What the clamping specs mean for your rail
The 1.5SMC91AS_R1_00001: The 77.8 V reverse standoff is the working rail the TVS sees in normal operation — the diode stays off and draws only leakage current. That 1.5 kW peak-pulse rating at the standard 10/1000 µs waveform is the 1.5SMC series envelope — it tells you the device survives a single-event surge of that energy without failing short. For a 77.8 V rail the clamping headroom is roughly 47 V — the downstream IC sees 125 V during a surge event, not the full open-circuit spike. The downstream protection rating of whatever sits behind it needs to survive that clamping level as a worst-case transient.
DO-214AB package and thermal integration
The DO-214AB (SMC) is the standard half-watt TVS package at this power class — the cathode bar is the thermal path to the PCB pad. A proper solder footprint with full-metallization under the cathode tab keeps junction temperature within the 150 °C rating under repeated surge events. Without adequate copper area the TJ ceiling hits first, not the peak-pulse energy rating. At high altitude the board runs cooler — the limiting factor stays the clamping energy, not the absolute temperature.