estimating junction temperature of a power mosfet

[Alain Coste]

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



---
L'absence de virus dans ce courrier electronique a ete verifiee par le logiciel antivirus Avast.
http://www.avast.com

 

[George Herold]

On Wednesday, January 21, 2015 at 1:14:05 PM UTC-5, Alain Coste wrote:

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



---
L'absence de virus dans ce courrier electronique a ete verifiee par le logiciel antivirus Avast.
http://www.avast.com

You know how much power it's dissipating, does the spec sheet give the thermal resistance between the junction and the case? It's hard to believe that the top would be much hotter than the tab on the back side. You could try sneaking a little thermal couple in there... maybe on the drain lead (as you suggest.)

George H.

 

On Wednesday, 21 January 2015 18:14:05 UTC, Alain Coste wrote:

I try to estimate the junction temperature of the mosfets, using an IR
camera to measure accessible temperatures.
....

You could use the parasitic diode in the MOSFET to measure the junction
temperature. You would first need to measure the forward voltage drop
of the diode at some convenient and fairly low current with the device
in an oven at various temperatures.

This would give you a calibration curve for the diode. Then use the
device as a load with your heat sink until the temperature is stable.
Finally, switch the device to your low-current source and very quickly
(this probably needs to be within a few milliseconds) measure the
parasitic diode forward voltage again. You will need a relay in order
to switch fast enough.

This measurement can then be used to calibrate one of your other methods.

John

 

[George Herold]

On Wednesday, January 21, 2015 at 1:46:50 PM UTC-5, jrwal...@gmail.com wrote:

On Wednesday, 21 January 2015 18:14:05 UTC, Alain Coste wrote:

I try to estimate the junction temperature of the mosfets, using an IR
camera to measure accessible temperatures.
...

You could use the parasitic diode in the MOSFET to measure the junction
temperature. You would first need to measure the forward voltage drop
of the diode at some convenient and fairly low current with the device
in an oven at various temperatures.

This would give you a calibration curve for the diode. Then use the
device as a load with your heat sink until the temperature is stable.
Finally, switch the device to your low-current source and very quickly
(this probably needs to be within a few milliseconds) measure the
parasitic diode forward voltage again. You will need a relay in order
to switch fast enough.

This measurement can then be used to calibrate one of your other methods.

John

I like it! You could make a few measurements (in time) and extrapolate back to when the power was turned off... assuming a linear temperature decrease or something.

George H.

 

[whit3rd]

On Wednesday, January 21, 2015 at 10:14:05 AM UTC-8, Alain Coste wrote:

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

This is a steady-state circuit? You can measure heat by comparing
the air inlet and outlet temperature (minor corrections with a barometer
and calibration for airflow might be helpful). Then, make a few
interruptions and check the substrate diode drop to get the true
junction temperature after long operation (few minutes).
That will give you a true value of 'thermal resistance' through case,
grease, heatsink, to the cooling airflow.

If you need to improve the airflow or thermal resistance, or can benefit
from redesigning the cooling hardware, this is the info that's required.

 

also IR readings can be in error due to the emmisivity of the material.

try using a small thermocouple

they make very small fine wire thermocouples that you can attach with a drop of glue, works well..


Mark

 

[DecadentLinuxUserNumeroUn]

On Wed, 21 Jan 2015 12:47:21 -0800, makolber wrote:

also IR readings can be in error due to the emmisivity of the material.

try using a small thermocouple

they make very small fine wire thermocouples that you can attach with a
drop of glue, works well..


Mark

The epoxy used on such a package very likely has a very high emissivity,
and good, well documented thermal conductivity data.

The rest can be extrapolated by how thick it is... & how far "up"
from the baseplate the actual device inside is mounted, that internal
mount's thermal coupling efficiency, etc.

Experiments could be done with a non-sink mounted device, in a very
controlled, slow work application, and observe rates with IR, and collect
data. Even though non-real, they can provide insight into thermal
efficiency of the entire device's construction technique, which may differ
from maker to maker, and from design to design.

But properly examined IR data can certainly be made useful, even on a
real world sink mounted scenario.

 

[John Larkin]

On Wed, 21 Jan 2015 19:14:00 +0100, "Alain Coste" <coste@irit.fr>
wrote:

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

For a beast like that, the peak Tt is probably pretty close to Tj.

