What the 1.5SMC56CA-Q actually does on your board
For a 48V automotive rail (nominal 12V system scaled up), a 53.2V minimum breakdown sits above the 48V bus but below what a 12V jump-start or load-dump transient would put through — that is the correct TVS sizing for this bus.
AEC-Q101 grade and thermal margin
If you are specifying for a 12V ADAS ECU or body-control module, this temperature ceiling gives you headroom above the 125°C ambient that typically terminates a Grade 1 design. The junction temperature rating is what matters for the clamping performance under a transient — the device self-heats during the event, and 150°C TJ maximum is the derating ceiling that sets your worst-case thermal budget.