[eagle] SDX

Dick Jansson-rr rjansson at cfl.rr.com
Tue Feb 20 10:31:36 PST 2007


Some time ago, Bob McGwier and I discussed the SDX thermal issue and pretty
well decided that this module would need to be one of our heat sink module
designs, either E05 25 or if you need it larger the E05 15 module. These
will be specially treated in their mounting in the spacecraft and not be
thermally isolated from the spaceframe as the other, non-power modules are
handled. In this module the power devices are clamped directly to the heat
sink using a space-rated thermal compound at their interfaces. The
interfacing of devices such as the PowerPAD from TI, will need some study of
the papers that you have referenced. That will not obviate the treatment of
the module as a power device, however. I shall be back to you on this. In
the meantime, please take the time to study the drawings on these modules so
that you become familiar with this design.

Dick Jansson, KD1K
(ex: WD4FAB)
kd1k at amsat.org 

-----Original Message-----
From: Howard Long [mailto:eagle at howardlong.com] 
Sent: Tuesday, 20 February, 2007 1117
To: 'Dick Jansson-rr'; 'AMSAT Eagle '
Subject: RE: [eagle] URx

Hi Dick, John,
> Device       Dissipation     Delivered 
> U16           0.340             0.850
> U17           0.411
> U18           0.384             1.200
> U19           0.108             0.090
> U20           0.040             0.224
>                 ------------        ------------
> Totals       1.283W          2.364W
> For a grand total power dissipation of the power system of 3.647W.

I am particularly interested in this as the DSP device on the SDX
(TMS320C6726) typically dissipates 857mW at its full 250MHz speed. It is a
144 pin LQFP PowerPad device and has a metal plate directly underneath the
die that is designed to be soldered directly to the PCB ground plane.

In order to conduct heat away, TI recommend the optimum to be 100 0.33mm
vias in a 10x10 matrix.

See http://www.g6lvb.com/dspthermal.jpg for the design I developed for the
prototype I use for my software development.

We will almost certainly need to conduct heat away from the device on the
underside, so that would suggest to me a direct metallic interface between
the board and the module housing, either milled out of the chassis itself or
with an interfacing block. IT should be noted that for electrical reasons
there are also 44 decupling capacitors on the bottom of the PCB that need to
be considered when providing the thermal interface.

We also would need some space rated goo to ensure good thermal conductivity
between the PCB and the heatsink - I am sure someone will know what this is!

One final thing - 3.647W seems very high for the receiver function. Is this
based on real figures or device spec sheets, and if the latter are these
worst case or typical values?

73, Howard G6LVB

PowerPADT Thermally Enhanced Package

PowerPADT Layout Guidelines http://focus.ti.com/lit/an/sloa120/sloa120.pdf

PowerPADT Made Easy http://www-s.ti.com/sc/psheets/slma004b/slma004b.pdf

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