The classic technique to measure Tj is to temporarily (and very
quickly) disconnect the fet from the circuit and use the substrate
diode as a thermometer. That is sort of a chore.

I like to test the fets to destruction, to see how much margin we
actually have.

You can't in general trust the heatsink's rated Tsa. The fet has a
relatively small footprint, so there will be a hot spot there.
Heatsinks are usually spec'd with a uniform heat load spread over the
surface. Your thermal imager may have enough resolution to see the
temperature gradients across the surface of the heatsink. People
sometimes drill a tiny hole under the fet, to get a small thermocouple
right up under the mosfet.



--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[John Larkin]

On Wed, 21 Jan 2015 12:47:21 -0800 (PST), makolber@yahoo.com wrote:

>also IR readings can be in error due to the emmisivity of the material.

Black whiteboard marker, or a bit of kapton tape, will get the
emissivity close to 1.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[John Fields]

On Wed, 21 Jan 2015 10:41:09 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:


You could try sneaking a little thermal couple in there...

---
Thermocouple.

John Fields

 

On Wed, 21 Jan 2015 19:14:00 +0100, "Alain Coste" <coste@irit.fr>
wrote:

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.

Is it covered by the heatsink ?

I have seen some TO-3 transistors, with a small hole through the
heatsink exposing the base of the transistor. A thermocouple or some
IR measurement equipment was then used.

Of course, the hole reduces heat conductivity from case to heatsink,
but at least it gives the worst case estimate of Tj.

 

[Alain Coste]

Hello
Thank you for your answers.
I can give some more details on my application.
The heatsink is a sort of tunnel filled with hollow fins, and there are two
fans to force air flow through the fins.
(the model is Fisher LAV7 for those who know this manufacturer).
So it is not easy at all to put a thermocouple against the base of the
MOSFET, through the heatsink.
The idea of using the parasitic diode of the junction to measure temperature
is very interesting, but the main problem is calibrating. I suppose that
calibration must be done with THE mosfet of my device, and not another one.
So I should unsolder the mosfet to put it in an oven as other parts of my
device cannot support high temperatures. While not unfeasible, it's a little
tricky...
For upsidedown, the plastic of the TO264 package covers totally the metal
tab, so once mounted on the heatsink, no metal is visible, and here is my
problem ! I can measure (with the IR camera) the temperature of the screw,
but this is closer to Ts than to Tc.

Here are results of my experiments:
The mosfet dissipates 150W
Ts measured with the IR camera = 55°
some black plastic tape on the heat sink gives an emissivity between
0.9 and 1
the camera was calibrated for 0.95
as John said, Ts decreases as the distance from the mosfet increases
55° is measured close to the mosfet
The datasheet gives theta-cs = 0.15 typical and theta-jc = 0.12 max.
I take theta-cs = 0.2 as 0.15 is typical value.
Tc should be 55 + (150 * 0.2) = 85°
Tj should be 85 + (150 * 0.12) = 103°

Now, the IR camera gives 100° as Tt (peak temperature of the top case of the
mosfet)
The emissivity of the black plastic of the mosfet case is supposed to be
close to 0.95

There are two possible interpretations:
- either Tt is close to Tj, and everything is ok
- or Tt is close to Tc (and then theta-cs is around 0.3) : the future
is bleak for my mosfet

I experimented with a reduced power, but I want the mosfet dissipate at
least 200W.
Measures with dissipated power = 100W give coherent results (measured Tt
close to calculated Tj).

The point is the difference between Tt and Tj, and it's difficult to get an
idea on the subject (John vs George...).
The litterature I found says Tt is not less than Tj-5° for power smt
packages (power-pack,...) using the PCB as heatsink.
But may I extrapolate for TO264 on a large heatsink?
Till now Ixys didn't answer my questions. I understand that a parameter as
Psi-jt is more difficult to garantee for a TO267 package as it depends on
the characteristics of the heatsink.

