The STMicroelectronics SM15T200CA is a 1500W bidirectional Zener TVS diode from the TRANSIL™ family, designed to clamp voltage transients on power rails and signal lines. It is a single-channel device in a DO-214AB (SMC) surface-mount package, rated for general-purpose surge protection.
The 1500W peak pulse power (10/1000µs waveform) is the headline spec — it tells you how much transient energy the diode can absorb before failing. For a 171V reverse standoff voltage, the device starts conducting when the line exceeds roughly 190V (breakdown min) and clamps the surge to 353V maximum at 28A peak pulse current. That 353V clamp sets the voltage rating your downstream capacitors and MOSFETs must survive. The 28A figure is for the 8/20µs waveform, a tighter pulse used in IEC 61000-4-5 testing; the 10/1000µs rating is the traditional telecom surge shape.
Bidirectional — one part covers both polarities
With one bidirectional channel, the SM15T200CA clamps positive and negative transients symmetrically. That saves a diode count on AC-coupled lines or bipolar supplies where you would otherwise need two series devices. No power line protection is claimed, so this is a signal- or secondary-rail protector, not a mains-entry suppressor.