Simple sample designs for ESD and other typical protection of 5V circuitry

[Infraviolet]

Has anyone got some sample schematic images of typical protection circuitry used in basic consumer devices (USB and that sort of thing) and in hot swappable module type boards to protect against ESD, spikes, surges and other common risks for devices in normal to slightly harsh environments? I've been looking at TVS diodes, but often the clamping voltage is well above the abs max of most ICs. So does a protection circuit usually involve quite a few separate steps? I'm thinking about protecting custom circuit boards which may be plugged and unplugged from their data and power lines (lines up to perhaps 2m length max) while the lines are powered. Perhaps fairly similar scenarios to what protection on USB connectors might be. Does a design usually start with TVS diodes of a fairly high clamping rating near the connector, then have a small resistance, then zener diodes, then more resistances, capacitors, more zeners...

I'm mainly just looking to make some custom circuit designs a bit more rugged, I've never actually been bitten by an ESD event, but I'm thinking it could be worth dedicating some board space to making sure the boards are atleast as protected as ordinary consumer and gentle-industrial* grade devices.

*I'm not interested in protection of mains powered stuff, or surviving sitting near a welding arc or blast furnace

Thanks

[exmadscientist]

Most ICs have protection built in. Look for "HBM" (Human Body Model) or "CDM" (Charged Device Model) or "MM" (Machine Model) specifications. But you're right that it is not wise to rely on those forever, and especially not if there will be repeated large strikes or if you are doing something sensitive.

Because you are not trying to protect against surge, EFT, or worse, a single TVS diode per line, perhaps with some series impedance to push against, is generally considered enough. You're right that it will not bring voltages to within an IC's "absolute maximum" rating, but that rating is at DC, not for single-pulse events. For rare single pulses, the built-in ESD structures will handle the rest. (If you have a particularly sensitive IC, you will need to do more.)

For protection of data lines, whenever possible I choose TVS diodes with a surge rating (8/20us), even if I am not trying to protect against surge. Surge rated parts are very common and affordable and I figure any part with this rating is likely to outperform any part without it. For power lines I even like to see the big surge rating (10/1000us), despite usually not really needing to deal with mains-level crud. Again, it's common and affordable, so why not? As well, be mindful of diode capacitance. Data lines prefer low capacitance, power lines prefer high capacitance. Just another reason to use distinct parts.

For USB in particular, consider the "flow-through routing" parts as they are much easier to physically lay out on your PCB. There are many, many parts out there marketed for USB, but most have dumb pin assignments that can make them very difficult to use. ECMF02-4CMX is one that's very, very good, if you can stomach the rather small DFN-8 package. It even has built-in common-mode filtering, which is great.

[Infraviolet]

exmadscientist: do you usually pick TVS diodes with clamping and max reverse voltages WELL ABOVE your logic levels. Because it seems that for form factors I do like working with (SOD323 ideally, or 0603 or 0805, or bigger if I have to) there aren'y many options for TVS diodes rated to start conducting at 6V or so, these tend to clamp to about 16V. If one can use diodes which start conducting above say 12V and clamp to something horrid like 24V then there's a lot more range to pick from, would it work to use some of those higher voltage TVS diodes, then a zener diode with a condution voltage of 5V to 6V to further filter out spikes which get clamped to whatever the TVS clamps to?

I'll try to find how to upload an image to this forum, still new enough here that I dont know if it supports uploads or if I have to find a hosting site, and show a schematic of my thoughts tomorrow.
Thanks

[David Hess]

For hot swap support, power connections use a current limiting circuit to prevent fast charging of the decoupling capacitors from glitching other circuits and damaging the connectors.  The connector stages the connections, so that ground connects first, then power, and then signals.

Signal lines are usually protected with a series resistance and then diodes or transistors to ground and power to limit signal excursions.  For hot swapping, a precharge circuit allows the inputs to charge to the input signal level before connecting to prevent glitching.

[exmadscientist]

Hot swap needs care and attention to work well, yeah. And not just in the protection. I've never really had to do a serious hot swap design so I'll leave that to others.

(The rest of this might be longer than you are looking for, but it took younger me longer to work out than I care to admit. So I'm writing down something so that it will hopefully take others less time than it took me.)

Back to the protection devices... and first, a bit about TVS parameters. You specify a TVS that starts conducting just above the maximum voltage you want to have on the line. Note that TVS diodes are not precision devices, so you can simply never use them as precision clamps. Sometimes you can't even use them as suitable protection devices, period, because their trigger voltage and their clamp level are too far apart. These cases are uncommon, in my experience, but very real, and usually involve DC or repetitive crud rather than simple ESD.

The manufacturer gives a "max reverse working voltage" V_rwm and if you read that as "use on lines of this nominal voltage" you won't go too far wrong. (V_br is actually defined as "highest applied voltage that gives a current through the diode of some given value or less".) But remember that this is not a fundamental characteristic of the part, it's a characterization, and sometimes exceeding V_rwm might be a perfectly good thing to do. For example, I once put a 5V TVS across a 5.5V power rail because I didn't care about a potential 1mA of leakage there.

The breakdown voltage V_br is usually the key specification of the part, and is the measured voltage to achieve a given current through the diode. You'll see V_br as minimum, maximum, typical, or some combination of those; min and max are most useful, in that order. This specifies the "knee" of the part, or when it actually starts to turn on. The reason I knew the 5V TVS I mentioned would be OK at 5.5V is that its minimum V_br was 1mA at 5.5V, and that was acceptable to me. If it was 100mA at 5.5V, I could not have used that part. V_rwm didn't really matter to me, V_br did. In other circuits this will be reversed. But that's what they mean.

Another thing to remember is that TVS diodes and zener diodes are (usually*) the exact same thing. It's just that they are optimized for different things (peak power vs continuous power/stability/binnability). You can swap one for another in a pinch, and there are several series that are rated for use as both. (Like my favorite SOD323 zeners, Vishay's PLZ series!) You'll do better with the correctly-optimized part, but don't let dogma stop you from using the good-enough choice.

Except... I said "usually*", with that ugly asterisk thing. Zener diodes are crap below, very roughly, five volts. And people like to protect data lines on 3.3V and 1.8V and lower parts, but you can't always get the performance you want out of zeners. So manufacturers started offering thyristor designs instead (aka SCRs aka "snapback diodes" aka I think there's others I'm forgetting). These are just fine if you (1) recognize you're buying a thyristor (2) recognize you're not buying a zener and you shouldn't treat it like a zener and (3) confirm that the snapback release point is going to be OK in your application and you're not going to get stuck in SCR latchup. (#3 is the important one. It's uncommon, but possible, especially if you're not aware to look for it.) Snapback diodes are rare above 5V, the definite minority for 5V TVS parts, common at 3.3V, and the majority below 3.3V. If not otherwise clearly indicated, the best way to recognize a snapback part is from the V-I curve having that characteristic SCR shape to it. If they bother to give you the curve. Now that I've mentioned that snapbacks are a thing, I'm going to forget about them, because that makes life easier.

So. Example time. Let's say I have a TTL (5V) UART leaving the board on a connector and I decide a TVS (per line) is an appropriate filter. (I might want more if the cable is long or the environment harsh, but usually, I consider a single TVS to be enough.) You like SOD-323, I like SOD-323 (though I'm going with smaller more and more often these days...) so let's search the catalog and come up with STS321050U182! (Also, it was in my library because I've used it before.) In my opinion this is a good part for this job. The 180pF is not great for a data line, but this is a UART and UARTs are slow so who cares. (Substitute actual analysis as required. Especially if you're doing more than writing a forum post.) We see it's rated to take a 30kV hit (that's a great rating, they don't really come any higher), and has a 350W 8/20us peak power rating (you can do better here but at the cost of worse C or being a snapback, so this is also very good). It conducts max 1uA at 5V and hits 1mA no earlier than 6V. It's unidirectional rather than bidirectional, which is appropriate because there are no negative voltages in our design. (Bidirectional devices are almost never appropriate unless you have a negative rail. Almost. So prefer to exclude them by default in your searches.) So far, so good. This is really about what it looks like to protect a data line with a TVS diode.

To finally start addressing your first question.... You see that I have V_rwm at 5V and my signal voltage is 5V. So, no, those aren't usually very different. Because TVS clamping is so coarse, I usually need the diode to kick in as soon as it can, and so the only reason I'd go for a V_rwm far removed from the signal max voltage is either availability (what I can get; or what I am willing to pay for, since some signals just aren't important enough to spend resources on), or if I really need to optimize leakage (this is not common).

Now, clamping voltages. Hit the diode with an ESD pulse and it'll see a lot of volts, break over, and start to conduct. A lot. Diode voltage drops scale with current. Put 1mA through the diode, it drops less voltage than if you put 1A through it. It's also exponential-ish, so 1A to 2A is a lot less trouble than 2A to 3A. These guys handle enough amps that we're definitely in the "more trouble" end of the curve. Our STS321050U182 is actually pretty clear about how it's been characterized when this happens: they used 8/20us tests (very common; still good to see stated; also 10/1000 is preferred with beefier parts), and they've been nice enough to provide us with two data points. (Not everyone gives you two.) Data point number one is that when you shove Ipp=1A into the poor thing (pulsed at 8/20us!) it drops Vcl=9.0V max. Data point number two is that when you shove 22A into it, it drops 12V typical or 15V max. The 350W P_pp number, incidentally, is from 15V × 22A = 330W, which is close enough to 350W in this business. (It tends to round better for some parts in the family than others, and they stick the same rating on 'em all.) So Digi-Key and Mouser both call this a "15V" clamping TVS diode. That's direct from the datasheet and completely correct. It's also bullshit because it lost the "22A" part of the conditions. You must, when you are comparing TVS diode performance, compare at the same conditions. Is a "10V" clamping diode better? Maybe! But probably not if "10V" means "V_cl typical 10V at 10A". And, for that diode I just made up, guess what Digi-Key will list its clamping voltage as? Yeah. "10V". Even though I'd take this "15V" diode any day (for clamping ability alone anyway), since if you hit them with the same pulse, this one's clearly going to do better. Because of this wrinkle, it is hard to comparison shop TVS diodes. I end up with a couple series I know well and just use those repeatedly if I can. Also, the footprints are common, so it's easy to change later if I must.

This stuff all tends to show up in peak power ratings, so when searching for a new TVS diode, I will typically start by picking a package, then a max capacitance I will tolerate, then start evaluating parts from highest peak power to lowest. (Along with the usual price and availability concerns, etc, etc.) Peak power rating is not everything, but it's a great way to direct the search.

OK... so... you're still exposing that poor UART pin to 15V on a hit? Right? That's bad? It's not as bad as you'd think. You put the TVS diode near the connector or ingress point (you did do that, right? seriously, this stuff is important), so it has a low-inductance shunt loop to get rid of the crap. You have the pin farther away (maybe), with at least a little inductance in between them. That helps a lot. The pin isn't defenseless; it has its own ESD network, and it is a lot easier for that small network to do its job now that there's a big mean signal TVS diode (don't tell it about power TVS diodes, lest it realize it's not big or mean) over there protecting it and taking the worst of the hit. And it's unlikely you actually managed to get that much energy into the system (for really fast events like ESD, you have to talk energy rather than just power -- power doesn't vaporize metal layers all by itself) with an ESD hit. See, most of those ratings I just discussed were IEC 61000-4-5 8/20μs surge test ratings. There's a lot of energy in them. ESD is fast. Lots of volts, lots of amps, yes, but it's over so fast it doesn't have time to do too much damage... if there's any amount of protection.

In general, if you stand a snowball's chance in hell of surviving surge, you are going to do just fine with ESD. No, this is not a "perfect" strategy. But, hey, a single diode is pretty easy, pretty cheap, and does pretty darn well. You really don't need a third layer of zeners in between, and they're not going to do any better even if you do have them unless you are super careful in your design. Remember, a TVS is just a differently-optimized zener, so you won't be able to clamp any tighter if you're handling any real power. And if it isn't handling real power, what's the point of your added part? The series impedance between your existing two protection networks is the first place to spend your time if you are worried. Making sure the nasties get dealt with by the big guy instead of the little guy can bring some real gains.

[Terry Bites]

https://www.electroschematics.com/esd-protection-for-usb/ ?
 For data buses and usb etc you need a scheme that only ads a very small aditional capacitance to the lines.
There are loads of off the shelf solutions.

[Infraviolet]

How would this do?
it is for protecting a device with various ICs in it (away to the right) and being powered from the connector (at the left) as well as outputting signals from the device towards other chips away to the left of the connector. The signal lines from the device only carry minimal current. The device usually consumes 20mA and sometimes 40mA, never anything more.

