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How it is possible to increase ESD protection of isolator ISOW7842DWER to tolerate 8KV ESD spike?

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    \$\begingroup\$ With a TVS, capacitor, series resistor. \$\endgroup\$ – Puffafish Nov 29 '19 at 8:44
  • \$\begingroup\$ I have used two big safety capacitors between two isolator parts and also series ferrites on each I/O ports of the isolator. I have passed esd test on 4 KV but on 8 KV the ESD protection is failed. \$\endgroup\$ – Ashkan Nov 29 '19 at 9:10
  • \$\begingroup\$ if you want us to help put ESD protection on this specific design, we'll need a schematic as well as layout. Datasheets for any unusual component and a design specification/requirement would also help. \$\endgroup\$ – Puffafish Nov 29 '19 at 10:21
  • \$\begingroup\$ You've now added an incomplete schematic. Where do the signals out of the IC got to? What is the IC? You've also not told us how it fails on ESD, where do the ESD get applied to cause the failure? What is the failure mode? \$\endgroup\$ – Puffafish Nov 29 '19 at 11:02
  • \$\begingroup\$ The IC is ISOW7842DWER and it is for isolating usb connections. After applying 8 KV esd spikes on two isolated parts of the board, the isolator do not work properly. \$\endgroup\$ – Ashkan Nov 29 '19 at 14:22
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To improve ESD protection you need a way to stop the discharge getting to the sensitive silicon. There are multiple ways to do this, you can either stop the signal, or put the signal back where it came from:

  • Series resistor: this is the cheapest way to do the job. It adds resistance and parasitic capacitance and inductance. This reduces the spike coming along the line. However this will have limited effect as it is just a resistor, and so will also effect the signal you want, as it is a resistor.

  • In line inductor: which will block (greatly reduce) the path for higher frequency signals, which is what an electrostatic discharge is. However this will block high speed signals which you may want, and is more expensive and takes up more board space than other options.

  • Capacitor to earth/ground/return line: this gives a route for high frequency signals to go back where they came from. Selecting the capacitance value and voltage range is tricky. Also, the package matters, 0402 capacitors will work a lot higher frequency (due to lack of parasitic inductance) compared to 0805 for instance. This is probably cheaper than inductor, more expensive than resistor.

  • Protection diode: this are silicon devices which are designed to do what the capacitor is doing, but only for high voltage signals. These are the "proper" way to do it, as they are designed for the task. This means they have the least effect on the signal, but also they cost the most. You can get TVS diodes, which are often designed for voltage spikes rather than ESD, so lower voltage but higher power, which also react slowly. If you get a very good ESD protection diode, it will cope with the high voltage and react quickly. When looking at these, don't forget to consider the diode capacitance will may effect your signal in ways you don't want.

I have used all of these for different reasons at different times. My go-to option is the protection diode. However, ESD is a very quick impulse so usually I do a protection diode in parallel with a suitable capacitor.

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    \$\begingroup\$ For diodes, their internal capacitance might need to be taken into account, depending on signal. In some cases, just the capacitance of the ESD protection diode can disrupt normal circuit operation. \$\endgroup\$ – AndrejaKo Nov 29 '19 at 9:18
  • \$\begingroup\$ @AndrejaKo You're right, I have added a quick sentence about it. \$\endgroup\$ – Puffafish Nov 29 '19 at 9:23
  • \$\begingroup\$ A 0402 capacitor lacks parasitic capacitance (compared to a 0805 capacitor). Could you please explain? \$\endgroup\$ – Huisman Nov 29 '19 at 9:28
  • \$\begingroup\$ @Huisman an 0402 case is significantly smaller than an 0805 (or 0603 etc), this massively reduces the inductive loop. A higher parasitic inductance will reduce the effect of the capacitor as the inductance blocks higher frequencies going through the capacitor and return to where it came from. \$\endgroup\$ – Puffafish Nov 29 '19 at 9:33
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    \$\begingroup\$ @Puffafish I know about parasitic inductance, thought I missed something about parasitic capacitance. Maybe you should change the word capacitance to inductance in your answer :D \$\endgroup\$ – Huisman Nov 29 '19 at 9:36

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