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In answers such as this one, https://electronics.stackexchange.com/a/647135/311631 I've seen suggestions that TVS diodes are not "reliable" under repetitive or continuous operation, such as a Zener diode might be used. (Assume the poorer tolerance compared to Zeners is not a concern. Also to be clear, this concerns avalanche diode type, uni- or bi-directional TVS.)

I have been unable to locate any references supporting this, and other answers seem to agree, e.g. TVS diode dissipation help

Can anyone provide more information to support or disprove this notion?

Clarifications:

"Reliable" means, specifications remain within tolerance: leakage, breakdown voltage, peak handling, etc.

I would accept an answer either from a majority of manufacturers showing how and why they are un/reliable, or a more general approach such as, what mechanisms if any cause these diodes to wear and fail (and, hopefully, if such differences can be inferred from the datasheet).

Put another way: one manufacturer rating their e.g. 1.5KE series with a power rating, and others not, preferably should have some explanation. Is it just laziness on their part, or does it mean something more fundamental? What do we know conclusively about this?

ocrdu
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Tim Williams
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    In my view, it's really a matter of specs and testing of TVS diodes vs. zeners for example. A TVS may not be tested or specified for continuous operation, so if you use it that way you have no guarantee that it will work long-term. Having said that, a TVS is really just a zener (avalanche) diode often with beefed up contacts and a possibly bigger die to handle the high peak power surges. So I don't see a reason other than specs/testing that it wouldn't work in place of a zener. – John D Dec 25 '22 at 00:26
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    If the TVS has a DC power dissipation spec (as some do) I wouldn't have a problem operating it with a continuous power dissipation safely under the rating. I've used TVS diodes in that way in the past without issues. – John D Dec 25 '22 at 00:35
  • Reliable as in constant voltage drop or not getting destroyed? – tobalt Jan 08 '23 at 18:43
  • (MO)Varistors are said to degrade with absorbed surges. (No references handy, sorry.)(Oh well, comments to this effect accumulate with [Jonathan S's answer](https://electronics.stackexchange.com/a/649246/274544).) – greybeard Jan 08 '23 at 21:58
  • @tobalt Yes, both; edited. – Tim Williams Jan 09 '23 at 04:18
  • Even if a power rating is given, it sometimes relates to lab-only situations, for instance keep the pins at room temperature at a specified distance from the case, or connect them to larger cooling planes. I doubt this wont fit on most real-world boards. Thus I would reduce a given dissipation a lot, depending on the cooling that could really be provided in your situation to keep Tjmax below limit. I would also compare with dissipation specs of other devices in the same package and with the same pin diameter... – datenheim Jan 13 '23 at 16:51
  • I would expect it would perform poorly as a voltage reference, with much more significant part-to-part variation than typical zeners, but other than that I'd be surprised if it didn't work, if you kept the power dissipation reasonable. – Hearth Jan 15 '23 at 17:30
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    You've put my answer as a basis for your question but misquoted it and appear to have misread it or misinterpreted it. I've posted an answer explaining this, along with a copy of my answer with sections highlighted in bold. **That clearly refutes the statement that my answer has "suggestions that TVS diodes are not "reliable" under repetitive or continuous operation"** but instead clearly states that **using a TVS outside of its rated behaviour will decrease its reliability and it will fail more quickly and unpredictably than its specified life**. – TonyM Jan 18 '23 at 22:46

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At least Diotec seems to be confident enough that their TVS diodes will not fail under continuous operation to give them a continuous power dissipation rating. See, for example, Diotec's 1.5KE series. The breakdown voltage tolerances also aren't that bad (they specify some diodes with 5%), so these are really just very rugged Zener diodes that can tolerate extreme short-time overloads. They won't have any trouble dissipating something like 2W continuously. Absolute maximum is 6.5W with (very) good cooling.

Note also that Diotec recommends this series of TVS diodes for use as free-wheeling diodes, so they're constructed in such a way that allows them to clamp inductive spikes repetitively without degrading.

So, to answer your question: There certainly are TVS diode types that are designed for reliable operation in continuous / repetitive pulse applications. Manufacturers spell this out in the datasheet by, for example, giving the diode a continuous dissipation rating, or recommending it for repetitive pulse applications.

If the datasheet doesn't give a continuous dissipation rating or recommendation for repetitive pulse applications, however, you should assume that the diode is constructed in such a way that doesn't permit continuous operation (i.e. it's not just a Zener, but maybe also contains transistors, or it develops thermal hotspots in its junction when operating continuously).

Semtech also claims in an application note:

TVS diodes will not wear out nor will there be any degradation of the electrical parameters as long as the device is operated within specified limits.

