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:
The Nusselt number is then calculated using empirical correlations.
Sides, front and back: