EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: forrestc on July 25, 2016, 09:02:23 am
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Quite a while back I used to use the 2n7000 and 2n7002 fets in everything. When I switched to 3.3V logic for my designs, the 2n7000 didn't really have a low enough Vgs(th)to make me happy, and the Ron at 3V wasn't very good either. I spent some time looking for a FET alternative, and didn't really find any which met my needs (which included high Vds). At some point someone asked the pointed question as to why I wasn't just using a jellybean NPN transistor instead, and after about 5 seconds of :-//, I switched to a MMBTA06 in everything.
Now I have an application where I need a FET - I.E. need to do basic slew rate control and need to go all the way to the negative rail. So I'm back to looking for a good jellybean part (cheap, widely available). Or at least something somewhat jellybean. I'm finding quite a few parts, including the FDV303N, which seems like a good part which is well-stocked. Does anyone else have a favorite N Channel FET driveable directly from 3.3V logic?
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BSS138, but be careful, check RDSon.
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BSS138.
Using bipolars is totally fine, if you don't mind the need for additional resistors, Vce voltage drop, and increased static power consumption.
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It's nice that all those ancient MOSFETs like 2n700x and BSS138 are still around for when you want sub pA gate leakage, but if you don't it's a bit silly to use them.
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Then what would you suggest? BSS138 has been my jellybean logic level FET for ages. It's 50V, available in automotive specs and is dirt cheap.
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The AO3400 is fairly cheap and has an RDSon of 52mOhm with 2.5V Vgs; 30V Vds max. It's quite a power-oriented MOSFET however, with 630pF of input capacitance.
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Use a BJT to drive your 2n7000. :popcorn:
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DutchGert : Not an endorsement, but searching for AEC-Q101 rated small signal MOSFETs in that range with gate protection ... DMN53D0xx? Or NVJD5121N, but that's a dual.
Not having to worry about blown gates is worth a few cents IMO.
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2N7002 (for when you don't need much load current, or operation at low temperatures)
BSS138
FDV301N
AO7400
Tim
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Hi,
I just happen to have the transfer characteristics of a randomly selected 2N7002 (Diodes Inc.) measured with my HP4155B Semiconductor Parameter Analyzer.
The 4155B is a collection of four Source Measurement Units (SMUs) that can be programmed to step and measure semiconductor parameters. Think of it as an accurate, programmable curve tracer.
(https://www.eevblog.com/forum/projects/jellybean-n-channel-fet-for-3v-logic/?action=dlattach;attach=243552;image)
Notes:
The horizontal axis is linear, the gate source voltage, Vgs, from 0 to 2V
The yellow trace is logarithmic that extends from 1 femto Amp (1E-15) to 100 milli Amp (1E-1) an astonishing 14 decades.
The blue trace is transconductance, 0 in the middle of the screen and 10 milliamps / volt at the top.
This is a single sample at room temperature.
With Vgs =0V, the drain current is 1pA
with Vgs = 1.6V the drain current is 1mA
The speed will be determined by the gate drive and the capacitances.
Regards,
Jay_Diddy_B
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2N7002 (for when you don't need much load current, or operation at low temperatures)
BSS138
FDV301N
AO7400
Tim
Tim's recommendations are on my "goto to" list too :-+ :- :-+
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FDV303N is my usual.
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FDV301N is a nice one indeed, Diodes even has a Automotive Qualified one: DMG301N (http://www.diodes.com/catalog/N_Channel_8V_to_29V_106/DMG301NU_10655 (http://www.diodes.com/catalog/N_Channel_8V_to_29V_106/DMG301NU_10655))
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The yellow trace is logarithmic that extends from 1 femto Amp (1E-15) to 100 milli Amp (1E-1) an astonishing 14 decades.
To make a device able to control leakage to that extent unprotected gate MOSFETs are nice, for everything else pick something with gate protection :)
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Also, 3LN01M-TL-H
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To make a device able to control leakage to that extent unprotected gate MOSFETs are nice, for everything else pick something with gate protection :)
It's a bit silly to add cost to every FET when you're driving them from a microcontroller, especially since the microcontroller would be good and fried well before exceeding the Vgs rating of the FET's...
(Talking from the standpoint of volume production here).
Although I will say I didn't even realize there were FET's with gate protection... so I do have some interesting ideas about using them for input protection (something I had never considered).
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I don't think I've seen MOSFETs rated for more than 2kV HBM. Input protection, uhhh...
Tim
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Often it's going to be the only component on a PCB with a HBM rating of 100V, with everything else being multiple kV. That represents an opportunity cost, change the 2n700x and suddenly the handling requirements for assembly are much looser.
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The cheapies I saw on ebay are: SI2302, Irlml0030 and BSS138
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The cheapies I saw on ebay are: SI2302, Irlml0030 and BSS138
You never buy discrete semiconductors on Ebay because they are over 90% inoperational fakes, and typically more expensive than the good parts.
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I like to use the SI2302 as well (bought from a credible source ofcourse).
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2N7002 is pretty easily damaged by ESD compared to BSS138.
