Clamping voltage and the protection window
The SMAJ150CA-13: That 243 V ceiling is what the downstream silicon sees during a surge; if the protected device's abs-max rating sits below that, the TVS absorbs the energy but the load still needs margin above the clamp.
Rated 400 W peak pulse power for the 10/1000 µs exponential waveform, the SMAJ150CA-13 handles the kind of surge you see from inductive load switching or secondary lightning transients on a 150 V bus. The 1.6 A peak pulse current at the clamp voltage confirms the power budget: 243 V × 1.6 A gives 389 W, just under the 400 W ceiling. For shorter pulses (8/20 µs) the peak current capability is higher, but the datasheet's derating curve above 25 °C ambient is the real limiter in a hot enclosure.
DO-214AC SMA — board-fit and thermal path
Housed in the standard DO-214AC (SMA) surface-mount package, the SMAJ150CA-13's thermal resistance to ambient depends heavily on the copper pad area under the cathode tab. A 25 mm² pad on a standard FR4 board drops the junction-to-ambient thermal resistance enough to keep the die below 150 °C during a 400 W pulse. The SMA footprint is shared across the whole SMAJ family, so a single PCB land pattern serves the full voltage range from 5 V to 188 V.