The 1.5SMC91CA M6G is a bidirectional Zener-based TVS (Transient Voltage Suppressor) diode in the DO-214AB (SMC) package — designed to clamp voltage spikes on a protected rail before they damage downstream circuitry. The 1500W peak pulse rating (10/1000 µs waveform) is the figure that sets this part apart from lower-power TVS families: it survives a single-event transient delivering 12.6A through the device without degradation, clamping the voltage to 125V maximum at that current.
Clamping voltage and standoff — the two specs that decide fit
The reverse standoff voltage of 77.8V typ means the TVS sits inactive during normal operation, drawing only leakage current. When a transient pushes the rail above the 86.5V minimum breakdown, the device avalanche-clamps and conducts the surge current to ground, holding the clamping voltage at 125V maximum. For a 77.8V rail, the 1.5× ratio between standoff and clamping ceiling gives a reasonable margin above the normal operating voltage — the protected node sees at most 125V during a 10/1000 µs event, well below what most downstream ICs can tolerate. The single bidirectional channel means this part protects a single polarity — both directions are symmetric. If you need to protect two independent rails, you need two devices.