What a 1.5 kW unidirectional TVS diode does on a 12 V rail
The 1.5SMCJ12CA is a unidirectional Zener-based TVS diode — it sits invisible on a 12 V rail and turns into a low-impedance clamp the instant a transient exceeds the 13.3 V minimum breakdown threshold, shunting the surge energy away from whatever is behind it. The 1500 W peak pulse rating describes the energy it can absorb during a single 10/1000 µs waveform — a 1 ms duration spike, not a sustained overload — so the spec maps directly to the surge immunity class of the downstream load.
Clamping voltage and the 75.4 A peak pulse rating
The 7.9 V difference between the 12 V standoff and the 19.9 V clamping ceiling is the protection margin; a downstream IC with an absolute-maximum input of 20 V is getting close enough that you run the margin calculation before committing the BOM line.
Thermal and environmental context
Surface-mount on a DO-214AB (SMC) footprint — the cathode bar on the package face is the identification for the biased-on direction; the unmarked face is the anode.