1500W pulse clamp for 40V rails
The STMicroelectronics SMCJ40A-TR is a 1500W unidirectional TVS diode from the TRANSIL series, designed to clamp transients on 40V nominal DC buses. Its 44.4V minimum breakdown voltage ensures it stays out of the way during normal operation, while the 84V maximum clamping voltage at 119A peak pulse current (8/20µs waveform) limits the energy let-through to downstream silicon.
Clamping voltage and peak current — the protection margin
The 84V clamp at 119A is the number that matters for the downstream circuit's survival. If the protected device's absolute maximum rating is, say, 80V, this TVS leaves only 4V of headroom — marginal for a repetitive event, so check the waveform energy against the device's avalanche capability. The 1500W peak pulse power rating (1ms, 10/1000µs) is the traditional metric; the 119A figure at 8/20µs is the more realistic lightning-surge waveform that field events follow.
Standoff voltage and breakdown threshold
At 40V reverse standoff, this part sits on a 48V nominal telecom bus or a 36V automotive rail with margin. The 44.4V minimum breakdown means it starts conducting around 45V, so transients that ride on a 40V rail with 5V ripple won't trigger it falsely. That matters for capacitive loads or hot-plug events where the bus rings up — a premature clamp can latch the supply.
Package and footprint — DO-214AB SMC
The DO-214AB SMC package is a standard JEDEC outline for 1500W-class TVS diodes. The PCB footprint is well-established, with the cathode band marking the unidirectional orientation. The package body is rated for 150°C junction temperature, so it can sit next to hot power components without derating — but the solder joint temperature during reflow is the limit, not the die.