The U8S-EXT (the 12V input version of the U8S) has an on-board 100W DC-DC converter to convert the 12V input to the 5.2V which the U8S requires for the downstream USB ports.
No DC-DC converter is 100% efficient therefore, as a result of the conversion, it generates some amount of heat.
The U8S-EXT may, in some instances, require forced cooling across its DC-DC converter in order to keep it within acceptable temperature limits. Without this forced cooling, the U8S-EXT firmware may sense a temperature level which is deemed too high. In this instance, the U8S-EXT would power-down the USB ports to reduce the load on the DC-DC converter and bring the temperature down to a safe level.
The temperature can be monitored via our API, LiveViewer App and also the command line interface. We have a shut off temperature of 65deg C.
We recommend a 40mm fan with a 10mm depth, these are readily available, low cost (around $2) and provide enough cooling when placed above or beside the DC-DC converter. Very little airflow is required to keep the DC-DC converter within limits, but fan position should be evaluated under your maximum operating ambient temperature to ensure that the U8S-EXT temperature (as read within LiveViewer) is maintained below 60deg C.
Please contact Cambrionix for advice about thermal considerations within your design and we will be happy to help.
Take a look at the PDFs below which show where you can attach a small 5V fan to a U8S-EXT along with where the airflow is required and how to monitor temperature during your product development phase.