Choosing a P-Channel MOSFET - Page 1
Forum section: electronics / beginners

author: cb900
author_rank: Newbie
timestamp: March 05, 2021, 10:08:38 am
content: Just joined - hello everyone.Just reworded, reposted and deleted my previous question which had no replies.Could you help me extract the information I need to choose a MOSFET from the following datasheet -Maxim MAX16914/16915 (https://datasheets.maximintegrated.com/en/ds/MAX16914-MAX16915.pdf) for a marine (diesel automotive mechanical injectors) based project.My max dc calculated current draw is circa 38A so 45A+ (Ids is it) minimum design value.I have tentatively picked a BUK6Y10-30PX from Nexperia (https://assets.nexperia.com/documents/data-sheet/BUK6Y10-30P.pdf) with a very low Rdson of 10mOhmsHave I chosen reasonably well and is it suitable??With the addition of some belt and braces TVS's on the input (before the first FET) and using a pair of BUK6Y10-30PX, I has used the circuit in the datasheetThank you

author: Doctorandus_P
author_rank: Super Contributor
timestamp: March 05, 2021, 10:47:33 am
content: First, I don't like maxim (No capital from me).They sort of specialize in chips for specific situations, and because of that they charge quite high prices.Take for example the very simple MAX7219. They charge over EUR6 for such a simple chip, no wonder the Chinese jump in with clones. (And it isn't even a good chip, with much to coarse brightment settings).They also obsolete parts fast and their chips were sometimes hard to actually find.Also, 10mOhm is not "very low" for today's P-MOSfets. You can get half of that or even lower without too much difficulty, and with 10mOhm and 40A you still need a (smallish) heat sink. With a bit lower RdsOn, a bit of a copper pour can be sufficient and the overall solution easier to make and thus cheaper, even with a more expensive MOSfet.P-channel MOSfets are often used in "Load Dump" protection. For this you need a quite high voltage. Some specs for load dump are around 80V, but others go over 100V. and your FET must be able to withstand that. Do a bit of research into load dump.https://html.duckduckgo.com/html?q=%22Load+dump%22+automotive+application+noteAlso, if you need 38A then I would not select a FET that is rated for only 45A. I do not know if there are standards for derating, but I'd much rather have a MOSfet that is rated for 80A or more.

author: sairfan1
author_rank: Frequent Contributor
timestamp: March 05, 2021, 02:38:35 pm
content: QuoteThere are some Chinese very cheep alternates, even include some additional features.

author: jmelson
author_rank: Super Contributor
timestamp: March 05, 2021, 05:20:25 pm
content: If you are talking about 38 AMPS (!!) why are you looking for a P-channel FET? Why not change topology and use an N-channel, it should have much lower on-resistance.jon

author: exe
author_rank: Unknown
timestamp: March 05, 2021, 05:33:00 pm
content: Why that part, and nothttps://www.ti.com/power-management/power-switches/ideal-diodes-oring-controllers/products.html#p236typ=0;0.69&p236max=0;0.79&!p2192=Solar%20Bypass or something else? Do you need it to be bidirectional, or unidirectional? Does it have to have AEC-Q101/whatever qualification?

author: cb900
author_rank: Newbie
timestamp: March 10, 2021, 02:19:08 pm
content: Hi thank you for the replies.Right some background!The application is marine but using an automotive type 12V charging utilising (3rd party) latching relays on the house battery bank and Starter battery to switch-out all the batteries to limit discharge and switch-in for emergency engine starting. The 'system' is connected to a true sign wave (Kisae) 2KW inverter.I am taking the supply from the control box (relays etc ) to the internal fuseboard and on. The customer has moved the goal posts from my basic 12V/240Vac electrical installation and requested 'things' which now require some basic switching and control, so I have moved into the electronics realm and need to implement a MCU based system.To this end I need to sort out a 'clean' supply with the normal automotive with tolerance for transients, load dumps etc levels to feed my inboard Buck/Boost voltage regulators. I assume the inverter would spit some form of high frequency noise onto the cable BUT I have 400Ahr battery bank connected which might smooth things out a bit.I need VERY low quiescent currents as the boat could be moored for months without shore supply, monitoring bilge switches, leak detectors and GSM communication.I have produced an initial design below (meant as a discussion document mainly), including a common mode choke to invite comments if actually needed, I have included my intended filtering (paralleled up to give 40A) and lastly found some 1.4mOhm pFETs. (can't seem to insert am image)QuoteStruggling to find what term is used in the datasheet to specify this value Vds? I included the TVSs with this in mind.Lastly, I do know my way around power electronics or filtering which is why I am here.Thanks

