170V standoff — the rail voltage it protects
The SMAJ170A-TR is a unidirectional TRANSIL™ TVS diode from STMicroelectronics, part of the SMAJ series. With a minimum breakdown voltage of 189V and a clamping voltage of 353V at the peak pulse current of 6.5A (8/20µs waveform), the part absorbs a 400W peak pulse (10/1000µs). The 353V clamp is the ceiling the downstream silicon sees during a surge — if your DC bus or signal line rides at 170V nominal, this part keeps the transient below 353V.
Package and temperature — board-fit realities
Housed in a DO-214AC (SMA) package, it is a surface-mount device with a standard footprint shared across the SMAJ family. The SMA package's thermal pad area on the PCB directly sets the junction-to-ambient thermal resistance; a 1 oz copper pour under the cathode tab helps sink the 400W pulse without exceeding the 150°C limit.