In conventional circuits you'd like to divert your strike to ground and through a path that diverts the current safely to a grounded chassis or output. That is not possible in hand held devices for example, i'd like to know what methods are chosen for these kind of floating hand held or otherwise devices. Do we prefer dumping charges into our ground plane or the power plane for an example?.

Hints on material related to this for further reading also appreciated.

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I dump the ESD into the ground plane. Most application notes for surge suppressors, TVS or similar do this.

Some sources:

Semtech pdf on ESD

Cooper pdf on ESD

Infineon inf on ESD

Consider that an ESD "strike" causes a large amount of current to flow from one node (wire, signal, etc.) to another, and that neither of these nodes might be ground. The goal of the ESD protection scheme is to provide a safe path for the ESD current, which can be many amperes for a short time, away from the sensitive circuits in the product. By providing paths from every node to a common point, usually ground, ESD currents are given a safe loop to flow through and dissipate.

In IC design, we pay much more attention to the input/output pads and their circuits than the interior circuits in the chip, but in a board level product design you need to pay attention to all the wires and traces on the board. We usually draw a schematic of the system and examine where ESD current will flow between every pair of nodes (choose one node as positive and another as the negative return point... even if the "loop" is closed outside the product the danger is from large current flow or large potential differences between nodes in the product). You don't need to worry as much about passive components, unless they are small RF transistors or diodes, but ICs are always ESD sensitive.

You can consider adding ESD protection devices (diodes or TVS's) to ground at points where ESD current would otherwise flow through a sensitive circuit. The diagram in the Semtech ESD .pdf linked in Dejvid_n01's post shows an example. Generally you want devices with a low forward voltage drop, since high voltages generated by the ESD currents can damage ICs. Also, it's important to consider current flow in either direction; the TSV devices provide low voltage drop in either direction.

Remember than in case of a floating circuit, you have to solve two problems:

  • the circuit shall not see an overvoltage event between its floating ground and any inputs-outputs - and the TVS diodes are a great for that
  • but the other problem is that the floating circuit will collect charges because of the lack of a direct connection to the earth ground. Those charges will eventually disappear due to a somewhat available capacitive coupling between the device and the Earth. BUT this charge can actually produce a secondary ESD event across your circuit just because these charges have no direct path to ground.

In other words, you ESD your device with 8kV. Your GND is at 8000V, your Vcc is at 8005V thanks to the TVS diodes, and your device is charged at 8000V to the protective earth (or coupling plane during an ESD test). Watch out for that.

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