Protection window and derating
The 1.5SMBJ40A-H: The reverse standoff voltage is 40V — the 40V rail sits safely below this, so the diode does not leak appreciably during normal operation. Breakdown occurs between 44.4V min and the clamp voltage of 64.5V max at the rated 23.3A peak pulse current. That 20V window between standoff and clamp is the margin the protected circuit must survive. Peak pulse power is specified at 1500W for a 10/1000µs exponential waveform. The junction temperature rating of 150°C means power derating applies above 25°C ambient — the datasheet derating curve shows the permissible power at elevated temperatures, which matters for sustained high-temperature environments like an engine bay.
Package and board integration
The DO-214AA SMB package is a standard surface-mount footprint compatible with reflow soldering. No power line protection feature is claimed — this is a signal or low-power rail protector, not a mains transient suppressor.