1500W peak pulse power — sizing the transient clamp
The STMicroelectronics SM15T39CA is a bidirectional Zener TVS diode from the TRANSIL™ family, rated for 1500W (1.5kW) peak pulse power per the 10/1000µs waveform. That 1500W rating is the energy the part can absorb in a single surge event without failing short — it sets the upper bound on the transient your protected rail sees. The clamping voltage maxes out at 69.7V at the peak pulse current of 143A (8/20µs waveform), which tells you the voltage the load will be exposed to during the event. For a 33.3V reverse standoff rail, that 69.7V clamp gives about a 2:1 margin above the working voltage — typical for industrial and automotive bus protection where the transient source impedance is low.
Bidirectional — one device, both polarities
Because the SM15T39CA is a single bidirectional channel, it clamps equally on positive and negative surges. The breakdown voltage starts conducting at 37.1V minimum, so the part stays out of the way until the transient arrives.
No derating needed for cold-crank or under-hood soak-back. The DO-214AB SMC surface-mount package handles the reflow profile and the thermal cycling without issue, as long as the board layout provides adequate copper for heat spreading during repetitive surge events.
For volume production or long-term supply agreements, the base product number SM15T39 covers the full bidirectional and unidirectional voltage variants in the SM15T family, so a second-source qualification across the series is straightforward.