It looks like every chip manufacturer has their own CAN bus TVS product nowadays, e.g. ON Semi NUP2105, Semtech 2492SQ, Bourns T24CAN, Nex PESD2CAN and many others.

What is common for all of them is surprisingly high stand-off and breakdown voltages, typically around 24 and 30V. These seem to be inline with ISO 11898 max bus voltage specification for 24V systems. Way too much for 12V systems.

Furthermore, most transceivers are designed to work of 5V supply and 3.3V devices becoming more common. There is no way these drivers can put up more than 5V on the line, and even if we add generous common mode tolerance it still be nowhere close to TVS specs.

While doing my research I've stumbled upon this interesting board from TI, demonstrating various ESD protection circuits. All of them use SM712 diodes, designed for RS-485. They have much lower and asymmetrical breakdown specs -7..12V, which seems to be ideally suited for CAN.

So, my question would be this: why manufacturers keep churning those high-voltage devices and why devices like SM712 are nowhere to be seen except on those obscure TI boards?

  • \$\begingroup\$ Voltage spikes on automotive systems are quite high and can easily surpass 12V. \$\endgroup\$
    – Lior Bilia
    Oct 7 '19 at 18:46
  • \$\begingroup\$ Automotive systems have nasty, noisy supply lines that can fry your delicate, unprotected electronics. You design for the worst case, real world scenario, not for the lab. \$\endgroup\$
    – Lior Bilia
    Oct 7 '19 at 18:54
  • \$\begingroup\$ Automotive applications at nominal 12V have to deal with steady 15V plus regular spikes from the ignition system. If CAN bus was shorted at, let's say 23V, you got a misfire each few minutes. \$\endgroup\$
    – Janka
    Oct 7 '19 at 19:05
  • \$\begingroup\$ I understand the comments re. automotive applications, however I think they miss one important point - TVS devices are NOT there to suppress noise, nor they can deal with steady overvoltage on the bus. They can clip some voltage spikes, but if these are regular then the bus is pretty much inoperable. Their primary purpose is to deal with huge (kV range) but short discharges, that simply do not happen during normal vehicle operation. Something like connecting trailer system to the vehicle is where they needed. At least that is my understanding, which might be wrong, of course. \$\endgroup\$
    – Maple
    Oct 7 '19 at 23:21
  • 1
    \$\begingroup\$ @LiorBilia TVS value should be picked after the voltage levels on the electronics you wish to save, no after the expected level of the spikes... From the point where the TVS starts to conduct, only the wattage spec matters. \$\endgroup\$
    – Lundin
    Oct 8 '19 at 7:41

These seem to be inline with ISO 11898 max bus voltage specification for 24V systems. Way too much for 12V systems.

According the datasheet of the NUP2105L, the maximum bus voltage for a 12V system can be up to 25V volts.

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  • \$\begingroup\$ That spec is for Honeywell SDS, it should not be taken into account for generic CAN bus designs. Common transceivers, like SN65HVD232 I am using, follow ISO requirements for 16V. \$\endgroup\$
    – Maple
    Oct 7 '19 at 19:23
  • \$\begingroup\$ Furthermore, they have much lower tolerance for negative voltage (-4V in my case), which is why asymmetrical SM712 is so appealing. Although SN65 does have +/- 25V transient rating and 16kV ESD rating. But I'd rather have extra layer of protection in easy to replace TVS diode. Maybe this is overkill and I should rely on transceiver's protection. After all it is there for a reason. \$\endgroup\$
    – Maple
    Oct 7 '19 at 19:36

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