The 1u and 10U caps in series with the 1 ohm resistor marked are there for helping suppress turn-on spikes (which I discussed in another thread here) due to similar MLCC caps within the device (away to the right).

Could any of these parts maybe be omitted? Perhaps the 10 ohm R5 and R6? What would the effects of omitting other parts be? If the design in the schematic is already excessive for normal ESD and ordinarily plausible turn-on spikes and and noise from nearby other conductors (12V to 18V referenced to the same Gnd, a cm or so away, currents of up to an amp maybe, sharp turning on and off) then what sort of events could it survive which a simplified version might not?

I think the TVS (TVS1) would also serve as reverse voltage protection, however this isn't needed on this design anyway, as the connector type used isn't one which can be plugged the wrong way.

Thanks

[exmadscientist]

What's the threat model and the signal bandwidth required? And what are the true limits that the other end can tolerate? It's hard to tell you if this is a clean solution without knowing that. If it's just ESD you're worried about, then most of this can go away. The diodes alone will handle it.

R5/Zener1 don't do a whole lot. R3 is going to be doing the work here, and similarly for the other line.

R2-C2-C4 can be replaced with a single electrolytic (if C2 and C4 are MLCC), if you can spare the volume. If not, well, at least C2 isn't doing anything. Large C here eats spikes all on its own, which lets you drop R1. (Most people don't like 10 ohms in their power lines.) If that doesn't work out, R1 can also be a ferrite bead. The bulk capacitor on the power rails does a lot of work dealing with ESD/surge events, which many people miss. The older approach of "steer crud into the rails, then protect the rail bulk capacitor with a massive clamp diode" works well, if you can live with steering diodes everywhere. (They're not that bad, but you need to tolerate ~1V rises above the rail for low leakage BAV199 pairs or ~0.3V with high leakage Schottkys.)

[Infraviolet]

How do I work out exactly what my ESD/spikes threat model would be?

This is operating in proximity to some 12V motors (stall current 1A), but NOT connected to them (that's what the two lines going off to the bottom of the page from the left connector are about) except in sharing a common ground.

It is potentially plugged or unplugged at many times while the stuff at the other end of the wires going to the left connector is powered.

It is subject to whatever ESD is plausible for exposed contacts on the lines going to that connector (but everywhere else is physically protected from anything being able to reach it).

The circuit without this protection definitely sees some spiking when powered on, due to the surge of current going in to the MLCCs (of the circuit, not the extra ones shown here) and the wire length, not huge but a volt or so above abs max ratings of the chips involved and for >10uS. I was using the 1 ohm and more MLCCs as electrolytics are a bit of a space issue and I've loads of MLCCs to hand. The reason I'd shwon 2 caps, perhaps I'd been mistaken, was due to the way one usually has multiple decoupling caps of differnt sizes, the smaller ones to keep up with the fastest stuff. Being able to omit the smaller of them is indeed helpful though.

Signal bandwidth needed: the edges rising/falling between 5V and 0V on the two output lines should be faster than 100uS.

The parts being protected include varous chips which are very hard to get hold of right now, so I'd like to protect them as much as feasible.

[exmadscientist]

"Threat model" means "what exactly do you want this thing to survive?" There are more and less formal ways to do it, but it means thinking about your environment (Lab? Factory polishing amber with cat fur? Boeing lightning lab?) and coupling mechanism (Connector? Cable? Direct contact?). Examples might include:

• Survive ESD strikes to any connector shell or pin once assembled
• Survive up to two IEC 61000-4-2 Level 2 air or contact strikes to any component during assembly by careless technicians
• Meet Telcordia surge standard whatever for connector J1 for up to 10 years installed life
• Survive typical abuse inflicted on industrial automation devices by installers and connected equipment
• Maintain Performance Class A with no degradation of function when used next to an operating electrosurgery device

Some of those are more difficult than others. Basically, you have to know what victory looks like, because no matter what you do to protect it, there will always be some way to kill your device.

Motor spikes are annoying but they're radiated EMI. They're not going to kill semiconductors. Regular old ESD is handled with just the TVS diodes, but the resistors are good practice and will help in a dirtier environment. So nothing looks too challenging here.

