estimating junction temperature of a power mosfet

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

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

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&#4294967295;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&#4294967295;....
>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

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.

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

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.

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

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