GDT surge life question.  - Page 1
Forum section: electronics / projects

author: Electr0nicus
author_rank: Regular Contributor
timestamp: October 25, 2018, 12:50:26 pm
content: HiI have a special problem designing a product.The product itself will be used in a very harsch environment where 50 lightning- induced overvoltage occurences are possible per week.Also the customer wants the device to last for at least 15 years, which means 39000 overvoltage events in total.The overvoltage pulse is defined by the customer as follows:1,2kV superimposed on the AC sine wave.500A surge current1,2/50us surge duration.So i need to use a GDT. I've searched Littelfuse's homepage, but all their products have a way to low surge life. Mostly 400 shots. The pulse in Littelfuse's datasheet is a 10/1000us, which is longer and has more energy. Does that mean, that this particular GDT can handle more overvoltage events, due to my shorter pulse.Also the overvoltage protection circuitry is not servicable, because the whole electronics will be potted and can not be accessed later.Is there a possibility to design a overvoltage protection circuit which can last that long, and for that many overvoltage occurences? Are there mayby other devices which are better suited for my application?How fucked am I?Cheers

author: IconicPCB
author_rank: Super Contributor
timestamp: October 25, 2018, 01:07:11 pm
content: There is no protection from lightning strike.Renegotiate the clients specification.Use a GDT in combination with a bit of series inductance and a MOV

author: BravoV
author_rank: Super Contributor
timestamp: October 25, 2018, 01:35:57 pm
content: There is no ABSOLUTE 100% guarantee when it comes to protection from the wrath of mother nature, as simple as that, period.Just tell your customer the story on Japanese Tsunami walls, there were built with centuries of wisdom & experiences, still, the last 2011 great earth quake wiped off thousands of human lives there, turned those walls useless, just google it, plenty of it.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 25, 2018, 02:08:46 pm
content: There's more than the GDT / MOV way to deal with overvoltages.GDTs reflect the pulse energy back to the source (by creating a short) and so they have to stand the surge current and suffer from electrode degradation or whatever.MOVs partially absorb and partially reflect the energy by limiting the voltage (but do not short the line). They suffer from repeated absorbing the pulse energy and somewhen explode.If you find a clever way to not have to absorb or reflect the energy (e.g. by shortly opening the circuit by an appropriate switching device), one can create a protective device that can stand way more usage than a GDT / MOV. If your device is low energy itself, an appropriate series impedance and a TVS (these don't degrage as long as you don't exceed its ratings) may work fine. In the end, usually some combination of these techniques does the job. Maybe you go research telecom subscriber line interfaces and railway power supplies, those usually have to stand a lot of "abuse".

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 25, 2018, 02:26:32 pm
content: There are some huge TVS on market rated even for kAmps surges, but prices are rather highDepend of what they want to pay

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 25, 2018, 02:29:20 pm
content: Consider a reliable clamping method. There's a patented method, this might be it?https://patents.google.com/patent/US5617284That doesn't quite look like it, but anyway the point is to use a series choke to increase the transient impedance, and rectifier diodes and electrolytic capacitors to absorb the current and energy.Note the inductor must be air cored, since any other type will saturate under surge conditions, and not present as much series impedance as intended.Diodes need to be rated for peak surge*, and capacitors should be alright as they aren't going to heat up. Finally, a bleeder resistor is sized to discharge the capacitors back to mains peak voltage before the next strike is expected (you might use enough capacitance to handle two or three strikes in rapid succession, just to make sure).*The short-duration peak rating is what matters. This is usually a bit above the surge (I_FSM, typically 8.3ms half sine) rating that is commonly specified. Some diodes have a short duration peak rating (e.g., 10us square), which is limited by device physics (probably electromigration). Exceedingthatrating will lead to eventual failure, even if the pulse duration is very short and the device does not heat up in operation (I've done it before, in a snubber application: even 12A diodes don't like 200A pulses, even just for fifty nanoseconds; 30A diodes were required). I suspect the failure method is electromigration.Anyway, all of those things are manifestly available, and reliable: inductors, rectifiers and capacitors. (The capacitors should also last quite long, since they're not heating up, and are maintained under bias.)Note that, if your main load already uses a rectifier and large filter caps, it probably already behaves this way, and can be made reliable with only small changes!There are also enormous (and pricey) TVS diodes that are capable of withstanding mains surge, which you may find useful, alone or in combination with other methods.Any number of MOVs can also be connected in parallel, reducing the energy per part and extending lifetime arbitrarily. Life vs. peak current curves are often provided in the datasheet, take a look.None of these methods will be terrifically cheap, i.e., $50+ in parts, plus design and assembly costs. Such is the price of a specification which one reply suggests the customer reconsider.....Tim

