Question to the experts of MOSFET's - Page 1
Forum section: electronics / projects

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 02, 2021, 04:39:10 pm
content: Hello experts,does anyone here in the forum know about MOSFET transistors and has some types in mind?I am looking for a BS170 that has a slightly lower threshold voltage VGS Max.Values of the BS170:VGS Gate Threshold Voltage Min Typ Max Unit0.8 2.1 3 VAnd I need a specimen ( N-Channel) with max 2.5V, which is easy to get - nothing exotic pleaseAnd preferably no SMD.Drain Current - Continuous: 500mA or more.Purpose: switching a slightly larger load with the port-output of an AVR controller.Thanks,Frank

author: Terry Bites
author_rank: Super Contributor
timestamp: November 02, 2021, 04:57:04 pm
content: https://www.digikey.co.uk/en/products/filter/transistors-fets-mosfets-single/278?s=N4IgTCBcDaILYHsDOAzApgFxAXQDQgDYpQBLAEyhAEYBOGgZhHwActIQBlDAJxIDsA5kxAYAnszSUAhkgDGIAL4KgASort by Vgs th and package and cost..... eg BSS138W opr RTR020N05TL. A high gain npn, as Benta rightly point sout, is an alternative. O.7V Tun on. BC817? Or a darlington 1.4V turn on. eg FMMT634 hfe >1000

author: Benta
author_rank: Super Contributor
timestamp: November 02, 2021, 05:01:36 pm
content: Just use a BJT, Much cheaper and easier.

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 02, 2021, 05:14:49 pm
content: Ok, thanks.What is BJT ?

author: magic
author_rank: Super Contributor
timestamp: November 02, 2021, 05:15:26 pm
content: The smallest THT package offering a good selection of MOSFETs is IPAKStill better than TO-220BJT is an ordinary bipolar junction transistor. 2N2222 will do if you can live with the base current.

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 02, 2021, 05:28:49 pm
content: Ok, thanks.I think a MOSFET is better because of the lower voltage U drain-source.The voltage loss should be as low as possible.It is a battery project with 3V

author: Benta
author_rank: Super Contributor
timestamp: November 02, 2021, 05:30:47 pm
content: The saturation voltage of a BJT is 50...100 mV, that's all. You'll get BC847 parts for a couple of cents.

author: magic
author_rank: Super Contributor
timestamp: November 02, 2021, 05:40:15 pm
content: BC847 is SMD and too wimpy for 0.5A. Good old 2N2222 would work, but its saturation voltage may not be great at such high current either.Well, if you don't mind TO-220, IRLZ44N sounds like a type that should be easily available from hobbyist suppliers and the datasheet shows it working down to 3V and still kinda working at 2.5V. It's way overkill and still somewhat uncomfortably high threshold and I'm sure something better (and less bulky) could be found in IPAK, but you would have search around.Nevermind, it's crap compared to BS170.

author: Benta
author_rank: Super Contributor
timestamp: November 02, 2021, 05:59:31 pm
content: OK, then a BC817. Still cheaper and easier (3 cent). Note that the gate threshold voltage is NOT the same as the "fully on" gate voltage.

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 02, 2021, 06:27:04 pm
content: Okay, I'll take a look at these specimens in the data sheet.Thanks again

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 03, 2021, 10:16:51 am
content: Quote from: Benta on November 02, 2021, 05:59:31 pmwhat does that mean exactly:"the gate threshold voltage is NOT the same" ?And 2rd question:In the datasheet of BC817 I see:Collector-emitter saturation voltage: VCEsat - - 0.7 V at IC = 500 mA, IB = 50 mACan I assume that the 0.7 V becomes significantly lower with a current of IC=150mA or even only IC=50mA ?

author: Siwastaja
author_rank: Super Contributor
timestamp: November 03, 2021, 10:24:43 am
content: Vgs_th is likely an irrelevant parameter, I guess you are looking at the wrong number.What are you actually doing? Switching a load on/off? What kind of load? Max current draw? Max input voltage?Also, what IO voltage you have available to drive the gate? 5V? 3V3? Something else?

author: Zero999
author_rank: Super Contributor
timestamp: November 03, 2021, 10:52:05 am
content: Quote from: Frank_MV on November 03, 2021, 10:16:51 amThe gate threshold voltage is the point at which the transistor starts to turn on. A higher voltage is normally required to achieve the specified drain resistance.I would not recommend the BC817 for 500mA, because it doesn't offer any margin. I'd go for the BC337. It's true a base current of 50mA is required to drive it hard into saturation, but for most applications you can get away with under half of that.

