What the 1500W rating means for your board
The 1.5SMC550: That 1500W is the peak power the device can absorb in a single transient event — not continuous dissipation. For a given clamping voltage of 798V, the peak pulse current is 2A, so the energy handling is set by the pulse duration and the junction temperature. The reverse standoff voltage is 495V, meaning the diode will not conduct appreciably below that rail voltage. Breakdown occurs at a minimum of 495V, clamping the transient to 798V maximum. This suits it for protecting 480VAC or 400VDC bus lines from lightning-induced surges or inductive load dumps.
Littelfuse lists the 1.5SMC550 as obsolete. If a board redesign is feasible, a parametric search for a 495V standoff, unidirectional TVS in the same package may yield an active alternative, but no direct cross-reference is maintained by Littelfuse.
The 1.5SMC550 is supplied in Tape & Reel (TR) packaging, surface-mount in a DO-214AB (SMC) case. This is a standard two-terminal SOD-123FL-style body with a JEDEC footprint — reflow profile follows typical lead-free solder profiles for the SMC package. No power line protection is claimed, meaning this diode is intended for signal or DC bus transient suppression, not for AC mains primary protection. The single unidirectional channel clamps only one polarity — for bidirectional protection on AC lines, two devices or a bidirectional TVS would be needed.