Multiple decoupling caps of different values is bullshit (but, sadly, very commonly repeated bullshit... there are lots of cargo culting designers) and one should not do it in general. Just use the biggest cap you can afford in the smallest package you can tolerate and all will be well. (Of course, you might need some minimum value for bulk capacitance too.) In a nutshell, capacitor value doesn't drive decoupling performance, capacitor inductance does, and inductance depends only on the geometry/package size.

The power spiking is a normal MLCCs-as-bulk-capacitor thing and readily handled with damped capacitors (electrolytics or your explicit damping of MLCCs, both are fine). You should see the ringing stop with that configuration, but if you don't... well, then probably something is wrong, because damped rail bulk capacitors plus a TVS should eliminate that problem.

At 100us edge rates (us, not uS, uS is microsiemens... sorry, pet peeve ever since my teaching days....) you can honestly get away with just about anything. Hang a capacitor off those lines. Do CLC or CRC filters. Whatever you like, it'll probably work (at least SI-wise). If the ICs are truly unobtainable and protecting them is the true goal of all this mess*, I'm guessing (you still haven't said what they are, or even what sort of thing...) what they actually want is to never have their inputs exceed their Vcc by more than a diode drop (0.3-0.7V). So... give that to them. Use a diode to clamp each input to the rail, and another to clamp it to never go below ground. I'd start with dual series Schottky parts (BAT54S or one of its smaller relatives) unless you know their leakage won't work. Then just make sure there's enough rail capacitance and a big fat TVS on the rail to handle the cleanup, and you're done. This arrangement has the major advantage that you never violate the chip input specs even when unpowered. It sounds like a good fit for your actual requirement. If the rail clamping is not precise enough, there are options there, but precision clamping of a power rail is a different problem.

*That would define your threat model then! "Protect the 555 and 741 against damage from ordinary contact ESD strikes to connector J1, motor radiated pickup on J1's cable when connected, or hot-plug inrush spikes on their power rails."

[Infraviolet]

Clamping the 5V rail to itself isn't possible. That's why I was using the zeners set for 5.6V, and they should naturally serve to protect against reverse voltages anyway I'd strongly assume.

Would you say then that, although perhaps a bit excessive, the schematic I showed would work well for a threat model of:

"Protect some ATMEGA328s, some MCP6024s, and the logic pins of some TB6612s, as well as some photodiodes against damage from ordinary contact ESD strikes to connector J1, nearby 12V motor radiated pickup on J1's cable when connected (motor related conductors 1mm away at some points), and hot-plug inrush spikes on their power rails."

Thank you

[David Hess]

Clamping the 5V rail to itself isn't possible. That's why I was using the zeners set for 5.6V, and they should naturally serve to protect against reverse voltages anyway I'd strongly assume.

Since bipolar transistor base-emitter breakdown is higher than 5 volts, a PNP bipolar transistor can be used in place of the diode with the collector tied to ground, so a voltage excursion above the positive supply gets shunted to ground.

For more sensitive circuits, another transistor or diode can be used to make a reference which is 1 Vbe below the positive supply, so that the input gets clamped at exactly 5 volts.

[Infraviolet]

Further to my note in post #10:
Would TVS diodes of 5.5V Max Reverse Standoff, 6.2V Min Breakdown and 16V Max Clamping be good? Or is this something where anything would do so long as the Min Breakdown is above 5.5V, and a higher clamping would be absolutely fine as the other parts in this circuit, particularly the zeners, would prevent >5.6V reaching the various chips?

[Infraviolet]

Would a 6.2V Min breakdown or lower be necessary then? or could one get away with higher voltage breakdown's TVS diodes, this latter sort being easier to find options for, and let the 5.6V zeners do further voltage reduction? Thank you

[ledtester]

I'm thinking about protecting custom circuit boards which may be plugged and unplugged from their data and power lines (lines up to perhaps 2m length max) while the lines are powered. ...

I would also get some input from the E2E forum at ti.com:

https://e2e.ti.com/

Of course they are going to recommend TI parts, but they should also give you some good general advice in case you want to use another vendor's parts.

[Infraviolet]

Does TI do zeners and TVS diodes, or only bigger "all functionality for everything in one black box" general protection ICs?

[ledtester]

Here's the overview of the circuit protection offerings:

https://www.ti.com/interface/circuit-protection/esd-surge-protection-ics/overview.html

Clicking on a package like "SOT-23" should bring up individual discrete devices, for instance.