I am looking at the Littlefuse 5KP series. It says on the datasheet:
I am trying to understand how they arrive at \$P_D\$ = 8.0W.
Given thermal resistances of 40°C/W or 8.0°C/W, and 100°C temperature difference, should \$P_D\$ not be either 2.5W or 12.5W?
Oddly enough for the Littlefuse SMBJ series, they use the thermal resistance junction to lead to calculate \$P_D\$. (The equivalent of 12.5W above.)
Where does the discrepancy come from? Is it due to the difference of leaded vs SMD components? Inconsistency in the datasheet? Or am I misunderstanding something?
Also: What is the point of specifying \$P_D\$ "on an infinite heat sink" for a round component with leads? Where is the heat sink supposed to go? Why is this seemingly purely theoretical value useful at all?