--
Alain Coste



---
L'absence de virus dans ce courrier electronique a ete verifiee par le logiciel antivirus Avast.
http://www.avast.com

 

[George Herold]

On Thursday, January 22, 2015 at 8:35:11 AM UTC-5, Alain Coste wrote:

Hello
Thank you for your answers.
I can give some more details on my application.
The heatsink is a sort of tunnel filled with hollow fins, and there are two
fans to force air flow through the fins.
(the model is Fisher LAV7 for those who know this manufacturer).
So it is not easy at all to put a thermocouple against the base of the
MOSFET, through the heatsink.
The idea of using the parasitic diode of the junction to measure temperature
is very interesting, but the main problem is calibrating. I suppose that
calibration must be done with THE mosfet of my device, and not another one.
So I should unsolder the mosfet to put it in an oven as other parts of my
device cannot support high temperatures. While not unfeasible, it's a little
tricky...
For upsidedown, the plastic of the TO264 package covers totally the metal
tab, so once mounted on the heatsink, no metal is visible, and here is my
problem ! I can measure (with the IR camera) the temperature of the screw,
but this is closer to Ts than to Tc.

Here are results of my experiments:
The mosfet dissipates 150W
Ts measured with the IR camera = 55°
some black plastic tape on the heat sink gives an emissivity between
0.9 and 1
the camera was calibrated for 0.95
as John said, Ts decreases as the distance from the mosfet increases
55° is measured close to the mosfet
The datasheet gives theta-cs = 0.15 typical and theta-jc = 0.12 max.
I take theta-cs = 0.2 as 0.15 is typical value.
Tc should be 55 + (150 * 0.2) = 85°
Tj should be 85 + (150 * 0.12) = 103°

Now, the IR camera gives 100° as Tt (peak temperature of the top case of the
mosfet)
The emissivity of the black plastic of the mosfet case is supposed to be
close to 0.95

There are two possible interpretations:
- either Tt is close to Tj, and everything is ok
- or Tt is close to Tc (and then theta-cs is around 0.3) : the future
is bleak for my mosfet

I experimented with a reduced power, but I want the mosfet dissipate at
least 200W.
Measures with dissipated power = 100W give coherent results (measured Tt
close to calculated Tj).

The point is the difference between Tt and Tj, and it's difficult to get an
idea on the subject (John vs George...).
The litterature I found says Tt is not less than Tj-5° for power smt
packages (power-pack,...) using the PCB as heatsink.
But may I extrapolate for TO264 on a large heatsink?
Till now Ixys didn't answer my questions. I understand that a parameter as
Psi-jt is more difficult to garantee for a TO267 package as it depends on
the characteristics of the heatsink.

--
Alain Coste



---
L'absence de virus dans ce courrier electronique a ete verifiee par le logiciel antivirus Avast.
http://www.avast.com

Thanks for the update. At ~$25 a piece for the FETs, I would try pushing one and see where it fails. That could give you some measure of comfort.

I measured the diode drop on ~10 IRF820's in my part box and they were all within 1 mV of each other. (Not bad) (Don't touch with hands while measuring.)

George H.

 

[John Larkin]

On Thu, 22 Jan 2015 14:34:58 +0100, "Alain Coste" <coste@irit.fr>
wrote:

Hello
Thank you for your answers.
I can give some more details on my application.
The heatsink is a sort of tunnel filled with hollow fins, and there are two
fans to force air flow through the fins.
(the model is Fisher LAV7 for those who know this manufacturer).
So it is not easy at all to put a thermocouple against the base of the
MOSFET, through the heatsink.
The idea of using the parasitic diode of the junction to measure temperature
is very interesting, but the main problem is calibrating. I suppose that
calibration must be done with THE mosfet of my device, and not another one.
So I should unsolder the mosfet to put it in an oven as other parts of my
device cannot support high temperatures. While not unfeasible, it's a little
tricky...

That's the way to do it. A DVM, on its "diode" range, can do the
measurement, but you'd have to calibrate the fet, or at least one from
the same batch. Close enough.

For upsidedown, the plastic of the TO264 package covers totally the metal
tab, so once mounted on the heatsink, no metal is visible, and here is my
problem ! I can measure (with the IR camera) the temperature of the screw,
but this is closer to Ts than to Tc.

