The STMicroelectronics SM6T150A is a 600 W peak pulse power (10/1000 µs) unidirectional TVS Zener diode from the TRANSIL™ series, housed in a DO-214AA (SMB) surface-mount package. It is designed for general-purpose transient voltage suppression on 128 V nominal rails, with a minimum breakdown voltage of 143 V and a maximum clamping voltage of 265 V at the rated peak pulse current of 15 A (8/20 µs waveform).
The 128 V reverse standoff voltage (working voltage) is the DC or peak-AC voltage the protected line can carry without the diode conducting appreciably. Above that, at 143 V minimum, the device begins to avalanche. The 265 V maximum clamping voltage is the highest voltage the load sees during a surge event — the design must ensure the downstream components (caps, ICs, FETs) have a voltage rating above 265 V, or add a second stage of protection. The 600 W peak pulse rating (10/1000 µs) is the energy-handling capability; for shorter pulses (8/20 µs) the peak current is 15 A, and the device can absorb higher instantaneous power.