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I would like to understand in which places one should be using a unidirectional TVS diode and a bi-directional TVS diode.

In the USB power line, VBUS (5V), is it enough to place an unidirectional TVS diode? Or is a bidirectional TVS diode required? I went through this Q&A over here & got confused with the 2 upvoted answers & subsequent comments.

Hence, can someone write an answer for clarification on where to use each one & which is better for what reason?

I'd also be grateful if someone offered a clear explanation of the above Q&A link. Thanks.

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3 Answers 3

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It's a bit of a misnomer.

All TVS diodes conduct in both directions.

The difference is:

  • Unidirectional TVS:

    It's like a high current, low capacitance, high pulse power Zener diode: in the forward direction, it's a diode, so it will conduct at the usual diode threshold voltage. In the reverse direction, it will conduct only when voltage exceeds the rated breakdown voltage. It is meant to protect signal voltages of only one polarity (usually positive, like your USB lines). It is wired in the circuit in reverse bias, like a Zener diode, so it limits voltage between -0.6V and the breakdown voltage. This is what you need for USB or the usual logic signals which are always positive or zero volts.

  • Bidirectional TVS:

    It's like two of the above in series, back to back, so it will conduct in any direction if breakdown voltage is exceeded. This is meant to protect signals which can go positive and negative, for example analog signals like audio. It will limit voltage between negative breakdown voltage and positive breakdown voltage.

To select the diode for 5V, consider:

  • It must not conduct at 5V ("standoff voltage")

  • It must conduct at a voltage that is below the absolute maximum rating input voltage of the LDO that will follow ("breakdown voltage", pay attention to the current it is specified at)

  • For a power supply, capacitance is not important

So don't just type "5V TVS" in the search engine, you have to read the link at the beginning of this answer and select the proper part.

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    \$\begingroup\$ Hm, I wouldn't say "low capacitance"; even the smallest ESD rated pure-zener TVS are some 10s of pF; surge rated parts reach many nF. For the fastest signal purposes, a very small diode is wired in series, which charges up the capacitance during part of the signal, then loads it little otherwise; or it can be biased to a supply without loading the signal ever. (This is irrelevant for supply application of course.) \$\endgroup\$
    – Tim Williams
    Commented Sep 11, 2023 at 15:55
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Bidirectional TVS is used when both negative and positive voltages normally occur. It will clamp anything above +Vbr or anything below -Vbr. It doesn't matter which way you solder it.

Unidirectional TVS is used when only positive (or only negative) voltages normally occur. It will clamp anything above +Vbr or anything below -Vfwd. You have to solder it right way around.

Diagram from Littelfuse SMAJ datasheet

Source: Littelfuse SMAJ TVS datasheet

Vbr = specified breakdown voltage, depends on diode type.

Vfwd = diode forward voltage, typically about 1 volt.

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As reference.

Bidirectional TVS must be used when overvoltage can occur in both senses and must be suppressed.

Unidirectional TVS are used when overvoltage occurs in the "positive" area.
The "diode" effect occurs in the other sense and protects "immediately" in the "negative" area.

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  • \$\begingroup\$ Thank you for the answer. So, since, for USB connectors/receptacles, we do not know whether we will get a positive overvoltage or negative overvoltage on the pin, you say a single unidirectional TVS diode would be sufficient? \$\endgroup\$
    – Freshman
    Commented Sep 11, 2023 at 7:41
  • \$\begingroup\$ I think the pin's voltages are always "positive", although it can be 5V ... 12 V, or perhaps "more". \$\endgroup\$
    – Antonio51
    Commented Sep 11, 2023 at 7:53
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    \$\begingroup\$ I don't agree with this answer. It is not whether the overvoltage can occur as both positive or negative (since we are mainly talking about ESD here, that would always be the case, rendering unidirectional pretty much useless). It is whether the signal voltage can be both positive and negative or not. For USB, that would not be the case, signal voltages are always positive with regard to ground so unidirectional TVS is sufficient (unless you intend to place it between D+ and D- for whatever reason). \$\endgroup\$
    – Klas-Kenny
    Commented Sep 11, 2023 at 8:42
  • \$\begingroup\$ @Klas-Kenny, thank you for the comment/clarification for this answer. So, for power lines or , to be more general/specific, if the voltage on a line is positive, then unidirectional diode is sufficient. If the voltage on a line can be positive or negative wrto ground, the bidirectional diode is required. Am I correct? \$\endgroup\$
    – Freshman
    Commented Sep 11, 2023 at 8:51
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    \$\begingroup\$ @Freshman theoretically no more than -0.7V but if you look at the specs it is dependent on the current. Similarly with the positive voltage the clamp voltage can be much higher than the advertised voltage. Real components are rarely as good as you’d hope them to be, this is why you need to read the datasheets closely. \$\endgroup\$
    – Kartman
    Commented Sep 12, 2023 at 10:56

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