Here are results of my experiments:
The mosfet dissipates 150W
Ts measured with the IR camera = 55°
some black plastic tape on the heat sink gives an emissivity between
0.9 and 1
the camera was calibrated for 0.95
as John said, Ts decreases as the distance from the mosfet increases
55° is measured close to the mosfet

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

The datasheet gives theta-cs = 0.15 typical and theta-jc = 0.12 max.
I take theta-cs = 0.2 as 0.15 is typical value.
Tc should be 55 + (150 * 0.2) = 85°
Tj should be 85 + (150 * 0.12) = 103°

Now, the IR camera gives 100° as Tt (peak temperature of the top case of the
mosfet)
The emissivity of the black plastic of the mosfet case is supposed to be
close to 0.95

There are two possible interpretations:
- either Tt is close to Tj, and everything is ok
- or Tt is close to Tc (and then theta-cs is around 0.3) : the future
is bleak for my mosfet

If the hot spot Tt is only 100C at 150W, you should be fine.

It would be fun to somehow remove the epoxy and image the actual chip,
to see its temperature profile.

I removed the epoxy from a bunch of mosfets, but the process was sort
of violent.

https://dl.dropboxusercontent.com/u/53724080/Parts/ExFets.jpg

I think there are organic epoxy removers that might not trash the
silicon.

I experimented with a reduced power, but I want the mosfet dissipate at
least 200W.
Measures with dissipated power = 100W give coherent results (measured Tt
close to calculated Tj).

The point is the difference between Tt and Tj, and it's difficult to get an
idea on the subject (John vs George...).

It could be measured, for a given fet, but it would be an all-day
project.

The litterature I found says Tt is not less than Tj-5° for power smt
packages (power-pack,...) using the PCB as heatsink.
But may I extrapolate for TO264 on a large heatsink?
Till now Ixys didn't answer my questions. I understand that a parameter as
Psi-jt is more difficult to garantee for a TO267 package as it depends on
the characteristics of the heatsink.

We have found the Ixys mosfets to be good for surviving linear-mode
high-dissipation pulses, out in the northeast corner of the SOAR
curve. We blew up a lot of fets to learn that.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[John Larkin]

On Thu, 22 Jan 2015 11:56:15 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

On Thursday, January 22, 2015 at 8:35:11 AM UTC-5, Alain Coste wrote:
Hello
Thank you for your answers.
I can give some more details on my application.
The heatsink is a sort of tunnel filled with hollow fins, and there are two
fans to force air flow through the fins.
(the model is Fisher LAV7 for those who know this manufacturer).
So it is not easy at all to put a thermocouple against the base of the
MOSFET, through the heatsink.
The idea of using the parasitic diode of the junction to measure temperature
is very interesting, but the main problem is calibrating. I suppose that
calibration must be done with THE mosfet of my device, and not another one.
So I should unsolder the mosfet to put it in an oven as other parts of my
device cannot support high temperatures. While not unfeasible, it's a little
tricky...
For upsidedown, the plastic of the TO264 package covers totally the metal
tab, so once mounted on the heatsink, no metal is visible, and here is my
problem ! I can measure (with the IR camera) the temperature of the screw,
but this is closer to Ts than to Tc.

Here are results of my experiments:
The mosfet dissipates 150W
Ts measured with the IR camera = 55°
some black plastic tape on the heat sink gives an emissivity between
0.9 and 1
the camera was calibrated for 0.95
as John said, Ts decreases as the distance from the mosfet increases
55° is measured close to the mosfet
The datasheet gives theta-cs = 0.15 typical and theta-jc = 0.12 max.
I take theta-cs = 0.2 as 0.15 is typical value.
Tc should be 55 + (150 * 0.2) = 85°
Tj should be 85 + (150 * 0.12) = 103°

Now, the IR camera gives 100° as Tt (peak temperature of the top case of the
mosfet)
The emissivity of the black plastic of the mosfet case is supposed to be
close to 0.95

There are two possible interpretations:
- either Tt is close to Tj, and everything is ok
- or Tt is close to Tc (and then theta-cs is around 0.3) : the future
is bleak for my mosfet

I experimented with a reduced power, but I want the mosfet dissipate at
least 200W.
Measures with dissipated power = 100W give coherent results (measured Tt
close to calculated Tj).

The point is the difference between Tt and Tj, and it's difficult to get an
idea on the subject (John vs George...).
The litterature I found says Tt is not less than Tj-5° for power smt
packages (power-pack,...) using the PCB as heatsink.
But may I extrapolate for TO264 on a large heatsink?
Till now Ixys didn't answer my questions. I understand that a parameter as
Psi-jt is more difficult to garantee for a TO267 package as it depends on
the characteristics of the heatsink.

