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Application: We have a PCBA inside a vehicle's embedded system. The PCBA receives DC voltages from a power input connector. We do have access to signal ground aka 5V_return at the power connector and we have a mounting hole nearby that will be chassis ground. Note chassis ground and signal ground will be isolated on our PCBA, but the may be connected in some manner within the larger system. (Outside the scope of our design)

Plan: Place ESD (Electro-Static Discharge) suppressor (TVS) diodes near the power input connector to protect the voltage rail inputs from sending ESD strikes "downstream" to the various circuits/ICs on-board.

enter image description here

Questions:

  1. Is it better to connect the ESD diodes to chassis ground or signal ground?
  2. Should we use unidirectional or bidirectional diodes?
  3. Should a more complicated array of diodes be used for better protection? i.e. a diode from +5V to chassis and a diode from GND to chassis?
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  • \$\begingroup\$ Unwanted energy goes to chassis ground (which should also be safety ground) rather than adding noise to the signal path. \$\endgroup\$ Oct 27 '20 at 15:36
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    \$\begingroup\$ I would say too an unidirectional TVS diode and chassi ground. You may also consider varistors. \$\endgroup\$
    – Fredled
    Oct 27 '20 at 18:26
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For ESD events, these disturbances will appear referenced to chassis ground so surge suppression should be between the incoming +5 volt DC and chassis ground but, it's quite likely that you will need to do the same on the signal ground wire to chassis ground.

Should we use unidirectional or bidirectional diodes?

That depends entirely on whether your circuit can withstand a negative ESD event of the same clamped magnitude as a positive ESD event. So, I'd play safe and use unidirectional types but remember that there are other disturbances that can kill off ESD protection devices in a millisecond hence I ask this: does your circuit need to survive indirect lightning protection (EN 61000-4-5)? I ask because there are many systems that are designed to cope with this type of extreme surge.

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  • \$\begingroup\$ Is "power ground" synonymous with "signal ground" in the OP's diagram? \$\endgroup\$
    – Reinderien
    Oct 27 '20 at 16:11
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    \$\begingroup\$ @Reinderien I've cleared up the anomaly in names by exclusively referring to chassis ground (and not power ground). My mistake so thanks. \$\endgroup\$
    – Andy aka
    Oct 27 '20 at 16:18
  • \$\begingroup\$ Thanks for the response! I think our requirement is protection from 0 to +15kV. So the single Uni-directional diode is okay? If we required +/- 15kV protection you would suggest a uni-dir diode from the signal ground line to chassis ground? \$\endgroup\$ Oct 28 '20 at 12:53
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    \$\begingroup\$ Yes I would because the unidir diode would reverse clamp to about -1 volt rather than a bidir which would clamp negatively the same magnitude as positive and, of course, the negative voltage might be more trouble that the positive voltage for your circuit. But, ultimately, only you can make this judgement. If the supply is always positive then use a unidir device and fit one to both wires back to chassis ground. \$\endgroup\$
    – Andy aka
    Oct 28 '20 at 13:00
  • \$\begingroup\$ What you should consider doing next. if you need further clarification leave a comment. \$\endgroup\$
    – Andy aka
    Oct 28 '20 at 13:02

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