I purchased some sot23-3 mosfets for use on a board I’m making. They are way to small to deal with. I have the space, not the EYES! This is hobby stuff so not a pick and place issue here 'cause I have to do that manually.
What package is “one size” larger than a sot23-3? I could MAYBE use a TO-92 but that means lots of soldering. FYI, the mosfets I bought are RJU003N03T. 20 of them on a reel only takes up 3".
Hi,
Is that truly SOT-23? Datasheet suggests that it is packaged to UMT3 (SOT323), which is 2/3 of the size of SOT-23. SOT-223 is probably the next bigger case from SOT-23.
Cheers,
heke
I read your reply again and looked at the specs. Yeah, it’s UMT3. Was not aware this is the same as SOT323. So now the question is, how do I get same/similar mosfet in bigger package, eg SOT-23. Lots of confusion here because I downloaded a chart of relative SMD parts and it showed SOT-23 as 2mm width, with SOT-223 about 6mm across. No SOT323.
I ordered some….they are 2 mm across, about the size of a pen nib. I can hardly see them, let alone place them on a board. I’m new to smd…not aware of all the packages.
Hi queenidog,
Yep, you are right, those SOT-323s are rather tiny and difficult to handle.
Perhaps you could consider for example RK7002 from the same maker as a substitute.
https://www.digikey.com/en/products/detail/rohm-semiconductor/RK7002BMT116/4924083
Even with SOT-23 can recommend using a microscope for soldering, as, in addition to the easier part handling, it helps achieving better quality soldering result too.
The package naming is indeed a big mess.
Cheers,
heke
Thanks Heke, you’ve been a great help. I tried to replace with SOT223 (a different fet) but they are the size of a TO-220, way too big.
I made another board using To-92 Mostfets, ie through-hole types. Looks ugly but I managed to get all the parts on and only had to enlarge board by a few mm all around.
I looked at the one at link you sent. It is 1 mm wider (3mm vs 2mm). I really want something in between SOT-23 (3mm) and SOT223 (6mm) TO92 fits the bill but has leads….
Bob F
Hi Queenidog,
SOT-23 was one of the first plastic SMD packages, thus there has not been much a need for a larger package for discrete low power dissipation devices.
Based on your description, your application contains a large number of FETs. If they are doing load switching, you could perhaps consider FET arrays, that package multiple devices on one die. Like the device linked below serving 7 FETs in a SOIC package, taking certainly less space than TO-92s, yet being relatively easy to solder.
https://www.digikey.com/en/products/detail/texas-instruments/ULN2003V12DR/3455512
Cheers,
heke
AsamaLab
Actually, I only need 3 small FETS to fit on a board about 1” x ½”. I’m familiar with the FET and Darlington arrays, use the 2803 and 2981 all the time! My problem is an SMD pixel LED that takes up most of the space making it difficult to put through-wire parts on. Attached is screen print of two versions, one with the SOT23-3 (top and bottom) and other with the TO92 (both sides shown superimposed).
Other parts, in and out terminals, resistors and a WS2811 (smd) pixel driver IC. SOP8.
Is there such as thing as a 3 FET array? (I’ve got 4 unit Darlington high power arrays).
Hi Queenidog,
Thank you for the details. Makes sense. By the way, very nice layout designs! Can see that the huge LED takes the majority of the board area.
AFAIK, 3 FET arrays with a large current capability are not so common. May land to some brushless motor driver’s territory.
By the way, are you planning to drive each 3 colors as ON/OFF or adjustable? The I/F chips WS2811 seems to have sink type outputs, which may create some trouble driving NFET as a load. Depending of the internal configuration of the driver stages of the chip, you may not get the drive current either scaled correctly or at all (that is, the outputs are constant current, but the FETs would like to see voltages at the gates).
