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I want to add ESD protection on my circuits that interface with vehicle 12V. My understanding is vehicle battery can reach up to 15V. Looking at datasheet below, I can either pick 14.2V or 25V for min Vbr. I prefer this device because it supports multiple channels. Give the product intended for automotive, I assume the diodes should be compatible with vehicle voltages. What min breakdown voltage should I choose if my circuits can handle only up to 20V? (Let's just ignore the power of TVS for now). Thanks.

Datasheet: https://www.st.com/content/ccc/resource/technical/document/datasheet/6c/ab/7a/c8/ed/5a/4e/94/DM00231412.pdf/files/DM00231412.pdf/jcr:content/translations/en.DM00231412.pdf

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  • \$\begingroup\$ Are you looking to use this on your circuit's power supply input, on control signal inputs, or both? \$\endgroup\$
    – brhans
    Nov 1, 2021 at 21:40
  • \$\begingroup\$ @brhans, yes for both. Thanks. \$\endgroup\$
    – jsmith
    Nov 1, 2021 at 22:52
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    \$\begingroup\$ You might want to consider the ‘load dump’ condition. \$\endgroup\$
    – Kartman
    Nov 2, 2021 at 10:37
  • \$\begingroup\$ Kartman has pointed out the issue I suspected you've not considered. A little TVS will not protect your circuit from a 'load dump' condition where your vehicle's nominal 12V rail can briefly rise well above its normal voltage. Wikipedia says that you could expect as high as 40V for as long as 400ms. With a 14.2V or even 25V TVS trying to clamp that, the current from the alternator through your TVS will be limited only be the wiring and will likely turn your TVS into a smoking cinder (followed shortly by the rest of your circuit). \$\endgroup\$
    – brhans
    Nov 2, 2021 at 13:43

1 Answer 1

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These ESD diodes are meant for data lines, not supply lines. For automotive applications, supply lines and data lines are not subject to the same standards. ISO7637-2 for supply lines and ISO 7637-3 for data lines. In addition, you must take care of reverse battery, jump start and load dump.

I recommend reading this AN where they explained all the automotive transients (ESD and longer surges for both data lines and power rails) and how protection devices work: https://www.st.com/resource/en/application_note/an2689-protection-of-automotive-electronics-from-electrical-hazards-guidelines-for-design-and-component-selection-stmicroelectronics.pdf

To start somewhere, I can say that for most of the cases (but not all, it really depends on your application constraints, ICs vulnerability etc…) a 600 W TVS like SM6TY or SMA6TY can do the job for supply rails. Most of the time, VBR between 33V and 42V are adapted when a central load is implemented in the car. Maybe 36V VBR is the most popular. You have to place the protection as close as possible to the input supply connector. 2 options here:

  1. You select a unidirectional TVS like SMA6T36AY so in that case you place a diode for reverse polarity blocking between the connector and the unidirectional TVS.
  2. You select a bidirectional TVS like SMA6T36CAY so in that case you place this TVS just after the power input connector and then a diode for reverse polarity blocking. For less than 1A, you may look into STTH102AY. Here again if you need more than 1A you will need to select another current rating. It is just an example to give a starting point.

Then for datalines it depends on the signal frequency and voltage range. If it is for CAN bus or LIN bus it is quite well-defined and specific protection devices exist. If it is for other kind of signals, then we need more details.

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