estimating junction temperature of a power mosfet

On a sunny day (Fri, 23 Jan 2015 07:54:21 -0800 (PST)) it happened George Herold wrote in :

What current did you use? Any self-heating?

That's amazing. Lakeshore won't like that.

But I've found same-part same-manufacturer semiconductors to be very repeatable. Which sometimes lets one use measured specs that are way better than guaranteed specs.

S or D but not floating. You don't want the channel to conduct and short out the substrate diode. You can bias a mosfet ON and disconnect the gate, and it may stay on for days.

Reminds me of my fun-with-a-2N7000 experiments

Gate leakage on a good 2N7000 is not many electrons per second.

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John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

The unprotected 2N7002E parts are going away, though. Better buy a few reels while they last!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

I don't really care about gate leakage in a 2N7000, except for fun things.

But I hope the gate protection zener is symmetrical, in case I have some old application where I pull the gate negative.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

This was with my fluke DMM (I think ~ 1mA)

George H.

Cryo-diodes are usually run at 10 uA, but a big mosfet in LN2 probably doesn't self-heat much.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

So, it's up to plus/minus a part per thousand? That's encouraging, it translates to a fraction of a degree K (or C) . It's hard to be sure of 'room temp' matching, an oil or Fluorinert bath would be a prudent enhancement for that column. The 77K numbers are matched to 3 parts per ten thousand...

Screw torque and bubbles in the heatsink goo are a bigger variable than the thermometric precision of the MOSFET junction.

Torque on the screw fasteners and dust on the fan blades is a bigger uncertainty than the

Yeah, the room temperature numbers were drifting all over the place. Fet's just lying on my lab bench, taped to a piece of paper. Just bolting 'em down to a single hunk of metal would help a lot. I'm not all that surprised by the decrease in variation with temperature. I've seen that before.

George H.

PS. I need to post an epoxy update, the glass beads failed miserably, Cracks everywhere, as you predicted.

On a sunny day (Fri, 23 Jan 2015 14:35:30 -0800) it happened John Larkin wrote in :

1.0159 ^ Well he is specifying at 10^-4 say 100 ppm.... :-) 10uA would probably be OK though, 10uW * Rtj :-)

Math OK?

On a sunny day (Fri, 23 Jan 2015 12:36:54 -0800 (PST)) it happened George Herold wrote in :

OK, thanks.

I am building an electronic load using a single IXTN60N50L2. It uses an unconventional control scheme which in my case was easier to do with a single large mosfet than with many small ones. I am cooling the MOSFET with liquid. I was very surprised that nobody seems to sell water blocks already drilled for SOT-227 packages. I expect that if I drilled the required mounting holes on any of the widely available CPU or GPU water blocks, the drill would hit a water channel and it would leak. Therefore I expect I'll have to make my own water block.

Chris

McMaster sells cold plates, cheap compared to most others. No problem drilling this kind.

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John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

At LN2 temp, the tempco is probably around 2 mV/K, so he's seeing really good repeatability.

PN silicon diodes act like diodes down to about 20K. Below that the voltage drop goes way up and they get more resistive, but less predictable. Some companies, like Lakeshore, test diodes down to liquid helium temps and sell parts with dependable curves.

Thermal conductivities go to hell at those temps, hence the 10 uA convention.

A big mosfet in boiling LN2 is going to be very well temperature pinned, at least short-term, as long as the LN2 stays LN2.

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

On a sunny day (Sat, 24 Jan 2015 09:40:16 -0800) it happened John Larkin wrote in :

Nice! Is that the temp sensor in the back of thr last jpg with the black wires?

I am using a BJT as temp sensor here:

How about this for cooling?

Thanks. Since the "cooling" fluid that I will get arrives already at about 70 deg C, and I want to dissipate a lot of power, I want very low thermal resistance, so I think I will use something made from copper. I also need it to be not much bigger than the SOT-227 package because of the mechanical constraints. I would like to find something already made for this package, but otherwise I might as well silver-braze some pipes into a block of copper - either into grooves made with a ball-end mill, or drill long holes right through the copper (yuck!), drill manifolds in the other direction, and braze plugs into the unnecessary holes and braze on inlet and outlet pipes, then mill the mounting surface flat. I only need a couple of them.

If I could buy a small version of this CP25 thing with threaded inserts in the right place for SOT-227 then I would be tempted:

Chris

Yup, it's a snap switch type. Shuts things off if we lose cooling.

Well, that's one breadboarding style.

LM35s are nice temp sensors. Or thinfilm RTDs.

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John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

On a sunny day (Sun, 25 Jan 2015 08:00:32 -0800) it happened John Larkin wrote in :

Breadboard now been working 24/7 for > 2 years, say 2.5 years. temp control has been within a half degree C or so all the time. As _relative_ sensors these transistor junctions are great.

It looks so expensive it scares me to solder on it...

Yes I have some, or was it LM135 or LM335 is use cold side sensor on my thermocouple amplifier

it is next to the trimpot, I used the adjustment lead of the LM. In action here with the cryo-cooler:

You could get a small copper block and solder a zigzag copper tube to it. Mill some channels first maybe, like the one in my pic. Low hassle level, except that copper is a bitch to machine. We use Alloy 110, not quite as gummy as soft copper. Alloys generally have much reduced electrical and thermal conductivity.

One GPM has a net theta of 0.0037 K/W, so you don't need much water flow to cool a few hundred watts of mosfet.

Water has a very high specific heat, which is why long hot showers are expensive. But worth it.

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John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Gold plated FR4, never tarnishes, wonderful to solder. Cost me $100 per square foot, but a square foot makes a lot of breadboards.

The semiconductor sensors are convenient but not super accurate. We've found the thinfilm RTDs to generally be very accurate, for t/c reference junction sensing and such.

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com