Perhaps this site will be of some value...
http://www.engineersedge.com/heat_transfer/heat_transfer_table_content.htm


----- Original Message ----- 
From: "Moreira, Luis A" <Luis.Moreira@ccfe.ac.uk>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Tuesday, February 23, 2010 7:20 AM
Subject: [EE] Heat Dissipation calculations


> 
> 
> Hi All,
> I have an existing assembly with two power diodes in series mounted on
> an aluminium heat sink, I am increasing the power dissipated on them and
> I am trying to do the calculations, to make sure that the diodes will
> still be fine using the current setup. 
> I started by modelling the assembly:
> 
> I will have two branches in parallel of Rjunction-case in series with
> Rcase-heatsink and they will be in series with Rheatsink-ambient.
> 
> I am just having some difficulty looking at it and need someone to check
> if I am thinking correctly. Both diodes will dissipate, in this case,
> the same power. The heat will flow from both to the heat sink and then
> to ambient. 
> 
> Hence if I use the formula
> 
>                  Tj -(Tcase + Theatsink + Tambient) = Q*(
> Rjunction-case + Rcase-heatsink +
> Rheatsink-ambient)
> 
> Where Q is the sum of power dissipated on both diodes
> 
> 
> In this case as the diodes are identical is it correct to say that the
> Tj you get out of this calculation is the junction temperature for both
> the diodes which is also the Tj each of them?
> 
> How would you do it for two diodes dissipating different power? It seems
> to me that it would not matter, but at the same time I can not be sure,
> especially if this power is pulsed instead of continuous.
> 
> Any thoughts welcomed.
> 
> 
> Best Regards
>             Luis
> 
> 
> 
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