This board has been very helpful to me, I am stuck with a small problem again. The datasheet is here.
Part name: SMDJ series.
I have a pulse train coming from an ignition system, the power I calculate is about 4 Watts of Pavg. When I refer to the datasheet above, we can see it can withstand about 6.5W of Pavg. But when I calculate the Tj, it exceeds, I get \$75C\times4 = 300C\$ above ambient. Which spec should I rely on: Pavg as stated by manufacturer, or Tj as calculated?