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To find out which parts are the most jellybean-y, I got to some distributor such as DigiKey, and sort by "Quantity Available":
http://www.digikey.com/product-search/en/discrete-semiconductor-products/transistors-fets-mosfets-single/1376381?k=&pkeyword=&pv1236=717&pv1236=60&pv1236=82&pv1236=486&pv1236=332&pv1236=75&pv1236=762&pv1236=192&pv1236=15&pv1236=178&pv1236=13&pv1236=72&pv1236=48&pv1236=12&pv1236=84&pv1236=380&pv1236=245&pv1236=517&pv1236=23&pv1236=519&pv1236=370&pv1236=18&pv1236=17&pv1236=448&pv1236=284&pv1236=51&pv1236=24&pv1236=25&pv1236=11&pv1236=210&pv1236=32&pv1236=50&pv1236=676&pv1236=7&FV=1c0001%2C402447%2C403150%2C9780013%2Cfff40015%2Cfff8007d&mnonly=0&newproducts=0&ColumnSort=-1000009&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=250 (http://www.digikey.com/product-search/en/discrete-semiconductor-products/transistors-fets-mosfets-single/1376381?k=&pkeyword=&pv1236=717&pv1236=60&pv1236=82&pv1236=486&pv1236=332&pv1236=75&pv1236=762&pv1236=192&pv1236=15&pv1236=178&pv1236=13&pv1236=72&pv1236=48&pv1236=12&pv1236=84&pv1236=380&pv1236=245&pv1236=517&pv1236=23&pv1236=519&pv1236=370&pv1236=18&pv1236=17&pv1236=448&pv1236=284&pv1236=51&pv1236=24&pv1236=25&pv1236=11&pv1236=210&pv1236=32&pv1236=50&pv1236=676&pv1236=7&FV=1c0001%2C402447%2C403150%2C9780013%2Cfff40015%2Cfff8007d&mnonly=0&newproducts=0&ColumnSort=-1000009&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=250)
So, the winner would be IRLML6344 (for low RDS(on)), or FDV301N (for low gate charge).
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To find out which parts are the most jellybean-y, I got to some distributor such as DigiKey, and sort by "Quantity Available":
http://www.digikey.com/product-search/en/discrete-semiconductor-products/transistors-fets-mosfets-single/1376381?k=&pkeyword=&pv1236=717&pv1236=60&pv1236=82&pv1236=486&pv1236=332&pv1236=75&pv1236=762&pv1236=192&pv1236=15&pv1236=178&pv1236=13&pv1236=72&pv1236=48&pv1236=12&pv1236=84&pv1236=380&pv1236=245&pv1236=517&pv1236=23&pv1236=519&pv1236=370&pv1236=18&pv1236=17&pv1236=448&pv1236=284&pv1236=51&pv1236=24&pv1236=25&pv1236=11&pv1236=210&pv1236=32&pv1236=50&pv1236=676&pv1236=7&FV=1c0001%2C402447%2C403150%2C9780013%2Cfff40015%2Cfff8007d&mnonly=0&newproducts=0&ColumnSort=-1000009&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=250 (http://www.digikey.com/product-search/en/discrete-semiconductor-products/transistors-fets-mosfets-single/1376381?k=&pkeyword=&pv1236=717&pv1236=60&pv1236=82&pv1236=486&pv1236=332&pv1236=75&pv1236=762&pv1236=192&pv1236=15&pv1236=178&pv1236=13&pv1236=72&pv1236=48&pv1236=12&pv1236=84&pv1236=380&pv1236=245&pv1236=517&pv1236=23&pv1236=519&pv1236=370&pv1236=18&pv1236=17&pv1236=448&pv1236=284&pv1236=51&pv1236=24&pv1236=25&pv1236=11&pv1236=210&pv1236=32&pv1236=50&pv1236=676&pv1236=7&FV=1c0001%2C402447%2C403150%2C9780013%2Cfff40015%2Cfff8007d&mnonly=0&newproducts=0&ColumnSort=-1000009&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=250)
Or they just stocked up on those......
So, the winner would be IRLML6344 (for low RDS(on)), or FDV301N (for low gate charge).
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Now I have an application where I need a FET - I.E. need to do basic slew rate control and need to go all the way to the negative rail.
If low Vebo and low current gain are acceptable, then you can use a bipolar transistor with the emitter and collector reversed to provide a much lower saturation voltage. They used to make special bipolar transistors for this application but no more.
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Ah yes, back to OP's situation -- BJTs saturate to < 0.2V very easily (saturation is roughly as resistive as a FET's, so it depends on Ic), so getting a rail-to-rail output isn't any harder than with FETs.
With discrete FETs, you have the additional problem that, if you wire them up as a standard CMOS inverter (gates and drains tied, sources to respective VDD/VSS), you get massive shoot-through current (100s of mA, worse than a NE555!). We're talking several-ohms MOSFETs here! They aren't small!
So either way, you can get comparable switching speed and saturation (and shoot-through, if you're careless!).
Perhaps the best advantage to a BJT is how easy it is to make a current-limited output. Connect to the base(s) with a voltage divider, and add an emitter resistor. (Bonus points: add a diode in series with the supply-rail side divider resistor, compensating for Vbe. Current is more stable with temp.) (You can do this with MOSFETs too, but the dropout voltage is much higher and the current less well defined, because Vgs is higher and more variable, and Gm is lower.)
For slew rate limiting, you want to add an external "Miller" capacitor (i.e., drain to gate, for each transistor that's pushing around the output voltage -- for a complementary output drive, you need one each for P and N), and supply a modest (base/gate) drive current, so that the current acts to charge/discharge the capacitor, thus giving you the limited slew rate. Additionally, you may find an LC (ferrite bead and 100pF+) on the output helps smooth things out even more, especially for pins connecting to the outside world (also, add clamp diodes or zener/TVS) or that connect to long cable runs (minimize EMI).
Tim
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Another vote for BSS138 (and the P-channel BSS84 for its twin). Used in dozens of designs at 3.3V. Also recommend the NTZD3155 if you need a "use a low-side signal to switch a high-side thing" (e.g. micro at 1.8V needs to turn on LED at 5V).
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I'll add a vote for the BSS138.
Also: use Digi-Key. Select the specs you need, select on price (reel quantities) and see what pops up as cheapest. Whichever one is available in the milions and preferably from several manufacturers gets my vote.