• [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 convection:

where:

The Nusselt number is then calculated using empirical correlations.
Top surface:

Bottom surface:

Sides, front and back:

(2 votes, average: 4.50 out of 5)

3 Responsesso far.

1. Yin Xiong says:

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.