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In this marketing material by Littlefuse they explain how to use e.g. their TPSMA6L TVS diode for automotive load dump protection: TVS Diodes can provide secondary transient voltage protection

Question 1: What makes e.g. the TPSMA6L20A more suitable to this application than any standard SMBJ20A? I cannot find any appreciable differences in the datasheets.

Question 2: More generally, is a TPSMA6L20A / SMBJ20A sufficient for protecting automotive circuits? Littlefuse speaks of TVS as "secondary protection", but they show no primary protection in their example circuits. What do they regard as the "primary protection", the Zener diodes in the alternator?

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Question 1: What makes e.g. the TPSMA6L20A more suitable to this application than any standard SMBJ20A? I cannot find any appreciable differences in the datasheets.

Immediately the main difference is one is SMA and the other is SMB. This indicates a different package size as well as power handling. However, the TP variant is the high reliability variant from Littlefuse

The difference here is this part is AEC Q101 qualified

If a component comes with AEC Q101 (or Q100, Q200 for IC and passives) it means certain aspects of the manufacture have a proven quality control plan. Traceability, defect analysis, guaranteed availability for a significant time as well as notice to distributors of any changes

What you have identified here is the "standard" variant and the "AEC Q101" variant. From a datasheet perspective they should be identical, from a good received the AEC Q101 comes with more guarantees

This is important for the Automotive, aerospace, marine etc industries, but less so for the hobbyist or consumer designers

Whether this part is suitable for your needs... that is a different question

Question 2: More generally, is a TPSMA6L20A / SMBJ20A sufficient for protecting automotive circuits? Littlefuse speaks of TVS as "secondary protection", but they show no primary protection in their example circuits. What do they regard as the "primary protection", the Zener diodes in the alternator?

The primary means of dealing with bus surges is done at the alternator. This can be classed as the primary protection. This is never enough and there for local per-load protection is needed. This is what is referred to as "secondary protection"

Currently, most of the alternators have zener diodes to protect against load dump surges; however, these are still not sufficient. During the powering or switching of inductive loads, the battery is disconnected, so that unwanted spikes or transients are generated.

https://www.littelfuse.com/~/media/electronics/application_notes/littelfuse_tvs_diode_automotive_circuit_protection_using_automotive_tvs_diodes_application_note.pdf.pdf

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  • \$\begingroup\$ Thanks, I appreciate you taking the time to explain the certification issue. \$\endgroup\$ – ARF Jan 12 at 18:07
  • \$\begingroup\$ This isn't entirely accurate. There are auto qualified SMBJ series parts from other manufacturers. \$\endgroup\$ – Matt Young Jan 12 at 18:58
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    \$\begingroup\$ So? This is the littlefuse AEC product line. Never said they were the only ones existing \$\endgroup\$ – JonRB Jan 12 at 19:25
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Question 1: What makes e.g. the TPSMA6L20A more suitable to this application than any standard SMBJ20A? I cannot find any appreciable differences in the datasheets.

Looking through the data, internally it is probably the same part. But they are not the same part. There are differences in the package and in ratings. Continuous power is different. 3W compared to 5W. This may be because of the differences in the packaging. DO221-AC / DO-214AA

The packages are different. The peak surge current is 60A compared to 100A.

As I said, the guts might be the same. Externally it is different, with different ratings, and was qualified with different test criteria. The standards met.

Question 2: More generally, is a TPSMA6L20A / SMBJ20A sufficient for protecting automotive circuits? Littlefuse speaks of TVS as "secondary protection", but they show no primary protection in their example circuits. What do they regard as the "primary protection", the Zener diodes in the alternator?

The part is intended to do local protection only. It is not intended to control the rest of the electrical system from transients or other excessive voltage issues. What they mean by primary protection is, "Its up to you to make sure there is protection".

For primary protection you need a large clamp that can clamp the output of the alternator until the regulation circuit can compensate for the change in the demand that was occurring. The clamp might be a part of the alternator's regulator. How it is accomplished is up to the manufacture.

These devices are clamping devices, zener diodes. And a zener diode needs to have some series resistance to control the voltage across the zener. (inductance in the case of short duration transients)

There needs to be something in series to handle the excess voltage for a small device like this.

One product I designed was a speed controller for electric motor powered sand&salt spreaders used for sanding roadways/parking lots. A conveyor motor (55A continuous) and a spreader motor (35A continuous). Mosfet controlled, surge current (turn on) for the conveyor motor was limited to 150A. I couldn't put any series resistance inline with the motors. I ended up using big TVS devices at a higher voltage. Clamped to a level the mosfets could tolerate. The funny thing was that the motors in our product were causing some of the worst transients on the electrical system.

The controller electronics (other than the motor power circuits) only needed 100mA so that was easier to design protection for.

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