Additionally, Semtech shows the internal circuit of a "deep snapback TVS device" in a blog post, showing that some TVS diodes aren't just Zeners:

Semtech Thyristor TVS

Such TVS diodes are of course totally unsuitable for continuous operation or free-wheeling applications. An example of a TVS diode that's not just a Zener is the Semtech RClamp0502A. Its datasheet does not specify a permissible continuous power dissipation, so you shouldn't use it for that purpose.

Another thing that gives this away quite clearly is the junction capacitance: The Diotec (plain Zener) devices have typical junction capacitances in the 100pF to 1nF range, while the Semtech (complex) device has just 0.9pF, which is achieved with additional active circuitry inside of the device which assists in clamping while not adding much junction capacitance.

Another example: Here's a paper that details the construction of a TVS device using MOSFETs, designed for integration into GaN ICs.

In short: You never know what's inside a given TVS "diode" - it may be just a diode, it may be transistor-based, and there might even be FETs inside of it. For that reason, you can't know whether it's safe to operate the TVS device continuously unless the datasheet explicitly says so, just like it's not safe to operate a BJT continuously at high voltages without first taking a look at the SOA graph (due to second breakdown).

Depending on their construction, TVS devices may be susceptible to the same failure mechanisms as BJTs, MOSFETs, Zener diodes, and whatever else you might try to build a TVS device from. BJTs can fail from second breakdown when they conduct a current while simultaneously having a high voltage across them. MOSFETs can fail in a similar way, especially cell-based ones ("HEXFETs"), where imbalances in current sharing can cause individual FET cells to fail. Diode junctions can be uneven too and have spots that break down earlier than others, which is not a problem when you whack the diode with a short high-current pulse (which raises the voltage across the junction so much beyond breakdown that every part of it breaks down evenly), but it'll become a problem at low continuous currents (where some parts of the diode may break down while others don't).

Jonathan S.
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    My impression was also that the long term (under repetitive use) reliability was exactly an advantage of TVS vs. MOVs – tobalt Jan 08 '23 at 20:08
  • Yeah, MOVs are really just designed to be cheap and absorb a LOT of energy a few times. Lots of thermal mass, poorly defined characteristics. TVS diodes can and do still fail, of course, but not nearly as quickly as a MOV, which degrades with every operation. – Jonathan S. Jan 08 '23 at 20:20
  • MOVs themselves get a bad rap, I think -- they are in fact rated for infinite cycles (or at least, a limit approaching such for practical purposes), just at much lower than nameplate ("two shots and it explodes") energy levels. It's my understanding, TVSs fail along a MUCH sharper curve, having more or less unlimited life up to some threshold, then quickly (exponentially? hyperbolically?) less beyond that. – Tim Williams Jan 08 '23 at 20:48
  • @JonathanS. See: "to be clear, this concerns avalanche diode type" -- am aware of snapback diodes and such; they are out of scope here. (Fantastic for low voltage protection though!) – Tim Williams Jan 08 '23 at 20:50
  • The RClamp0502A isn't a snapback type, but it still contains additional active circuitry to keep the junction capacitance low. Semtech calls it a "silicon avalanche" type; yet it's not suitable for continuous operation. – Jonathan S. Jan 08 '23 at 21:17
  • I suspect those types are still suitable, there's just no point because they'd be rated same as a zener of comparable size, i.e., 300mW or so, say. Or less due to the extremely small clamp diodes, I don't know. Incidentally, I do expect some kind of wear effect on those (electromigration perhaps?); I've seen low lifetimes in datasheets before (e.g. 3 or 10 ESD strikes). But yeah, we can consider such composite devices out of scope as well. – Tim Williams Jan 09 '23 at 04:22
  • For many TVS devices, the datasheet doesn't actually specify what kind of circuitry the device contains, and it's not really relevant anyway. It's absolutely possible to build a composite TVS device that can operate continuously (i.e. Zener + pass transistor), and it's also possible to build a single-junction avalanche TVS diode that can't operate continuously (i.e. due to development of hotspots). The only thing that really matters is whether the datasheet tells you that the device is suitable for continuous operation or not (by giving a continuous dissipation rating). – Jonathan S. Jan 15 '23 at 17:31
  • To clarify your edit, Zener effect (Vbr < 6V or so) is NTC, avalanche is PTC. So the types in scope are not subject to runaway hotspotting in an analogous way to transistors for example (mind, they may still be subject to spot failure due to defects, or other effects I'm not aware of). – Tim Williams Jan 15 '23 at 21:04
  • @TimWilliams That is not true. The Littelfuse SMBJ series, for example, specifies a forward voltage drop of 5V. That means it's constructed from 8 to 10 series-connected diodes, which will still operate in Zener breakdown mode even at moderate voltages (60V). Of course, as you mentioned, effects other than the negative tempco of Zener breakdown may occur in other diode types - it all depends on the devices's construction. The big problem is that you never know what's inside a given TVS device unless the manufacturer tells you, and most don't. – Jonathan S. Jan 15 '23 at 21:13
  • Check the note: it's 3.5V max *at 50A* for single-die, 5V for stacked; the whole series, it seems, is single-die, with stacked *available* for 300-440 only (asterisk suffix). Interestingly, Fig.8 suggests the max figure is close to typical. Notice the Vf is a more typical 0.8 / 1.5V down at 1A, as we'd expect for a single die. Further evidence suggestive of single-die construction is the constant energy across the family; more dice should handle more energy, even if they're thinned greatly compared to a single die version. – Tim Williams Jan 15 '23 at 21:24
  • Anyway, I'm afraid none of this addresses the meat of the question; the closest I think is the Diotec being "designed" for it, but the question then is how. Could you find a patent perhaps that explains this? – Tim Williams Jan 15 '23 at 21:25
  • I've found a paper that details a TVS device built from MOSFETs. Of course, this is just a single example, but it shows that TVS "diodes" can really contain *anything*, and you have no idea what's actually inside unless the manufacturer tells you. TVS diodes are anything but simple diodes. – Jonathan S. Jan 15 '23 at 22:09
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It is extremely common in industrial automation control circuits to find TVS diodes used in a continuous manner. Instances were one wishes to protect the data communication channels from potential problems in the power control modules. Problems with a 24v or 48v supply can sometimes go "undiagnosed" for weeks on end. When the EtherCAT or rs485 lines are near the supply voltage lines (most of the time crammed in a box together); you account for the potential for that long duration surge / power excursion or intermittent short circuit.