--
Alain Coste



---
L'absence de virus dans ce courrier electronique a ete verifiee par le logiciel antivirus Avast.
http://www.avast.com

Thanks for the update. At ~$25 a piece for the FETs, I would try pushing one and see where it fails. That could give you some measure of comfort.

I measured the diode drop on ~10 IRF820's in my part box and they were all within 1 mV of each other. (Not bad) (Don't touch with hands while measuring.)

George H.

That's impressive, less than 0.5 C.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[Jon Elson]

Alain Coste wrote:

For upsidedown, the plastic of the TO264 package covers totally the metal
tab, so once mounted on the heatsink, no metal is visible, and here is my
problem !

Solder a test transistor to a piece of copper block, big enough to attach
a thermocouple to. Clamp the transistor-block asssmbly as normal, it can
be just a couple mm thicker. The block should be at almost the exact
temperature of the transistor metal heat conductor, and the junction will be
just a little hotter than that. This should get you the info you need,
unless I misunderstand the problem.

The soldering between transistor and copper block needs to be a full-area
joint, but tinning both parts and then collapsing the solder while molten
until close contact is made should do it.

Jon

 

[John Larkin]

On Thu, 22 Jan 2015 14:46:19 -0600, Jon Elson <jmelson@wustl.edu>
wrote:

Alain Coste wrote:


For upsidedown, the plastic of the TO264 package covers totally the metal
tab, so once mounted on the heatsink, no metal is visible, and here is my
problem !

Solder a test transistor to a piece of copper block, big enough to attach
a thermocouple to. Clamp the transistor-block asssmbly as normal, it can
be just a couple mm thicker. The block should be at almost the exact
temperature of the transistor metal heat conductor, and the junction will be
just a little hotter than that. This should get you the info you need,
unless I misunderstand the problem.

The soldering between transistor and copper block needs to be a full-area
joint, but tinning both parts and then collapsing the solder while molten
until close contact is made should do it.

Jon

Alternately, run the fet in free air, or even sitting on a bit of
styrafoam or bubble wrap. Dissipate a watt or two and compare the top
and bottom temps. The bottom will be Tj, and the top is, well, Ttop.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[George Herold]

On Thursday, January 22, 2015 at 3:18:56 PM UTC-5, John Larkin wrote:

On Thu, 22 Jan 2015 11:56:15 -0800 (PST), George Herold
gherold@teachspin.com> wrote:

On Thursday, January 22, 2015 at 8:35:11 AM UTC-5, Alain Coste wrote:
Hello
Thank you for your answers.
I can give some more details on my application.
The heatsink is a sort of tunnel filled with hollow fins, and there are two
fans to force air flow through the fins.
(the model is Fisher LAV7 for those who know this manufacturer).
So it is not easy at all to put a thermocouple against the base of the
MOSFET, through the heatsink.
The idea of using the parasitic diode of the junction to measure temperature
is very interesting, but the main problem is calibrating. I suppose that
calibration must be done with THE mosfet of my device, and not another one.
So I should unsolder the mosfet to put it in an oven as other parts of my
device cannot support high temperatures. While not unfeasible, it's a little
tricky...
For upsidedown, the plastic of the TO264 package covers totally the metal
tab, so once mounted on the heatsink, no metal is visible, and here is my
problem ! I can measure (with the IR camera) the temperature of the screw,
but this is closer to Ts than to Tc.

Here are results of my experiments:
The mosfet dissipates 150W
Ts measured with the IR camera = 55°
some black plastic tape on the heat sink gives an emissivity between
0.9 and 1
the camera was calibrated for 0.95
as John said, Ts decreases as the distance from the mosfet increases
55° is measured close to the mosfet
The datasheet gives theta-cs = 0.15 typical and theta-jc = 0.12 max.
I take theta-cs = 0.2 as 0.15 is typical value.
Tc should be 55 + (150 * 0.2) = 85°
Tj should be 85 + (150 * 0.12) = 103°

Now, the IR camera gives 100° as Tt (peak temperature of the top case of the
mosfet)
The emissivity of the black plastic of the mosfet case is supposed to be
close to 0.95

There are two possible interpretations:
- either Tt is close to Tj, and everything is ok
- or Tt is close to Tc (and then theta-cs is around 0.3) : the future
is bleak for my mosfet

I experimented with a reduced power, but I want the mosfet dissipate at
least 200W.
Measures with dissipated power = 100W give coherent results (measured Tt
close to calculated Tj).

