Question 19.5: A power FET operates with a maximum continuous drain current...
A power FET operates with a maximum continuous drain current ( I_D ) of 0.5 A at a maximum continuous drain-source voltage ( V_{DS} ) of 40 V. Determine the minimum acceptable thermal resistance rating for a heatsink for use with the device under the following conditions:
| Maximum junction temperature: | 175°C |
| Thermal resistance of case to surface: | 1.35°C/W |
| Thermal resistance of washer and mounting kit: | 1.45°C/W |
| Maximum ambient temperature: | 55°C |
First we need to determine the maximum total power dissipation. Since the device is a FET, the gate dissipation can be neglected. Thus the total power dissipation will be:
P_{Tmax} = V_{DS} × I_D = 40 × 0.5 = 20WApplying equation (7) gives:
\theta _{SA}=\frac{T_{Jmax}-T_{Amax}}{P_{Tmax}} -(\theta _{JC}+ \theta _{CS}) \\ \theta _{SA}=\frac{175-55}{20} -(1.35+1.45)=6-2.8=3.2^\circ C/WAll of the previous calculations assume that the heatsink is subject to natural convection. Increasing the airflow across the surface of the heatsink can dramatically increase the amount of heat radiated and consequently it will also significantly reduce the effective thermal resistance of the heatsink.