11.3 Power Dissipation and Junction Temperature

The low ON-resistance of the N-channel MOSFET allows small surface-mount packages to pass large currents. It is required design practice to determine power dissipation and junction temperature. The below analysis gives an approximation for calculating junction temperature based on the power dissipation in the package. However, it is important to note that thermal analysis is strongly dependent on additional system level factors. Such factors include air flow, board layout, copper thickness and surface area, and proximity to other devices dissipating power. Good thermal design practice must include all system level factors in addition to individual component analysis.

Begin by determining the r_DS(on) of the N-channel MOSFET relative to the input voltage and operating temperature. As an initial estimate, use the highest operating ambient temperature expected and read r_DS(on) from the typical characteristics graph. Using this value, the power dissipation can be calculated using Equation 7:

Finally, calculate the junction temperature:

Compare the calculated junction temperature with the initial estimate. If they are not within a few degrees, repeat the calculation using the refined r_DS(on) from the previous calculation as the new estimate. Two or three iterations are generally sufficient to achieve the desired result. The final junction temperature is highly dependent on thermal resistance θ_JA, and thermal resistance is highly dependent on the individual package and board layout. The table provides example thermal resistances for specific packages and board layouts.