Of all the transients on a 12 V vehicle rail, load dump is the one that destroys parts. It happens when the alternator is charging hard and the battery is suddenly disconnected — a corroded terminal, a cable knocked loose, someone removing a battery with the engine running. The alternator field cannot collapse instantly, so its stored energy unloads into the rail as a long, high-energy pulse. This is ISO 7637-2 pulse 5, and it is measured in joules over tens to hundreds of milliseconds, not the microseconds of an ESD or surge event.
Why a normal TVS will not survive it
A data-line TVS like an SMAJ or SMBJ is rated for peak pulse power over a 10/1000 µs waveform — about a millisecond. Load dump can last 400 ms. The energy is hundreds of times larger, and a 400 W or 600 W part simply vaporizes. Load-dump suppression needs a device with a large die and a kilowatt-class rating, which is why these parts come in a big DO-218 package: the package is the heatsink that absorbs the pulse.
Suppressed vs unsuppressed alternators
ISO 7637-2 splits load dump into pulse 5a (unsuppressed alternator — the full event) and pulse 5b (centrally suppressed — the alternator has its own clamp and the residual is lower). If your system guarantees a suppressed alternator, you can design to the milder 5b level. If you cannot guarantee it across every vehicle the module ships into, design for 5a and the full pulse. When in doubt, assume unsuppressed; it is the cheaper mistake.
Picking VRWM for a 12 V system
The standoff voltage (VRWM) has to sit above everything the rail does in normal life so the TVS never conducts during operation: ~14 V while charging, and up to 24 V during a double-battery jump-start. That puts the working voltage in the 24–28 V band for a 12 V system. Go too low and the device runs hot on a normal day; go too high and the clamp lets too much through. These are the standard picks:
| Part | VRWM | VBR (min) | VC (max) | PPK | IPP | Package |
|---|---|---|---|---|---|---|
| SM8S22(C)A | 22 V | 24.4 V | 35.5 V | 6600 W | 186 A | DO-218 |
| SM8S24(C)A | 24 V | 26.7 V | 38.9 V | 6600 W | 170 A | DO-218 |
| SM8S26(C)A | 26 V | 28.9 V | 42.1 V | 6600 W | 157 A | DO-218 |
| SM8S28(C)A | 28 V | 31.1 V | 45.4 V | 6600 W | 145 A | DO-218 |
| SM8T24A | 24 V | 26.7 V | 38.9 V | 8000 W | 205 A | DO-218 |
Check the clamp against what is downstream
Selecting the standoff voltage is only half the job — the clamping voltage (VC) is what your circuit actually sees during the event. An SM8S24A clamps at 38.9 V. Your reverse-polarity MOSFET, the DC/DC converter input, and any bulk capacitor on that node all have to survive that 39 V for the duration of the pulse. If your front-end DC/DC is only rated to 40 V, you are riding the edge; step up to a part rated 45 V or higher and re-check against the next TVS up the table. This back-check is where most load-dump designs actually get decided.
Where it goes
Place the load-dump TVS right at the module power input, after the reverse-polarity protection and before the DC/DC converter, with a short, wide path to chassis ground so the package can dump the pulse. It is the first line of defense on the rail; everything behind it is protected by its clamp.
Load-dump suppressors are an automotive part by definition — confirm the AEC-Q101 status you need for your program with your Magnias FAE, who can also point you at the bidirectional (C) options where the rail can swing negative.