Heat sink maximum temperature for RF Power amplifer.

hi

I am using MRFMRFE6S9125N GSM and MRF8S18120HR3 DCS amplifier at 70watts continuous output power. I have issue regarding heat dissipation. I have a heat sink with fan. I want to know how much cooling is required ? What should be the maximum heat sink temperature (50c, 60c,70c or more) for safe operation of amplifiers ?

Attached document contains calculation of MRF8S18120HR3 at 120 watts. Kindly correct it if there is any mistake.

Heasink temperature is not the concern.Instead, junction temperature should be considered.
By doing a backward calculation, you can find the junction temperature if you know thermal resistances of Heatsink to Package (including insulator,mounting plastics,other mechanical parts etc),Package to Die,Die to Junction.
The junction temperature shouldn't exceed the permissible temperature range that is most important.If you use heatsink fan, you have to consider Heatsink thermal resistance to packin different cage with consideration of this fan.It may be very variable but manufacturers generally give these datas in different cooling conditions.
Don't forget to pur a margin to have guaranteed working environment.



I have forgotten..
Die to junction thermal resistance is not very clearly defined because of junction position,dimensions of the die,used materials and metal layers etc.
Therefore Die to Junction thermal resistance can be simulated in a thermal simulator like Ansys and this resistance value is dedicated to known componenets. I mean Die to Junction thermal resistence shows a wide variance between devices..

From the dat sheet :- http://html.alldatasheet.com/html-pd...S18120HR3.html , the maximum case temperature is 150 degs C. - says it all, make an allowance for the max ambient temperature and for dust build up on heat sink and uncertainties in heat sinking compound/fan performance. . .
Frank