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I am designing a product that should meet ATEX requirements (Intrinsically safe design). Part of the design is a voltage limiter (overvoltage limit of buck-boost converter). Typically Zener diode is the only component acceptable by ATEX standard as a safety voltage limiter (no other circuit composed of more components would be accepted). I am selecting the Zener diode which should meet the following requirements:

  • min. total power dissipation 1.5W
  • Nominal zener voltage below 4.3V
  • Reverse current at 3V below 100uA (or close to this number)

I can not find a suitable component. Searching is difficult as the reverse current is often specified very generally (eg. 5uA @ 1V). My current choice was 1SMB5916BT3G but measurement shows that the leakage current is around 2mA at 3V. Would anybody be able to advise? Maybe try to search for some TVS diodes? Not sure if this would be accepted by the certification authority.

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  • \$\begingroup\$ Can you tell us a bit more about the buck-boost converter? What are the input/output voltages? Can the converter shut down when an over voltage occurs? Is it required to continue to output for a certain amount of time? \$\endgroup\$ Sep 14 '20 at 17:31
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I suggest you limit the energy to an intrinsically safe level using a higher voltage zener diode before dealing with other requirements.

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The 3.8 to 4.8V range is problematic for Zener diodes. It is the transition between avalanche and Zener effects. You will always get a soft transition between conduction and clamping, and a relatively high leakage currents. Spehro's suggestion to use a higher voltage (5.1V) and reducing the fuse current rating is the correct solution to this problem.

A bit of history: Microsemi used to have a Zener diode exactly for this voltage (1N5337) but it was discontinued.

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  • \$\begingroup\$ Ok, thanks for your comments. Yes, I have noticed that Zeners above 5V have relatively lower leakage current compared to their clamping voltage. But in this case, I need to limit voltage because it is related to total capacitance I can use in my design. This is given in EN60079-11 table A.2 \$\endgroup\$
    – Jan Riha
    Sep 15 '20 at 9:25

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