The STMicroelectronics SMCJ28A-TR is a unidirectional TRANSIL™ TVS diode rated for 1500 W peak pulse power, with a 28 V reverse standoff voltage and a 31.1 V minimum breakdown.
Breakdown and clamping — where the protection window sits
The 28 V reverse standoff is the maximum DC or peak-AC voltage the diode can tolerate without conducting appreciably. Above that, the device breaks down at 31.1 V minimum and clamps the surge to 59 V at the rated current — that 59 V ceiling is what the downstream circuitry must survive.
The SMC (DO-214AB) package is a standard surface-mount outline with a JEDEC-registered footprint. The copper pad area under the package on the PCB sets the thermal impedance.