This entry is a supplement to the article Low Power Intel® Architecture for Small Form Factor Devices, by Ram Chary, Pat A. Correia, Raviprakash Nagaraj, and James Song. Additional technical information presented here was provided by Scott Noble.
The attached Microsoft PowerPoint* slides, included in the file small_form_factor.zip below, are provided to help answer two questions received from a developer by our support team:
Q. Radiation should be related with the Surface area, why are you saying '300 Cubic centimeters .... Maximum power is about 5W?' I have another question. Restricted in radiation, what is the Upper limit of the mobile device's total power loss. how to calculate?
A. The first question asks how we determined 5W thermal limit for a 300cc device. This is based on a number of assumptions:
1. There is no fan and heat is dissipated by being conducted to the enclosure surface and then radiated out into the ambient air
2. Ambient air is 35°C
3. There is an ergonomic limit to the temperature of the enclosure (for people to hold the device, it cannot exceed 50°C). The challenge for maximum heat dissipation is to get all the surfaces up to 50°C. However, heat spreading is not ideal and there is a thermal gradient across the device. In addition, hot spots from components under the enclosure cover can limit the power dissipation capability by up to 50%.
4. A graph in the presentation shows an ideal situation of over 6W if there was ideal heat spreading to all the surfaces of the device. However, this is not practical, so another graph is shown with more realistic heat spreading (still requires very good enclosure and system design). This graph shows the 4.5W to 5W capability.
We calculate the thermal capability based on thermal radiation by creating a flowtherm model. We calculate total surface area of the enclosure and estimate the total effective heat dissipating surface area. We calculate based on some heat spreading capability and also factor in hot spots.
Some experiments confirmed this.
Added features that can extend the power capability would be the addition of a small fan, or a double walled back cover (as shown in slides) that enables some surfaces to reach higher temperatures without contacting the user and thus allow more heat dissipation.