The point is the difference between Tt and Tj, and it's difficult to get an
idea on the subject (John vs George...).
The litterature I found says Tt is not less than Tj-5° for power smt
packages (power-pack,...) using the PCB as heatsink.
But may I extrapolate for TO264 on a large heatsink?
Till now Ixys didn't answer my questions. I understand that a parameter as
Psi-jt is more difficult to garantee for a TO267 package as it depends on
the characteristics of the heatsink.

--
Alain Coste



---
L'absence de virus dans ce courrier electronique a ete verifiee par le logiciel antivirus Avast.
http://www.avast.com

Thanks for the update. At ~$25 a piece for the FETs, I would try pushing one and see where it fails. That could give you some measure of comfort.

I measured the diode drop on ~10 IRF820's in my part box and they were all within 1 mV of each other. (Not bad) (Don't touch with hands while measuring.)

George H.

That's impressive, less than 0.5 C.

Yeah the diodes (transistors) I use for temp sensors have ~5-10 mV
of variation at room temp.
Hey, I've got a dewar of LN2 next to me..
I'll dunk some in and see how well they track each other.

later...
George H.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[George Herold]

On Friday, January 23, 2015 at 8:31:21 AM UTC-5, George Herold wrote:

On Thursday, January 22, 2015 at 3:18:56 PM UTC-5, John Larkin wrote:
On Thu, 22 Jan 2015 11:56:15 -0800 (PST), George Herold
gherold@teachspin.com> wrote:

On Thursday, January 22, 2015 at 8:35:11 AM UTC-5, Alain Coste wrote:
snip
I measured the diode drop on ~10 IRF820's in my part box and they were all within 1 mV of each other. (Not bad) (Don't touch with hands while measuring.)

George H.

That's impressive, less than 0.5 C.

Yeah the diodes (transistors) I use for temp sensors have ~5-10 mV
of variation at room temp.
Hey, I've got a dewar of LN2 next to me..
I'll dunk some in and see how well they track each other.

later...
George H.

OK first the numbers are almost too good to believe, but here they are.
(I only tested 4 FET's I had to solder wires on to go down into the dewar. The first one I had clip leads, but the dang thick plastic on the leads froze up, and I was afraid I'd have to break it to get it back out of the dewar neck.)

V V
Room temp. 77K
0.5500 1.0154
0.5504 1.0156
0.5508 1.0157
0.5509 1.0159

Hey, what should I connect the gate to? (S or D or doesn't it matter?)
I left it floating, which was kinda fun 'cause if I scuffed my shoe and touched the gate the channel would conduct.

George H.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[Alain Coste]

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.

> If the hot spot Tt is only 100C at 150W, you should be fine.

If I limit the power to 300W (two mosfets in //) I have enough confidence
that Tj max will not be exceeded.
But when I designed the electronic load I had counted on around 400W.
Well, 300W are not so bad for my needs, but I would like to know more
precisely "how far I can go too far".

It would be fun to somehow remove the epoxy and image the actual chip,
to see its temperature profile.
I removed the epoxy from a bunch of mosfets, but the process was sort
of violent.

https://dl.dropboxusercontent.com/u/53724080/Parts/ExFets.jpg

I think there are organic epoxy removers that might not trash the
silicon.

Now I see what _violent_ means...
Considering the price of my fets, I didn't really want to try it...

We have found the Ixys mosfets to be good for surviving linear-mode
high-dissipation pulses, out in the northeast corner of the SOAR
curve. We blew up a lot of fets to learn that.

Yes, my selection of Ixyx mosfets owes nothing to chance. If you want at the
same time very low theta-jc and guaranteed SOA for DC (and not only for
switch mode) the choice is rather limited. International Rectifier and
Infineon had some fets with guaranteed SOA for DC, but for theta-jc Ixys was
the best. The counterpart is the price, which doesn't encourage to
destructive tests...



---
L'absence de virus dans ce courrier ĂŠlectronique a ĂŠtĂŠ vĂŠrifiĂŠe par le logiciel antivirus Avast.
http://www.avast.com