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I have some questions on ESD tests performed when device connected with power and not and searching for help here. I am asking this question because I think ESD can go into the power through TVS diodes, if the device is powered.

  • It seems that most ESD tests are performed when device is powered(connected with power). Am I correct?
  • For same ESD, will the harm be same to the device when connected with power and when not connected with power?
  • If the board is not powered, for a single ESD shock, the ESD protection designs shall be still effective. Especially if there are power planes, since the planes can absorb charges. Am I correct?
  • If the board is not powered, for accumulated ESD shocks, will ESD protection designs(schottky diodes, TVS diodes and etc.) still be effective? The charge will accumulate on the power lines.
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Some answers:

  • When ESD testing ICs they are powered OFF. For an IC most ESD problems can occur when it's not yet soldered on a PCB so during handling. It would make the ESD test more complex and not have any advantages to ESD test with the IC powered up. An ESD pulse is such a stressfull event that it doesn't matter much if the IC is on or not. What matters is that the ESD protection handles the pulse and dissipates the energy in a non-damaging way.
  • A finished product is usually tested both on and off. When on it can be checked that the product remains functional. Maybe it can enter a locked-up state or reset which might be acceptable or not.
  • I don't see how an ESD test on a powered device makes much difference in relation to damage to the device. The ESD pulse is so destructive when not dissipated properly it will do damage anyway.
  • Yes on a PCB the decoupling should absorb the ESD pulse, if you need extra protection (there's not much decoupling for example) you might want to add some device to absorb the pulse on the power rails. I don't think a zener diode is fast enough, there are certainly devices available that will be fast enough though. On all ICs there are so called crow-bar circuits which short the power supply lines when the voltage rises too quickly (as in the case of an ESD pulse). Also there are diodes that protect against the supply from being reversed polarity. An ESD pulse can be positive or negative !
  • The charge of an ESD pulse is not very large so on an board be it powered or not, after the ESD pulse is absorbed you will NOT notice any voltage change on the power rails. So do not worry about the ESD pulse accumulating on the power lines, the charge is simply too small to notice.
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  • \$\begingroup\$ Some ESD tests are done powered, where the equipment class may matter: some equipment may 'not function' during the event but return to normal functionality afterward. There are numerous such classes. \$\endgroup\$ – Peter Smith Nov 6 '15 at 11:00
  • \$\begingroup\$ It is also possible for an ESD event with little energy to cause a device to "latch up" into a low impedance state, allowing the power supply to destroy the device. The same event when unpowered may have been survivable or less damaging. \$\endgroup\$ – Brian Drummond Nov 6 '15 at 11:08
  • \$\begingroup\$ Is it correct to say ESD is always more dangerous when devices are powered on than when devices are disconnected from power? My initial thoughts were the opposite, considering power supply provides a path for ESD to go away. \$\endgroup\$ – richieqianle Nov 7 '15 at 6:43
  • \$\begingroup\$ No I think it does not matter. An ESD pulse is too fast anyway for the power supply circuits to supress. The pulse must be "eaten up" by the decoupling caps or a fast recovery diode. These will also work when power is off. Also the crowbar circuits inside ICs will work with power on or off. They should otherwise they would provide no protection when the IC is not mounted on a PCB yet. \$\endgroup\$ – Bimpelrekkie Nov 8 '15 at 12:18

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