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 03, 2021, 11:35:48 am
content: Quote from: Siwastaja on November 03, 2021, 10:24:43 amYes, I want to switch a load via ATtiny, about 150mA.The battery voltage is 3V.Therefore the voltage Uce should be as small as possible.Zero999: I need only in about 150mAHowever, I am not quite clear from which diagram I can read the magnitude of the base current as a function of the collector current.I am not so familiar with "hef".

author: Siwastaja
author_rank: Super Contributor
timestamp: November 03, 2021, 12:02:36 pm
content: If I understood correctly,Vcc of the MCU is ~ 3.0VVcc of the load is ~ 3.0VIload_max is ~ 150mA.Both BJT and MOSFET work, but I'd pick MOSFET. With a BJT, you should pick a special low Vce saturation voltage part because you can't likely afford to lose a large part of your 3V supply on Vce(sat). A BJT also needs a resistor (unless you buy a co-packaged special part).So I usually go with a MOSFET. Vds_max=20V is enough, which also means market is saturated with parts that fully turn on at mere Vgs=2.5V.I don't recommend limiting yourself to through hole parts because that makes this hobby or profession much more difficult than it needs to be in this world of modern and good SMD parts.Picking Vds_max>=20V, Idmax>=1A from Digikey parametric search finds for example, just ordered by price,DMG2302UK-7 120mOhm @ Vgs=2.5VSI2310B-TPDMN2058UW-7BSH103BKR... and hundreds more.Almost any MOSFET does it at such low current.It can go wrong by picking one with excessively high Vds_max because then Vgs=3.0V might not be enough even at such low Id.

author: 2N3055
author_rank: Super Contributor
timestamp: November 03, 2021, 12:04:56 pm
content: Integrated Load Switch for example FDC6323L...

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 03, 2021, 12:14:45 pm
content: Vcc of the MCU is ~ 3.0VVcc of the load is ~ 3.0VIload_max is ~ 150mA.Yes - correct.Thanks for all tips :-)

author: Zero999
author_rank: Super Contributor
timestamp: November 03, 2021, 12:16:48 pm
content: Quote from: Frank_MV on November 03, 2021, 12:14:45 pmThen I'd still recommend the BC337. A base current of at 5mA or more should be enough to ensure it's pretty well saturated.A MOSFET isn't a bad idea either, if you can afford the extra cost and difficulty in finding parts.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 03, 2021, 12:38:02 pm
content: Yes, BC337 at Ic=150mA, Ib=5mA gives Vce of approx. 60-70mV. You can do better with a MOSFET but likely this much isn't a problem.Chip crisis is bad yes but availability of a simple transistor capable of switching a 3V 150mA load has no issue whatsoever, whether it's MOSFET or BJT. There are literally hundreds of suitable part numbers from dozens of manufacturers.It's true such jellybean BJT is like 5 cents while the MOSFET is like 10 cents but don't forget the cost of adding the base resistor!Likely nothing of this matters at all.

author: CaptDon
author_rank: Super Contributor
timestamp: November 03, 2021, 12:44:58 pm
content: Those ultra-sensitive gate mosfets need to be understood if you want to use them correctly. That threshold voltage will not get you to the low RDS you are looking for!!! You still need 4 to 5 volts gate to source to achieve good saturation. Those LLFET's work real nice when driven from TTL buffers like 74ls374 or 74ls245. I have done it and even paralleled the buffer outputs to get faster switching speeds since LLFET's have heavy gate capacitance loading. Also, most of those sensitive FET's will see gate punch-through and permanent damage if the gate goes more than 10 volts above or below the source voltage. They are extremely static sensitive!!

author: Siwastaja
author_rank: Super Contributor
timestamp: November 03, 2021, 12:51:06 pm
content: Quote from: CaptDon on November 03, 2021, 12:44:58 pmBS. "Good" depends on what is needed. It is irrelevant if the part can perform even lower Rds_on at even higher Vgs.Look at the datasheet of DMG2302UK-7 for example. Front page, actual Rds_on defined at mere Vgs=2.5V. Yes, things are easy today, they tell you this directly. Also look at Fig1. and Fig.3. Maybe some two decades ago we only had Fig.1. available and beginners didn't know how to interpret it so they tried to apply Vgs_th in some weird ways, or looked for "magic words" like "logic level" which are just hand-waving. Things are better today.120mOhm at 150mA is 18mV, some 4x better than the BJT in the previous example. If you need even lower voltage drop, no problem, just pick a MOSFET with even less Rds_on at your whatever Vgs=3.0V. You start needing 4-5V only when Vds_max goes higher (>25-30V), or Id goes significantly higher (> a few amps), or both.Most small MOSFETs (for general purpose switching applications) on the market today include ESD clamping TVS but of course they are still somewhat ESD sensitive. I have never had any problem but I do always have at least some mediocre (primitive) minimum level of ESD control available. It is true BJTs are less sensitive to ESD.For special applications which need ultra low gate charge (including RF parts), small MOSFETs are available without the ESD protection diodes, but AFAIK you don't accidentally buy those when shopping general purpose switching transistors, they are in special categories or are more expensive.