For a high power RGB LED you may need to go with a switching driver instead of constant current driver in order to keep the thermal losses low… If the LED power is lowish, an alternative is to use a LEDs that have the driver embedded. For example:
https://www.digikey.com/en/products/detail/sparkfun-electronics/BOB-13282/6163655
Cheers,
heke,
AsamaLab
That’s my problem…the LED power is NOT lowish. I live across from a cove on Atlantic ocean, highway where viewing of my displays (whatever they are) is about 700 feet away. I need POWER in the LEDs and the WS2812 doesn’t have it. (I have some sitting on my bench). BTW, the WS2812 is a WS2811 (the chip I’m using) with a 5050 RGB pixel attached. All I’m doing is removing the low power pixel, replacing it with a 3W jobby and adding drivers. Not really a FET guy, I’ve always used BJTs.
Having said this, all I need is FETs with 200-300 mA capability for each pixel, not large current by any means. I haven’t breadboarded the design yet so not sure if it will work (as you attest to in your email). I can still use BJTs if I have to, e.g. 2N3906 in TO92 package. I bought some quad Darlington arrays (STA475) from Digikey, and just love using them: lots of juice, easy to layout, minimal wiring.
I’ve constructed an 8 x 32 matrix (second photo) using 2981->FETs for columns and higher power FETS for the rows (since they have to drive 32 pixels). I breadboarded this circuit to make sure it works. Attached is logic board (first photo) to drive it using an Atmega 328, same as in Arduino UNO, but I made space for an ESP32 for more punch if required. (I use slower I2C because of my familiarity with it. Previous version used 74HC575 latches)
Here’s a video I made of two 8 foot Christmas light displays (can be seen from 700 feet away) using the high power pixels as in the design we are talking about, but using 2981 drivers and medium power BJTs. I limit the LED current to about 100 mA (determined empirically) and use 12Volts for the pixels. If I used 15 volts….brighter.
I may have finished talking about it, but it might be a good idea to look at the list of packages.
I hope someday good parts will come out.
http://rohmfs.rohm.com/en/products/databook/package/spec/discrete/transistorpkg_1.pdf
One tip if intending to hand solder is to make a version of your footprints that are ‘hand solder’ friendly. Your resistors and SOP have pads that slightly extend, so had this been done for the FETs, it might not have been such a dread, somehting like this-
Magnification is your best friend when soldering tiny items (along with a 45 degree tweezers) and wait with caffeine until after soldering 
Or find some teen/20 something who loves a challenge.
Great work. Your video came out nice! Feel free to post here again when you get this working.
Hi Queenidog,
Nice display!
The best approach is probably to build one pixel and see how it works.
With 3W LED the system power dissipation issues may arise (with 8x32 array whopping 1kW is not far away). A chance is that you’ll need to drive the LED with a switching driver instead of linear. I’ve used in some projects AL8805, which offers both PWM and voltage dimming option.
https://www.digikey.com/en/products/detail/diodes-incorporated/AL8805W5-7/2426149
Cheers,
heke
AsamaLab
The LED is 3W but I only allow it about 1 watt (100 mA/12volts). I’ll be multiplexing the display so don’t expect too much current draw.
Yes, testing is the best way.
Yes, I did that for the pixel led… made my own pattern ‘cause the supplied one from Diptrace was exactly pad size. To solder by hand, I definitely needed the larger lands, but now that I know how to cook these things in my wife’s oven, I could make the pads on my led smaller, thus saving a bit of space.
Yesterday I received a microscope with integral LCD display that I bought just for this task. I put one of these little guys under it to see how easy it was to manipulate. Scope worked very, very well but I’m thinking to do a couple is okay, but to do a couple hundred is going to be a chore. I’m going to check if my PCB house can manufacture for me (ie place the small fets). I’m working on a Darlington solution too since I can get 4 darlingtons in one chip about ¾” wide. I’ve already lost two of those little suckers.
When my son was young (40 years ago…) I had him soldering a whole bunch of things for me. Now that he has his PhD, this is beneath him….
More stuff I built: http://www.raiderracing.com/MainPage/Engineering.html
Thanks for your input.
Bob Found
