One of my project architectures looks like this:


GPIO / 595 output / 12 VDC wire lengths will not exceed 25 cm.

I have faced MCU lockup issues due to EFT in the past and want to make sure this design is protected against all such threats.

This is what I intend to do for protecting my system:

1) Protection against surges - MOV on the input ac lines in the ac DC converter circuit which handles 4 kV surges. Do I need to do anything more besides this? I have heard that TVS diode offers better protection due to fast reaction time but MOV turns out to give best performance per unit money.

2) EFT/ESD - TVS diode (uni-directional) between 12V and GND at the entry point on main board (PCB which has MCU, 595 and relay). Uni-directional TVS diodes on all GPIOs leaving the board in form of wires.

There are some GPIOs that control 74HC595. Do I need to put TVS diodes on those GPIOs as well? Or should I put TVS diodes on the 595 output wires? Or both GPIO and output wires?

For DC lines, is uni-directional TVS diodes OK?

For such a system, what values of breakdown voltage, standoff voltage and peak power should I select? - for 12 VDC power line and 3.3 VDC GPIO lines.

  • \$\begingroup\$ Uni-directional TVS diodes for unipolar lines is PREFERRED over bi-directional. I've found 3.3V TVS diodes to be rather rare. The cutoff for most parts seems to be 5V. Your minimum standoff voltage needs to be at least your operating voltage. You won't have very much choice about breakdown voltage after that since it will vary with technology and the current capacity of the device as well as the actual current flowing through the device at the time of clamping. TVS diodes are expensive though. If you want them on all lines you should figure out the characteristics of the EFT you expect \$\endgroup\$ – DKNguyen Feb 21 at 13:41
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    \$\begingroup\$ You should also note that MOVs have a finite lifetime due a wear out mechanism and there have a limited number of clamping cycles. They degrade after each clamping and there is a point where they are deemed to have worn out. TVS diodes do not this. From your diagram, shouldn't you be isolating those (long?) lines running out of the MCU? Or at least buffer them series resistors or something for voltage-only signal lines (which also helps the TVS diodes do their job if the resistor is in between the TVS diode/EFT entry point and the MCU pin. \$\endgroup\$ – DKNguyen Feb 21 at 13:43
  • \$\begingroup\$ @toor - On input ac side, I have never seen a TVS because of their less power handling capacity as compared to MOV. For input lines, I can put series resistors. What value do you suggest? There are some SPI lines and IO which can be either input or output depending upon situations. What do you suggest for those? Should I try and find 3.3 V standoff TVS or is it okay to use a 5 V standoff TVS? \$\endgroup\$ – Whiskeyjack Feb 22 at 5:53

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