Experimenting with MOSFETS: Threshold Voltage

RE: Experimenting with MOSFETS: Threshold Voltage

jc2048 — Tue, 10 Sep 2019 22:00:16 GMT

Finally, this is a BSS138 device in an SOT23 package.

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Here they are again reduced to a dV per degree C value:

BSS138 -2.05mV/C
BS170 -3.25mV/C
STP55NF06L -6.06mV/C
IRF730 -6.67mV/C

RE: Experimenting with MOSFETS: Threshold Voltage

jc2048 — Mon, 09 Sep 2019 15:51:15 GMT

Here are two more. The STP55NF06L is a 'logic level' power device in a TO220 package, the BS170 is a 'small signal' device in a TO92 package. By coincidence, they had similar threshold voltages, so I put them both on the same graph so we could see how they compare. As you can see, the power device varies much more than the small-signal part.

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RE: Experimenting with MOSFETS: Threshold Voltage

jc2048 — Sat, 07 Sep 2019 12:19:38 GMT

When I saw in datasheets that the threshold voltage was normally measured with the drain voltage the same as the gate, I was curious about how much effect varying the Vds voltage would have on it. Simple way to find out was to try it. Here's a graph of threshold voltage versus Vds for an IRF730 part.

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As you can see, the threshold voltage dropped as the voltage between the drain and the source increased, but the effect was very small: it only fell by just over 3mV as the Vds went from 1V to 10V.

I did that test quite quickly, after allowing the whole circuit to warm up for several minutes, and stepped down the Vds values as well as up with a comparison between the resulting Vgs(th) values to make sure I wasn't just measuring the effect of the temperature rise from the increased dissipation at the 10V end. But I wasn't very confident in the result - I really needed a better idea of the way the threshold voltage changed with temperature.

The next day I did a simple test where I measured the Vgs(th) for Vds=10V as the whole circuit warmed up. Here's the curve I got over a period of 15 minutes. The MOSFET dissipation is 2.5mW (10V x 250uA), so not too much in the way of self heating. That gave a change of 6mV before it settles.

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Something worried me about that curve. It seemed to go in the wrong direction - the curves in the datasheet imply that the Vgs(th) temperature coefficient is negative - so what was I looking at? It's more likely that it's the way the constant-current circuit behaves - the meter I was using to show the drain current could only show it to 3 digits.

That wasn't very helpful so, finally, I've rigged up an ad-hoc temperature test. It isn't very sophisticated - the TO220 package is sitting on the top surface of a ceramic power resistor and I'm measuring the tab temperature with a thermocouple - but it's good enough to give me an indication of how the threshold varies with temperature. This one was done with Vds being the same as Vgs.

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To return to the first plot, the 3mV change I saw there is nothing compared to how the threshold voltage moves with temperature and it's highly likely that some proportion of it is down to self-heating anyway. In a real circuit, if the threshold voltage was important to you in some way [it's not really if the device is used as a switch or inside a feedback loop], you are going to be battling the change with temperature and the change with Vds looks to be quite minor.

I'm having fun with these blogs. I know all the information should be available in books or on the web, but it is interesting doing the experiments for myself.

RE: Experimenting with MOSFETS: Threshold Voltage

jw0752 — Sun, 01 Sep 2019 06:26:11 GMT

Hi Jon,

I decided to make a little more permanent test jig. I ended up using a TLE2142 OP amp since I didn't have an OP177. I also adjusted the 4k7 and the 2k7 in series with the pot so that i could use a 1K pot. I have found that the 250 uA remains quite stable so a small 10 turn trimmer was used. I originally thought that the pot would need adjusting with each different MODFET but of course this just depends on the stability of the series 3k9 Source resistor. Any how it has been a lot of fun and I still have quite a few MOSFETs to go.

[View:/resized-image/__size/620x465/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-9a558521-b863-41eb-9bda-7c48c5021384/8358.contentimage_5F00_207765.jpg:620:465][View:/resized-image/__size/620x465/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-9a558521-b863-41eb-9bda-7c48c5021384/7853.contentimage_5F00_207766.jpg:620:465]

CLICK IN THE BOX TO SEE THE PICTURE

I use alligator clips with wires and pins to attach to the board.

John

RE: Experimenting with MOSFETS: Threshold Voltage

three-phase — Thu, 29 Aug 2019 19:11:21 GMT

Thanks for the informative blog, I too will try and set this up to carry out my own tests.

Kind regards

RE: Experimenting with MOSFETS: Threshold Voltage

jw0752 — Thu, 29 Aug 2019 15:53:28 GMT

Hi Jon,

I find this an interesting experiment and I hope to duplicate it on my bench. Thanks for the post.

John

RE: Experimenting with MOSFETS: Threshold Voltage

clem57 — Thu, 29 Aug 2019 15:30:06 GMT

Thanks [mention:4f57fc9d538949ad9eb336ddb9469bb8:e9ed411860ed4f2ba0265705b8793d05] for me to learn about power MOSFETs.

RE: Experimenting with MOSFETS: Threshold Voltage

dougw — Thu, 29 Aug 2019 13:14:15 GMT

Nicely done. I'd be interested to know if the gate voltage on these parts is more (or less) consistent with 100 mA through the source.

RE: Experimenting with MOSFETS: Threshold Voltage

cstanton — Thu, 29 Aug 2019 12:43:25 GMT

Huh, this's pretty good - thanks for the blog series [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-9a558521-b863-41eb-9bda-7c48c5021384/contentimage_5F00_2516.png:16:16]