What this TRANSIL TVS does in the board
The STMicroelectronics SM6T200A is a 600W unidirectional TRANSIL TVS diode in a DO-214AA SMB package. It clamps transients on a 171V nominal rail — the reverse standoff voltage — and fires at 190V minimum breakdown, holding the clamp to 353V at the 11.3A peak pulse current (8/20µs waveform). That 353V ceiling is what the downstream silicon has to survive; if your DC bus regulator or downstream cap is rated for 400V, this part buys you the margin. Rated for junction temps from -55°C to 150°C, so it handles hot engine bays or outdoor cabinets without derating drama.
Field-swap reality: orientation and soldering
The SMB package has a cathode band — the end with the stripe is the cathode, same as a standard signal diode. On a unidirectional part like this one, the cathode goes toward the protected rail, anode to ground. Get it backwards and it conducts at the standoff voltage, not the breakdown. Hand-solderable with a fine-tip iron if you have steady hands and a magnifier; the SMB body is small but the leads are wide enough to wet. No lab bench needed, just a clean tip and some flux. If you are swapping one on a live board, kill the power first — the 600W rating means it can dump a lot of energy into a short if the board is still hot.