author: Electr0nicus
author_rank: Regular Contributor
timestamp: October 25, 2018, 03:24:11 pm
content: Thank you for your answers so far, I will look into it.The customer is really stubborn concernig their specifications.Good for me is, that price is not necessarily a problem. If the circuit does what it should do, it can cost some money, because it's no consumer product I'm developing, and the volume is really small.Cheers!

author: ConKbot
author_rank: Super Contributor
timestamp: October 25, 2018, 03:37:17 pm
content: If money isnt an issue, sounds like they need a service contract for someone to come out every month or two and swap GDT modules, and ensure functionality of the equipment protecting their super important equipment.

author: TimNJ
author_rank: Super Contributor
timestamp: October 25, 2018, 03:45:13 pm
content: It seems like the parallel MOV route probably won't be good enough for your application, but thought this would be cool to share anyway. Littelfuse has iTMOV/TMOV line with integrated thermal fuse to disconnect the MOV from the line instead of lighting on fire. The iTMOV parts have a third lead which can be used to sense whether the MOV has failed or not. Would indicate its time to replace the unit.Anyway, probably of no use.http://www.littelfuse.com/products/varistors/thermally-protected/itmov.aspxhttp://www.littelfuse.com/products/varistors/thermally-protected/tmov25s.aspxhttps://www.mouser.com/ProductDetail/Littelfuse/TMOV20RP320M?qs=nXjUL3qLHyXKFeeGMyVnwQ%3d%3d

author: joeqsmith
author_rank: Super Contributor
timestamp: October 25, 2018, 04:51:55 pm
content: Quote from: Electr0nicus on October 25, 2018, 12:50:26 pmI am interested in knowing a little more about the environment you are designing for. The above would seem to the standard surge test for the AC mains. Are they suggesting their mains gets hit like this? Do they use the normal 60 second interval?The 500A is what they have available from the combo generator into a very low impedance load. It does not mean that they are putting 500A through your circuit. Also, keep in mind that the waveform you listed is the open circuit voltage. There is another waveform for the current.Why do you "need" to use a GDT? You have provided no information about your load. Let's assume you are connected to some AC lines, are you pulling a 200A or 2uA? If you're limited to 2uA, that's fairly low energy for a GDT.Provide a bit more information about what you are attempting to do and you may get some better answers.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 25, 2018, 06:42:58 pm
content: Quote from: Electr0nicus on October 25, 2018, 12:50:26 pmYou may take a look at the Bourns 2027-xx-SM or 2027-xx-XX series which are heavy duty GDTs.A GDT would have to be complemented with additional protection means (current limiting, TVS diodes...) IMO.Still, with your possible number of strikes...Quote from: Electr0nicus on October 25, 2018, 12:50:26 pmYou are.I wonder what kind of environment that would be?Seaside? Or mountain tops?

author: coppercone2
author_rank: Super Contributor
timestamp: October 25, 2018, 07:20:58 pm
content: Won't movs need to be balanced too? Some might end up getting insane peaks.Do they kinda wear out evenly or something? (like sanding metal or wood). Even if they wear down to trigger at the same voltage, won't the leakage of the worn ones be much higher?

author: jmelson
author_rank: Super Contributor
timestamp: October 25, 2018, 08:23:00 pm
content: Quote from: BravoV on October 25, 2018, 01:35:57 pmTokyo Electric knew about tsunamis, and CHOSE to just IGNORE all the centuries of experience. They KNEW that they'd get hit with a Tsunami EVENTUALLY, but just didn't want to deal with it. Incredibly, the Japanese government just let them do something WAY stupid.As for GDTs, there is NO WAY you can have a tube survive 39000 events without failure. I use them on my incoming phone lines, and every ten years or so, they get "leaky" and have to be replaced, when they hold the lone off-hook.Jon

author: floobydust
author_rank: Super Contributor
timestamp: October 25, 2018, 10:00:24 pm
content: Engineering always gets silly requirements and demands from customers or product management.You have to push back, say "no" and explain the cost/design time required to implement the silly feature.A product in a perpetual lightning storm? Maybe on Mars or Jupiter.For an ignitor application, I needed 100K lifetime with low current <0.1A discharges.I talked to Bournes and they advised, for low current discharges, that the GDT aging mechanism is electrode etching.You can't get long life with high discharge currents, something about the plasma reacting with electrode metals.OP is better off to "ride through" the surge instead of hard clamping it entirely.Add series resistance or inductance (good for HV), then clamp that lesser current with MOVs, or something without negative resistance.

