BTX System Thermal Considerations
Thermal and Mechanical Design Guidelines 109
Appendix F BTX System Thermal
Considerations
There are anticipated system operating conditions in which the processor power may
be low but other system component powers may be high. If the only Fan Speed
Control (FSC) circuit input for the Thermal Module Assembly (TMA) fan is from the
processor diode then the fan speed and system airflow is likely to be too low in this
operating state. Therefore, it is recommended that a second FSC circuit input be
acquired from an ambient temperature monitor location within the system.
The location of the System Monitor thermal sensor is best determined through
extensive system-level numerical thermal modeling or prototype thermal testing. In
either case, the temperature of critical components or the air temperature near critical
components should be assessed for a range of system external temperatures,
component powers, and fan speed operating conditions. The temperature at the
selected location for the System Monitor Point should be well correlated to the
temperatures at or near critical components. For instance, it may be useful to monitor
the temperature near the PSU airflow inlet, near the graphics add-in card, or near
memory.
The final system integrator is typically responsible for ensuring compliance with the
component temperature specifications at all operating conditions and, therefore,
should be responsible for specifying the System Monitor thermal sensor location.
However, it is not always possible for a board supplier – especially a channel board
supplier – to know the system into which a board will be installed. It is, therefore,
important for BTX board suppliers to select a System Monitor thermal sensor location
that will function properly in most systems.
A BTX system should be designed such that the TMA exhaust is the primary airflow
stream that cools the rest of the system. The airflow passes through the chipset
heatsink and its temperature will rise as the memory controller chipset power
increases. Since chipset power will increase when other subsystems (e.g. memory,
graphics) are active, a System Monitor thermal sensor located in the exhaust airflow
from the chipset heatsink is a reasonable location.
It is likely that a thermal sensor that is not mounted above the board and in the
chipset exhaust airflow will reflect board temperature and not ambient temperature. It
is therefore recommended that the Thermal sensor be elevated above the board.
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