually I think the on semi part maybe better, I misread the on resistance.. it's miliohms.

author: zaphod5
author_rank: Contributor
timestamp: April 21, 2021, 06:41:53 am
content: Got it, thanks again.

author: Zero999
author_rank: Super Contributor
timestamp: April 21, 2021, 07:47:23 am
content: Quote from: Algoma on April 21, 2021, 06:32:16 amBear in mind that the on resistance at the top of the data sheet, is specified with a much higher gate-source voltage, than the threshold.Out of the parts listed so far, the MCH3484 is the only one which is suitable. It has a maximum on resistance of 330mΩ, when VGS= 0.9V, but the original poster is only using if for 80mA, so the volt drop is a non-issue. The other parts have a much higher on resistance. The RYM002N05 for example has an on resistance of at least 3Ω, 9Ω maximum, when VGS= 0.9V.

author: Algoma
author_rank: Frequent Contributor
timestamp: April 21, 2021, 01:53:57 pm
content: Late night searching Mouser's catalog with a mobile browser was a terrible idea. And Mouser seems to lack a search filter for Gate Drive Voltage. Only Threshold and Maximum VgsDigikey's search is better results, but there appears to be only a tiny selection of parts that operate at such low gate drive voltage.

author: Peabody
author_rank: Super Contributor
timestamp: April 21, 2021, 01:56:33 pm
content: I don't know what led to the change to one battery and a boost converter, but keep in mind that the boost converter may draw significant current even when the reset of the circuit is sleeping. So you may find that your battery life isn't very good. There's a lot to be said for powering directly from the batteries, with no regulator at all.

author: Zero999
author_rank: Super Contributor
timestamp: April 21, 2021, 03:50:16 pm
content: Quote from: Algoma on April 21, 2021, 01:53:57 pmIt's physics. Assuming everything is equal, to get a low threshold, given the same on resistance, the leakage current will go up. It's possible to make such a MOSFET with a threshold of 0.4V, but it will still be slighly on, when VGS= 0V and would need a operate in the depletion region, i.e. a negative VGS, to turn off properly.

author: zaphod5
author_rank: Contributor
timestamp: April 21, 2021, 04:27:51 pm
content: QuoteOne of the sensors onboard is very picky about voltage and won't operate reliably once the batteries (non-boosted, just 3V out of two alkalines) drop below 2.9V, so an MCP16251 was installed in this revision. The customer requested a single large alkaline. I haven't tested operating current as the board isn't up and running yet.

author: Algoma
author_rank: Frequent Contributor
timestamp: April 21, 2021, 05:50:35 pm
content: What about an single 18650 cell. Bit bigger. But there is plenty of supported protection devices, and a compatible voltage range to the sensor, the device could even be rechargeable from USB.

author: mariush
author_rank: Super Contributor
timestamp: April 21, 2021, 07:33:31 pm
content: ... and if 18650 is too big, you can always go with one of those pouch style cells ... ex :https://www.digikey.com/en/products/detail/jauch-quartz/LP523450JU-PCM-WIRES-70MM/9560988

author: zaphod5
author_rank: Contributor
timestamp: April 22, 2021, 01:30:44 am
content: I would love to be using a lithium or LifePo4 battery for this project, the customer wants Alkaline as they are readily available anywhere. The future waste is concerning.I've desoldered and jumped the AO3402 to find that the MCP16251 sometimes produces 3.3V, sometimes 2V. Capacitors are good, voltage setting resistors are good, inductor is good. Can only determine the IC is bad, and no replacement stock in North America, 45 weeks out for Digi. Feels like a monday

author: Peabody
author_rank: Super Contributor
timestamp: April 22, 2021, 04:06:12 am
content: What is the battery voltage when the output is 2V?

author: mariush
author_rank: Super Contributor
timestamp: April 22, 2021, 04:57:20 am
content: Quote from: zaphod5 on April 22, 2021, 01:30:44 amThe problem with the first paragraph is that most aa battery chargers charge batteries in pairs, and if your device is gonna use a single aa battery, few people are gonna have a readily available single aa battery that's charged and they won't wait a few hours for a battery to charge, so they're just gonna buy non-rechargeable alkaline and add to the waste pile.With a lithium battery, the end user no longer needs to replace batteries.A simple alternative could be to just use 2 AAA batteries instead of a single AA. You get slightly less energy, but user can buy 4 aaa rechargeable, and swap 2 at a time when they're discharged an you get your minimum 2v-ish.if your device uses little current you may want to check out ratiometric chips, voltage doublers, triplers ... some should handle reverse voltage by design so you could put your mosfet after and use therefore use cheaper mosfetbenefits low quiescent current, high efficiency, low switching noise, no inductor needed just a few ceramic caps and diodes.Most doublers can be extended to do 3x, so your rechargeable 1.1...1.35v could end up to around 3..3.5v in triple configuration.Seehttps://www.digikey.com/short/p28mnpvh

author: Zero999
author_rank: Super Contributor
timestamp: April 22, 2021, 08:32:32 am
content: Quote from: zaphod5 on April 21, 2021, 04:27:51 pmWhy not use the output from the boost converter to help switch the MOSFET in the polarity protection circuit on?