Please refer to my project architecture in the image below:


Power supply is isolated flyback smps type. GND is not connected to Earth or Neutral of 220 VAC. All parts of circuit are powered up with the same power supply unit.

Some GPIO lines will go from one PCB to another PCB with a wired connector. GPIO wires will be running along with 220 VAC wires. GPIO wire length will be max 25 cm.

I want to have some protection against EFT, ESD, induced voltage spikes etc on the GPIO lines. On some research, I came to know that TVS diodes are best for the purpose. However I am having some difficulty selecting the best TVS diode for protecting my 3.3 V lines.

First parameter is whether to go for unidirectional or bidirectional. Since these GPIOs work between 0 to 3.3 VDC, unidirectional appears to make more sense. Am I correct in thinking so?

Second parameter is: Voltage - Reverse Standoff (Typ). Should I go with 3.3 V rated TVS or is it safe to increase this parameter to 5 V or 12 V?

Third parameter is: Voltage - Breakdown (Min). I think this parameter is tied up with second parameter. What value is recommended for this parameter?

Fourth parameter is: Voltage - Clamping (Max) @ Ipp. Even if I select 3.3 V on first parameter, this parameter is around 10 V which means my GPIO will be exposed to 10 V during a esd event. Is this OK for the MCU? What values should be selected for this parameter?

Fifth parameter: Peak power pulse. There are options between 30 watts to 300 watts in 0402 package (I have PCB space constraints so I am planning to go for 0402 package). What value for this parameter is good enough for my application?

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  • \$\begingroup\$ You should extend the description about the PSU and the potential of MCU's GND point. Further how a GPIO is connected to the mains. \$\endgroup\$ – Marko Buršič Feb 23 '19 at 14:04
  • \$\begingroup\$ @marko - Updated. PSU is isolated flyback smps. GPIO is not connected to mains. However GPIO signal wires are placed near the 220 VAC mains wire and there will be some capacitive coupling between the two. GPIO wires are unshielded. Coupling might caused voltage spikes which I want to protect against so that MCU doesn't lock up. \$\endgroup\$ – Whiskeyjack Feb 23 '19 at 14:18
  • \$\begingroup\$ I am not sure I would use a TVS here; more details about the specific usage of the GPIO (are the outputs to relays buffered, for instance) would be helpful. \$\endgroup\$ – Peter Smith Feb 23 '19 at 16:04

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