Gas station without pumps

2017 December 20

Ron vs Vgs for pFETs and nFETS

Filed under: Circuits course — gasstationwithoutpumps @ 22:16
Tags: ,

My son has joked that I don’t have an electronics hobby—I have a hobby of characterizing transistors.  There is an uncomfortable amount of truth in that assertion—I spend more time measuring things with my tools than building things.

Today I wanted to characterize all the nFETs and pFETs that I have, so that I could choose the right ones for the class-D amplifier lab in spring quarter—also to see whether the on resistances were as low at the data sheet claimed.

I used just the Analog Discovery 2, some 10W resistors (to make the 24Ω load), and a Schottky diode as a fly-back diode (because the power resistors have a fairly large inductance).  I chose the load so that the power supplies on the Analog Discovery 2 would not exceed their 2.1W limit. In the past I’ve used a wall wart with a higher current limit to do power FET tests, but I figured that the 400mA test current was enough for what I wanted to do today.

The setup for measuring the pFETs.

I swept the gate voltage with a 1-second ramp, turning the transistor on abruptly but off slowly.  I determined the gate voltage just from the timing, as I used the two oscilloscope channels to measure the drain-to-source voltage Vds and the drain current Id.

The results of measuring the pFETs. Only the low-threshold IPP45P03P4L11AKSA1 turns on at –3.3V. The funny knees in the curve around 10Ω are where the test jig switches from being roughly constant current (~400mA) to having a current dependent on Ron (Vds is no longer nearly 0).

To get the on resistance, I fit the data from –10V to –9V with a straight line and used that straight line at Vgs=–10V to get the value.

IPP45P03P4L11AKSA1 is no longer available, but NTD2955-1G and IRFU9024NPBF still are—it’s too bad that they aren’t really turned on at —3.3V.  Perhaps I should get myself some IRLIB9343PBF pFETS and test them—they supposedly have a very low threshold and reasonably low Ron. They cost a bit more, but they do have the longer leads that allow breadboarding.  I looked at the data sheet for the IRLIB9343PBF, and it does not look very promising—the resistance shoots up at about –5V.  The SPP15P10PLHXKSA1 looks more promising on the datasheet.

I have a lot more nFETs than pFETs, so I tested all of them also. The circuit is essentially the same as for the pFETs, just swapping the voltage sources and turning the fly-back diode around. I also changed the direction of the ramp, so that I was still turning on the FET abruptly and off slowly.

The nFETs clustered more than the pFETs did, with three outliers.

One of the outliers, IPU50R950CEAKMA1, is a high-voltage nFET. It has both a high threshold and a high Ron, but it can handle much larger voltages than the others—it was intended for switching rectified mains voltages (about 350V), but I’ve not used it yet. The other two outlier nFETs (2N7000TA and TN2106N3-G) are not power FETs—I had to switch to a 48Ω load to measure them, to avoid putting too much current through them.

The AOI514 and AOI518 parts are obsolete. The NTD4858N-35G is still available, but it has very short leads, and we had problems last year with them popping out of the breadboards. The NTD5867 is now only available as a surface-mount part, but the PSMN022-30PL,127 is still available and has long leads that work in the breadboard.


  1. I bought some for breadboarding and prototyping with SOT-23 FETs. I was going to design my own since they are about as trivial as possible, but found that someone else had already done it. Makes it easy to use current parts. I also got for doing the same with SOIC op-amps; I just use as many pads as I need.

    Comment by Michael Johnson — 2017 December 21 @ 03:47 | Reply

    • I’ve thought about using breakout boards for using SOT-23 FETs as you mention, but the extra soldering time for the class would really be undesirable. The cost per chip goes up about 20¢ for the 3-pin header and the PC board (in 100s), but that is still cheaper than through-hole parts. Maybe I should consider SOT-23 FETs again.

      Comment by gasstationwithoutpumps — 2017 December 21 @ 08:53 | Reply

      • It is starting to feel like if you are going to do electronics, you need to know how to solder SMT as well as TH. I kind of gave in when all the FETs with the characteristics I was looking for showed up with no TH variants at mouser and digikey.

