thermal resistance _ junction to ambient

[mariuszoll]

Hi,

Could you help me please with the formula of calculating the thermal resistance for junction to ambient?
I have to do some calculations for power LEDs, and unfortunately the datasheet provides me just the thermal resistance junction to solder point.

Thank you in advance.

 

[erikl]

Both thermal resistances are in series, i.e. have to be added.

 

[schmitt trigger]

Rja = Rjs + Rsh + Rha

Rja: junction to ambient, total resistance
Rjs: junction to solder point, the value that you already know
Rsh: solder point to heatsink, estimated from typical values
Rha: heatsink to ambient, you have to select a heatsink to meet your requirements

those requirements are:

Tj(max) > (Pled * Rja) + Tamb(max)

 

[crutschow]

You select a heatsink as determined from the power dissipated by the LED, the highest ambient temperature, and the maximum LED junction temperature (Tj) you want (should be below the maximum allowed for good reliability). From this, and the thermal impedance calculation given by schmitt trigger above, you determine the thermal resistance to air you need and select the appropriate heat sink.