The STMicroelectronics SM6T220A is a 600W unidirectional TVS diode from the TRANSIL™ family, built on Zener technology. It is designed to clamp transients on a 188V continuous DC rail — the reverse standoff voltage — and responds to overvoltage events by breaking down at a minimum of 209V, limiting the clamped voltage to 388V at the peak pulse current of 10.3A (8/20µs waveform).
The base product number SM6T220 covers the full family; the 'A' suffix denotes a tighter breakdown tolerance variant.
Package and mounting — standard SMB footprint
The DO-214AA (SMB) package is a surface-mount, two-lead form factor with a standard land pattern.
No power line protection — use it on signal or DC bus
The SM6T220A is explicitly marked as having no power line protection. That means it is intended for data lines, low-voltage DC buses, or auxiliary rails — not for direct AC mains clamping. The 188V standoff is high enough for a 150V DC bus or a 120VAC rectified rail, but the 600W pulse rating and the unidirectional construction point to a single-polarity transient clamp on a DC supply or signal line.