[SOLVED] Junction Temperature calculation of LM317 using surface temperature rise

[knikhil271083]

Hi,
I am using LM317MQDCYR from TI as a current limiter in my circuit. I want to estimate the Junction temperature of LM317 when it is limiting the current. I have measured the top surface temperature rise above ambient of LM317. I have Θja from the datasheet of LM317 and maximum power being dissipated in LM317.
I can use the formula Pd = (Tj - Tamb) / Θja to theoretically estimate the junction temperature, but how can I estimate the junction temperature practically if I have the top surface temperature rise figure available.

Any pointers to the solution would be appreciated.

Regards
Nikhil

 

[chuckey]

Rj-amb = 50 deg C/W, so P watts, Tj = 50 X P + Tamb (not Tc). ( https://pdf1.alldatasheet.com/datasheet-pdf/view/22768/STMICROELECTRONICS/LM317MDT.html)
Frank

 

[dick_freebird]

The top surface is a poor place to measure, it has the
most convection-variable temperature. The base or
tab is better by far, especially on a plastic package.
If you get a case measurement then you can use the
thetaJC number without as much uncertainty.

 

[knikhil271083]

The thermal metric Psi-jt (Junction to top of Package) has been adopted by the industry (EIA/JESD 51-2) which helps to estimate the junction temperature of component based on the surface temperature and power dissipated.
The actual junction temperature can be very easily estimated using the equation;
Tjun = Tcase + (Psi-jt x Power) where Psi-jt is in oC/W and can be found in datasheets or from the IC manufacturer.

Regards
Nikhil

 

[FvM]

Means you are using datasheet θjc value as ψjt estimation?

 

[knikhil271083]

Means you are using datasheet θjc value as ψjt estimation?

No. The value of ψjt was not given in the datasheet. I found it on the TI E2E community forum. Somebody had already asked for the details and it was posted on TI forum.
As I said earlier, if its not given in datasheet, contact the manufacturer for the values.

 

[FvM]

Just out of curiosity, at which value did the arrive?

 

[knikhil271083]

Refer below link:
http://e2e.ti.com/support/power_management/linear_regulators/f/321/p/241901/847118.aspx

Psi JT (standard datasheet value) = 1.7 oC/W