estimating junction temperature of a power mosfet

[John Larkin]

On Sun, 25 Jan 2015 17:04:03 GMT, Jan Panteltje <panteltje@yahoo.com>
wrote:

On a sunny day (Sun, 25 Jan 2015 08:00:32 -0800) it happened John Larkin
jlarkin@highlandtechnology.com> wrote in
674acah94ih10mmk3ovtq884tf6p3cdb79@4ax.com>:

On Sun, 25 Jan 2015 11:16:34 GMT, Jan Panteltje <panteltje@yahoo.com
wrote:

On a sunny day (Sat, 24 Jan 2015 09:40:16 -0800) it happened John Larkin
jlarkin@highlandtechnology.com> wrote in
jnl7cadm33hf00saar3grl0lku1sjc8otd@4ax.com>:

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

https://dl.dropboxusercontent.com/u/53724080/Thermal/T222_Coldplate.jpg

https://dl.dropboxusercontent.com/u/53724080/Thermal/T222_Water_Cooled_Pulser.jpg

https://dl.dropboxusercontent.com/u/53724080/Thermal/T222_Copper.jpg

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

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


I am using a BJT as temp sensor here:
http://panteltje.com/panteltje/tri_pic/tritium_decay_experiment_black_box_electronics_top_view_IMG_3873.GIF

Well, that's one breadboarding style.

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.


https://dl.dropboxusercontent.com/u/53724080/Protos/D200_BB_4.JPG

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

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

LM35s are nice temp sensors. Or thinfilm RTDs.

https://dl.dropboxusercontent.com/u/53724080/Thermal/RTD_outside.jpg

Yes I have some, or was it LM135 or LM335 is use cold side sensor on my thermocouple amplifier
http://panteltje.com/pub/thermocouple_interface_with_sunshade_IMG_3394.JPG
it is next to the trimpot, I used the adjustment lead of the LM.
In action here with the cryo-cooler:
http://panteltje.com/pub/cryo/

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

 

[Chris Jones]

On 26/01/2015 05:21, Lasse Langwadt Christensen wrote:

Den sřndag den 25. januar 2015 kl. 12.37.15 UTC+1 skrev Chris Jones:
On 25/01/2015 04:40, John Larkin wrote:
On Sat, 24 Jan 2015 23:57:24 +1100, Chris Jones
lugnut808@spam.yahoo.com> wrote:

On 23/01/2015 23:31, Alain Coste wrote:
Thank you John for the interesting information. The use of mosfets in linear
mode is not very common, and it's more difficult to find data than for
switch mode.

It often makes sense to use more fets, spread out over the heatsink
surface, especially if the baseplate part of the heat sink is thin,
namely has high thermal spreading resistance.

This uses copper heat spreaders to transfer the heat into the aluminum
sink.

https://dl.dropboxusercontent.com/u/53724080/Thermal/Amp.jpg


Now I see what to use _more_ fets means...
For my electronic load I could have used more transistors, but this
increases the number of current sense resistors and operational amplifiers
to control them. For the power I wanted (400 .. 420W), I thought that two
mosfets was a good compromise.
[...]


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.

https://dl.dropboxusercontent.com/u/53724080/Thermal/T222_Coldplate.jpg

https://dl.dropboxusercontent.com/u/53724080/Thermal/T222_Water_Cooled_Pulser.jpg

https://dl.dropboxusercontent.com/u/53724080/Thermal/T222_Copper.jpg



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:
http://www.amstechnologies.com/products/thermal-management/liquid-cooling/cold-plates/vacuum-brazed-flat-tube-pin-fin-cold-plates/view/extended-surface-iiTM-cp25/

Chris

not exactly cheap but if you only need a few

http://www.customthermoelectric.com/Water_blocks.html

WBA-1.62-0.55-CU-01 drill and tap CPT-2.25-1.62-0.25-AL for the sot-227

or you could just take on of the cheap GPU coolers and reflow
solder a ~10mm plate of copper on top drilled and tapped for the sot-227

http://i01.i.aliimg.com/wsphoto/v0/2027141455_2/New-Water-cooling-Copper-Water-Block-40-x-40-x-10-mm-free-shipping.jpg

-Lasse

Thanks for those links. They are quite interesting.

 

[Alain Coste]

Thanks to all those who responded.
I decided initially to limit the power of the electronic load to 300W (two
fets in //), which seems safe.
Later I will make an experimental setup to measure the parasitic diode drop
at various temperatures, and see if I can safely dissipate 400 to 450W.
For now, I am fighting against parasitic thermo-electric effect of sense
resistors, which seriously compromises the precision of intensity
measurement and regulation athigh power dissipation.

--
Alain Coste
"Alain Coste" <coste@irit.fr> a écrit dans le message de news:
m9oq8a$sfu$1@dont-email.me...

Hello

I design an electronic load, using IXTK200N10L2 power mosfets (T0264
package).
I try to estimate the junction temperature of the mosfets, using an IR
camera to measure accessible temperatures.
Mosfets are directly mounted on a air-forced heatsink (Theta-sa around
0.08°C/W), with thermal grease.
With the camera, I can access the temperatures of :
- the heatsink near the mosfet (Ts)
- the package top of the mosfet (Tt)
- but not to the case temperature (Tc), as the metallic part of the
TO264 package is not visible.
Ts is in line with the calculated value, but the measured Tt is much
higher
than the calculated Tc (from Theta-jc, Theta-cs of the data-sheet, and
Theta-sa of the heat-sink).
I suppose this is normal, as much of the heat is evacuated through the
heatsink, and few watts flow from the package top to ambiant. So Tt is
probably closer to Tj than to Tc, but how much ?
Is there a way to estimate Tj from Tt. I am looking for some parameter as
Psi-jt, but nothing of the sort appears in the data-sheet....Or perhaps a
rule of thumb saying that, for a TO264 package mounted on a "serious"
heatsink, Tt is never less than Tj - 5°....
I found this kind of information for smt IC packages, but not for TO264 or
TO220.

NB : I could also measure the lead (drain) temperature, which seems to be
close to Tt. This reinforces my idea that the measured Tt is not too far
from Tj, but I would be sure before jeopardising such expensive devices as
IXTK200N10L2 !

Thanks in advance


--
Alain Coste


---
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logiciel antivirus Avast.
http://www.avast.com

---
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http://www.avast.com

 

[John Larkin]

On Thu, 29 Jan 2015 13:31:57 +0100, "Alain Coste" <coste@irit.fr>
wrote:

Thanks to all those who responded.
I decided initially to limit the power of the electronic load to 300W (two
fets in //), which seems safe.
Later I will make an experimental setup to measure the parasitic diode drop
at various temperatures, and see if I can safely dissipate 400 to 450W.
For now, I am fighting against parasitic thermo-electric effect of sense
resistors, which seriously compromises the precision of intensity
measurement and regulation athigh power dissipation.

Manganin has a low tc and a low thermoelectric potential against
copper. Zeranin is even better.

We make our own manganin shunts, which we epoxy to an anodized
aluminum block which is itself thermally regulated to sit at the flat
spot of the manganin's parabolic resistance/temperature curve. That's
extreme, but heat sinking a strip of manganin can make an awfully good
shunt resistor.

https://dl.dropboxusercontent.com/u/53724080/Parts/Shunts/Manganin_Bits.JPG

What's important is thermal symmetry of the two shunt-to-copper
junctions. With modern chopper opamps, you don't need a lot of voltage
to work with, which helps keep the shunt heating down. Interesting
geometries are possible.

You can buy thru-hole and surface mount low TC shunt resistors, which
are fine if the power dissipation is low.




--

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

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