What the 1500W pulse rating means on a 128V rail
The 1.5SMC150A-E3/9AT: Its peak pulse power of 1500W (1.5kW) at 10/1000µs waveform means it can absorb a 7.2A pulse before the clamping voltage reaches 207V — the point where the diode starts conducting hard to shunt the surge away from downstream silicon. The 128V reverse standoff voltage is the DC or peak-AC voltage the part can sit across without conducting in normal operation. When a transient exceeds the minimum breakdown voltage of 143V, the device transitions into avalanche and clamps the spike to 207V maximum. That 207V ceiling is the number your downstream MOSFET or controller needs to survive — if its abs-max rating is below that, this TVS alone won't protect it.
Package and board-fit for the DO-214AB footprint
Housed in a DO-214AB (SMC) surface-mount package, also referred to as the SMCJ footprint. The base product number is 1.5SMC150, so the -/9AT suffix identifies the lead-free plating and the reel quantity variant.
Active production and sourcing posture
No power-line protection rating (the spec sheet notes 'Power Line Protection: No'), so this is a signal-rail or low-voltage bus TVS, not a mains-side protector. For a 128V DC bus or a 100V AC rectified rail, it fits; for 240V mains, you need a higher standoff part.