        It’s a little silly that headers and adapter board together cost more than a good MOSFET, of course. By way of example, NXP’s PMV20XNER are $0.15 in 100s. I expect that you aren’t drawing enough power in your applications to care about derating from mounting SOT-23 packages on small boards; if you did I guess you would have to factor in also adding heat sinks to the cost.

        For SOIC breakout boards, you really want fine solder, drag soldering tips (that’s $8 right there!), and flux pens, and I’m not feeling 100% comfortable with drag soldering yet. But SOT-23 really isn’t bad. Just tape it in place and solder one pad, then remove the tape and solder the other two pads. It shouldn’t be particularly harder than the more finicky TH parts. I used the same solder and tips that I use for soldering TH parts for the SOT-23 adapters.

        Comment by Michael Johnson — 2017 December 21 @ 18:21 | Reply

        • I’ve hand-soldered worse packages than SOT-23, and I think that SOT-23 is within student capabilities. I’ll have to see whether tape works well enough in student hands—I’ve generally needed to use squeeze-to-open tweezers to clamp small parts while I solder them, but I’ll try tape.

          I’m seeing 28.46¢ in 100s for PMV20XNER (at Digikey), and they are 30V nFETs with 30mΩ Ron at Vgs=2.5V, which is quite good. I can probably get cheaper ones, as there are even pFETs that I can use for under 25¢ in 100s.

          My big question now is the best way to make the breakout boards. I’m considering doing a half H-bridge with bypass capacitor (through-hole) as a board, rather than separate breakout boards for each FET. The size would be about 10mm×16mm, which means about ¼sq in, or about 42¢ a board from OSH Park (but the same price for 2oz copper). If I went with a Chinese manufacturer like Seeed, I could get 600 boards for under $25 (so < 5¢ each) including shipping or about $65 (11¢ each) for 2oz copper, but I'd have to cut the boards apart myself. I can also get right-angle male headers through AliExpress for about 2¢ a breakout board, so the boards plus headers would only be about 13¢ each, even with 2oz copper. That is much less than the difference in cost between SOT-23 and trhough-hole parts.

          Comment by gasstationwithoutpumps — 2017 December 21 @ 19:28 | Reply

  2. I do have access to board shears at work, which are handy for cutting lots of boards. When I just have one or two boards to cut, I use tin ships at home, which work ok.

    The Mouser FETs are surprisingly much cheaper (about half the price) than the same ones from DigiKey. This was true for about 9 different FETS that I checked (Mouser did not carry all the ones I looked at on Digikey). Other distributors are more in line with DigiKey’s prices, or higher, so Mouser’s prices are particularly anomalous.

    The pFET you recommended is a 20V part, and I was thinking of sticking with 30V parts, with my top choices being PMV20XNER for nFET and SSM3J332R for pFET, which have specs for fairly low resistance with Vgs at 2.5V.

    Comment by gasstationwithoutpumps — 2017 December 21 @ 22:46 | Reply

    • I think I saw lower prices, particularly at the low/mid breaks, for other tape parts at mouser as well. Might have been MCP6002/4 that I’m remembering? Full reels of PMV20XNER are within a couple cents/each between digikey and mouser, not half price. Makes me wonder whether mouser invested in automation that makes it much cheaper for them to fulfill smaller breaks of tape parts. The PMV20XNER was $0.153/each in 100s when I purchased them almost a year ago, so it’s not a new thing.

      I’ve also liked mouser because they have been consistent in my experience in keeping beaglebone variants in stock.

      Thanks for the tin snips tip (say that ten times fast!). I have some ganged boards that I haven’t cut yet, I’ll have to try that.

      Comment by Michael Johnson — 2017 December 22 @ 05:18 | Reply

  3. […] a discussion in the comments of Ron vs Vgs for pFETs and nFETS with Michael Johnson, I decided to design my own breakout boards for SOT-23 surface-mount FETs, […]

    Pingback by Breakout board for SOT-23 FETs | Gas station without pumps — 2017 December 22 @ 23:26 | Reply

  4. […] the afternoon checking that the boards were OK.  I used essentially the same setup as I used for Ron vs Vgs for pFETs and nFETS, with a 24Ω load and a 10V ramp that gradually turned the transistor off.  Because the test was […]

    Pingback by SOT-23 FETs for half H-bridge | Gas station without pumps — 2018 January 3 @ 20:53 | Reply

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