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Is there a reason to use unidirectional TVS diodes over bidirectional TVS on USB circuit protection apart from cheaper cost? As USB is a DC powered circuit, I feel like unidirectional TVS diode (such as Zener / avalanche diode) is the one to use. However, I'd like to check if this is a reasonable conclusion before I add a pair to my D+/- lines.

Doing some search, I ran into this article which goes into much details about electrical properties of both but failed to mention when or how to use one versus the other.

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If the expected surge has a polarity known "a priori" then why not to use uindirectional TVS.

But you can not teach the overvoltage to be just one polarity, it comes as is. So you'd better stay with bidirectional TVS.

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    \$\begingroup\$ But a bi-directional TVS will allow 'negative-polarity' signals to reach your sensitive DC-powered USB transceiver - which is not something you want to happen. A uni-directional TVS will clamp these, much like a zener. \$\endgroup\$ – brhans Oct 4 '18 at 13:30
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    \$\begingroup\$ This answer is logical, but incorrect for the devices offered in the market. The problem here is the the devices are not well named. "Bidirectional" applies to data polarity, not to overvoltage polarity. A bidirectional TVS will allow moderate reverse polarity inputs that can still cause damage as it doesn't have ordinary diode forward bias clamp behavior, but only a breakdown voltage in each direction. \$\endgroup\$ – Chris Stratton Oct 4 '18 at 14:55
  • \$\begingroup\$ @ChrisStratton, if you carefully read the OP and the link within, you will find out that the OP is talking about "bi-directional" in the sense overvoltage polarity, of "one diode" vs. 2-diode setup or TVS or something. \$\endgroup\$ – Ale..chenski Oct 4 '18 at 17:31
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    \$\begingroup\$ @Ale..chenski - I am not taking issue with the asker, but rather pointing out that Marko is using the terms in the opposite of the way that the manufacturers use them. \$\endgroup\$ – Chris Stratton Oct 4 '18 at 17:33
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Is there a reason to use unidirectional TVS diodes

If the I-V characteristics of "unidirectional" TVS are as the linked article means (avalanche breakthrough limit for positive spike polarity, and regular diode for negative polarity), then their "unidirectional" devices are better for USB than "bidirectional". The reason is that SOME USB devices (mostly some hosts) are very sensitive to negative part of ESD waveform, so the forward-biased diode offers better (lower) clamping voltage, which is good.

However, I would suggest not to guess and gamble on some theoretical limits, but rather rely on reference design schematics from manufacturer of your IC. Whichever they suggest will be a thorough tested solution with guaranteed results, while your own solution would require lengthy testing and special equipment (ESD gun, special table, etc.).

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