author: BravoV
author_rank: Super Contributor
timestamp: October 26, 2018, 04:14:43 am
content: Quote from: jmelson on October 25, 2018, 08:23:00 pmJon, its not the nuclear reactor I was talking about, its the tsunami barrier walls that were built at many of the coastal cities, and the wall's height was considered safe or good enough according to their knowledge gathered thru centuries of experience + from ancient geological water flood markings caused by ancient tsunamis, yet, they were breached big times at almost all cities, killing so many humans.The moral story, don't underestimate mother nature, thats all.

author: riccardo.pittini
author_rank: Regular Contributor
timestamp: October 26, 2018, 07:04:29 am
content: I designed the surge protection for outdoor /street light.We had a test running continuously and it took 5kV/2.5kA (1.2/50us standard surge) about 2500 times (then actually we stopped the test but it could have taken more-not any sign of visible degradation)It was embedded as part of the EMC filter based on a big MOV + GDT +small MOV.Degradation is mostly related to energy and current. As you have 500A, and degradation is not linear. I think it's feasible. It takes a detailed analysis (simulations and test) of your input circuitry.

author: Electr0nicus
author_rank: Regular Contributor
timestamp: October 26, 2018, 08:26:28 am
content: Yes it is a ship based system. Actually the hardware is mounted on deck. I can not disclose the exact application as this falls under a NDA with the customer. But the electronics, which is powered, is a heating application with integrated temperature regulation. So the temperature regulation is the sensitive hardware. The power intake is 4000W from a delta 115VAC system.I would consider using a multiple stage overvoltage protection, consisting of GTDs, MOVs and TVSs and add a bit of series resistance after each stage.The power components (rectifiers, MOSFETs etc.) are designed to withstand a minimum of 650VDC. So the protection circuitry has some room to fluctuate, it doesn't need to clamp the overvoltage to a exact narrow band value.Cheers.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 26, 2018, 09:42:30 am
content: I would assume the electronics that you need to protect draws very little current. The drivers are in series with a resistive load and the load itself could very well handle the surge. It may not be that big of a deal. Do they use the standard 1 minute interval?

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 26, 2018, 03:28:48 pm
content: Quote from: Electr0nicus on October 26, 2018, 08:26:28 amI was thinking of either that or an offshore platform.Quote from: Electr0nicus on October 26, 2018, 08:26:28 amProbably your best bet. I would tend to avoid MOVs, they are great but I don't like their failure modes for very long-term, maintenance-free devices.Hopefully you have means of testing this in your lab so you can iterate and don't have to wait till your design is fully completed and the client tests it, and it fails.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 26, 2018, 06:07:46 pm
content: Is this device subject to possible direct strikes, or is it shielded and supplied in such a way that the customer can guarantee the surges are maximum (not typical) the amplitude and duration given?Example: conventional mains distribution combines lightning arrestors on distribution lines, with transformers, and nominal air gaps, that ensures end-point surge (at 120/240V depending) has a maximum around 5kV, with the traditional 1.5/2.5kV surge levels being most common.Contrary example: telecom lines are typically not clamped periodically, so any induced or direct lightning surges are carried on the lines. Upside, the lines don't carry much power, the resistance is high (typically 20 ohms is used in the test). The lack of clamps means the surge is longer, hence the 10/1000us waveform.The catch is, you can construct a system that degrades more or less gracefully with increasing duration or amplitude. There may be useful gains in reliability or size/cost for a less graceful system. Example, the series-inductor, diode-shunt-capacitor method I mentioned above: ~unlimited life within ratings, but the clamp voltage will greatly exceed the capacitor's if the surge is too long, and the diode (and clamp voltage) ratings will be exceeded if the amplitude is too high. Or both.In short, the distribution of surge amplitude/duration matters on this level!Tim