author: exe
author_rank: Unknown
timestamp: March 10, 2021, 03:41:53 pm
content: Quote from: cb900 on March 10, 2021, 02:19:08 pmPlease define "VERY low". I have doubts that it needs to be super low because of self-discharge currents of typical lead-acid batteries (or whatever is used) and presence of other electronic devices onboard. Anyway, I'm pretty sure any modern part will work as they commonly specify quiescent currents of below 10uA (in shutdown mode). Plus some leakage from mosfets, but, again, that's insignificant for a battery that can provide 40A.PS what are the mosfets are you going to use? ntmfs002p03p8zt1g ?PPS You have some protection, right? So, no nasty inductive load can kick back and kill those poor mosfets.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 10, 2021, 05:53:16 pm
content: MAX are ok a long as you can live with the product being discontinued on a whim.So as indicated by others there are better ways. eg LM74810 and seehttps://www.ti.com/tool/TIDA-020040It's much more complicated obviously but then the auto /marine operating environment is very harsh and unforgiving.Ah, soldering on the high seas......... where is that Farnell helicopter?

author: cb900
author_rank: Newbie
timestamp: March 10, 2021, 05:56:58 pm
content: HiStruggling finding suitable MOSFETs (my ignorance and no experience with FETs) the NTMFS002P03P8Z (On Semi) seems a good bet but I have a concern over Vds rating of only 30V but like said I AM unsure of the terms used in datasheet29uA for the section posted.My initial go to was the TVSs as shown on schematic together with either a suitably sized Common Mode Choke / Inductor (I don't know how to spec those)The 40A is worse case mathematical scenario for all possible 12V equipment possible being on at once (unlikely). This includes a 7A water pump for a shower that will be used as long as there's water in the tank 15mins ish.

author: mvs
author_rank: Frequent Contributor
timestamp: March 10, 2021, 06:14:58 pm
content: NTMFS002P03P8Z is a low voltage MOSFET, with Vds_max of only -30V, and is not suitable for the task.TVS diodes do have limited energy absorption. Load dump pulse (ISO 7637-2 5a) is quite long, so do not expect them to survive on low impedance connection.TI has comparable ideal diode controller with integrated charge pump to drive N-ch. MOSFETs in high side configuration, LM7480-Q1/LM7481-Q1.

author: cb900
author_rank: Newbie
timestamp: March 10, 2021, 06:23:00 pm
content: Hi TerryI didn't realise MAXIM has this reputation.It's only on a narrowboat inland waterways it was easier to say marine for international understanding - main consumption is lighting and a occasionally a water pump motor and even rarer external running lights.I took the view to go with a low part count and therefore cost route and use JLCPCB /PCBWAY 5 board offer. I have to produce a manual for my installation, if there is also an easy way to test and / or swap out for troubleshooting documentation purposes it's also a win.I did see the TI reference design but the cost/grief of manufacture is prohibitive for me I'ts P***ing me off as it is not being able to find low RDSon heatsink mountable MOSFETS to make my life easier.The 16914/5 is £1.50 - I'll buy 10!!!.

author: cb900
author_rank: Newbie
timestamp: March 10, 2021, 06:34:04 pm
content: Hi thank you for your replyWow you chaps/chapesses really don't like Maxim do you. I'm think I'm picking up on a pattern here.OK so if I'm not mistaken if I use an N-channel I also have to include a gate driver which is operating permanently so contributes to the TI version running at 486uA or there would be a delay triggering the FETs?A P-channel will operate at battery Voltage so should be quicker yes/no?

author: mvs
author_rank: Frequent Contributor
timestamp: March 10, 2021, 07:18:00 pm
content: Quote from: cb900 on March 10, 2021, 06:34:04 pmCharge pump of LM7480 has typical consumption of 2.7mA. But it is controlled by comparator with hysteresis, so it would run during a small fraction of time, like few %.MOSFET turn-on with 20mA will take some time, but turn-off with 2670mA is very quick.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: March 11, 2021, 11:11:45 am
content: Have you considered bistable mechanical relays?These usually have two coils, one to turn it on, and another to turn it off, and they only need a short pulse (100ms or so) to change state and do not need any power to remember their state.Sites as Mouser, Digikey, Farnell, etc, have pretty extensive search engines in which you can enter parameters of components, and compare their prices.