Calculating the juction temperature of a diode

[ku637]

Hi,

I want to approximately calculate (theoretically) the junction temperature of a diode (part number is 15MQ040NPbF) the datasheet mentions only Rj-a=80 C/W

I don't have a test setup to measure the case temperature in actual ambient environment.My ambient temperature can go upto 85 degrees.

Power dissipation=0.149W

I read some literature and most of them are saying it is not accurate to use the following formula..
Tj = Ta + Rj-a*Pd

But i dont have Rj-c ..& can i assume Ta = Tc ..?Any idea abt how can i proceed?
Is there any generic vaules for Rj-c can be available for SMA package??


Thanks for any help

 

[dick_freebird]

Ta is worthless without an idea of the airflow and
the surface heat transfer relating to it. That mfr
value probably has some fine print stating conditions,
somewhere, but how your situation relates is unknown.

Ta=Tc only at extremely low power, where lead
conduction to the PCB (also at Ta) produces a trivial
temp rise.

 

[crutschow]

If you can measure the DC forward voltage across the diode with a constant current, then you can roughly calculate the temperature from the fact the the diode forward voltage decreases about 2mV/°C for silicon.