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.
estimating junction temperature of a power mosfet
Started by ●January 21, 2015
Reply by ●January 22, 20152015-01-22
Reply by ●January 22, 20152015-01-22
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
Reply by ●January 22, 20152015-01-22
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 mosfetIt 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 mosfetIf 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
Reply by ●January 22, 20152015-01-22
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.comThanks 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.
Reply by ●January 22, 20152015-01-22
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
Reply by ●January 22, 20152015-01-22
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
Reply by ●January 22, 20152015-01-22
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. > >JonAlternately, 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
Reply by ●January 23, 20152015-01-23
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 électronique a été vérifiée par le logiciel antivirus Avast. http://www.avast.com
Reply by ●January 23, 20152015-01-23
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
Reply by ●January 23, 20152015-01-23
On Fri, 23 Jan 2015 13:31:20 +0100, "Alain Coste" <coste@irit.fr> 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. > >> 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... >Given that heat sinks and huge mosfets are expensive, and opamps and resistors are cheap, it often makes sense to use a lot of small fets, to spread out the heat. And dump as much of the power as possible into resistors. A switchmode load box would be interesting. Fets switch and dump the power into a big resistor. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