Using a thyristor in series with a TVS diode allows one to account for this while still maintaining proper signal levels for the rs485 controllers. The TVS diode's working voltage ultimately gets set to a value greater than the nearby supply voltages. The thyristor will provide the maximum operating current allowing the TVS diode to do its job on the surge voltage protection for the transceiver.

Two helpful national standards associated with circuitry for surge protection are:

ISO 7637-2:2011

&

For TVS diodes specifically. IEEE C62.35-2010

JRogers107
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The [answer] you link to is mine and is duplicated below.

You quote it ('below' refers to the copy of [answer]) as:

In answers such as this one (below) I've seen suggestions that TVS diodes are not "reliable" under repetitive or continuous operation, such as a Zener diode might be used.

There's no such suggestion in the [answer].

I've highlighted the areas for you to examine in bold.

So the basis for your question "Can anyone provide more information to support or disprove this notion?" in [answer] appears to be false.

The referred-to [answer] was address this question:

Is there a down side to using bidirectional TVS diodes in 12 V DC applications, such as across the coil of the relay or solenoid, or to protect other sensitive equipment?

[ANSWER] GIVEN:

Usually the reason why not is reliability.

As its name suggests, a Transient Voltage Suppressor (TVS) is designed for suppressing the effects of infrequent and unusual transients into a board connector or cable. It's connected directly across that input, to dissipate ESD, lightning strikes etc.

It's not designed for dissipation of normal operating power in a circuit. Its body package typically cannot conduct power well compared to other components because it doesn't need to. It's datasheets reflect this, with discharges through it rated at a very low pulse width and repetition rate indeed to keep the peak power and average power dissipation right down.

So using a TVS outside of its rated behaviour will decrease its reliability and it will fail more quickly and unpredictably than its specified life.

A TVS is just an electronic component, like any other. And like any component, it has:

  • characteristics that define its behaviour
  • specifications that define its performance and reliability

The manufacturer will list only some of its characteristics as specifications, in datasheets and reliability data etc. The manufacturer will qualify the part's behaviour and reliability against those specs and guarantee against them.

Other characteristics aren't specified. And if you go outside the specs, and rely on unspecified component characteristics, you're on your own in uncharted territory with a good chance of sinking. For one or a few circuits, you may choose to take your chances and it may work out OK. But it may not and for many/most mass production situations, that's unacceptable because the cost of failures in the field swamps the cost of properly spec'd parts.

So, with a TVS as an operating current clamp, there simply are better alternative components and circuits to use that do give specified reliability.

And, as said above, usually lower business costs in the long and short term.

TonyM
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