[CP_CALCULATED_FIELDS id=”6″]
The power budget calculator estimates the maximum (or idealized) heat dissipation on a passive device. The calculation is based on the assumption that all surfaces dissipate heat at a uniform temperature.
The heat dissipation in a real device can be considerably lower. The idealized value can be adjusted by a factor (0 – 1).
Example: Determine the power budget for the new iPhone 6+. The product dimensions are, length 77.8 mm, width 7.1 mm, and height 158.1 mm. Assuming an ambient temperature of 25 °C and maximum touch temperature of 40 °C (metal/glass surface). The calculator estimates an ideal budget of 4.3 W. Considering the iPhone 6+ is a well designed product (thermally), a factor of 75% efficiency is applied to give a total power budget of 3.2 W.
These estimations help thermal designers to quickly determine if a product can be passively cooled.
The calculator determines the maximum heat removed by convection and radiation. The device is assumed to be floating in air.
Heat dissipation by radiation:
Heat dissipation by convection:
where:
The Nusselt number is then calculated using empirical correlations.
Top surface:
Bottom surface:
Sides, front and back:
I have similar calculator built in Excel based on the same equations you presented. A couple of suggestions:
1. different surface material has different emissivity. You can add more details here.
2. Show how you calculate characteristic length. This relates to the orientation of the box;
3. Show how you get the air properties based on which temperature.
Hi Yin
1 The calculator assumes all surfaces have the same emissivity. Not true for all cases.
2 Right now the calculator assumes the devices is floating in free air on vertical orientation (the height should be aligned with the gravity vector). Again not true for all cases.
3 Air properties are calculated from the given ambient temperature.
More details can be easily added but the main idea of the calculator is to get a quick estimation of the power dissipation assuming a Cu-block case (all surfaces dissipating heat at a uniform temperature).
Thanks for your valuable comment!
This article is about the thermal design envelope of microprocessors. For the general concept, see power rating .