Automotive Load Dump: What % of Vehicles Have a Centralized Clamp Diode? - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: September 07, 2021, 04:22:58 pm
content: Greetings,I've read through no less than five lengthy threads on this forum on the topic of automotive load dump. Some people say it's a problem of yesteryear; some people say it's still an issue. Simply, the issue is whether or not an electronic device attached to the automotive DC power bus needs to be able to withstand a high peak voltage (100-200V), long duration (200-400ms, decaying) waveform.Most of this discussion has been somewhat anecdotal, which is fine, but I'm interested if anyone has a more broad picture of the issue. i.e. Which manufactures build in centralized load dump suppression, and which do not? With the amount of electronics onboard new vehicles, I'd think it would be well worth to use centralized load dump...but I don't have a clear picture on this.LV 124 (Daimler, BMW, Audi) seems to imply the use of centralized load dump suppression. But, I am not sure about other manufacturers, namely those of American, Japanese, Korean, and Chinese origin. I assume many use their own in-house standards. Anyone familiar with the names of these other standards? Is the information publicly available?Thanks!Tim

author: Stray Electron
author_rank: Super Contributor
timestamp: September 07, 2021, 05:07:20 pm
content: I assume that you mean some kind of voltage clamp such as a zener diode or a crowbar circuit. I have the electrical manuals for a 2000 Mitsubishi Diamonte and the 2010 Ford F-150 and some other cars and there are no clamps mentioned in any of them. If there are any clamps they're probably internal to the various modules in the vehicles but they give very little detail about the internals of those. I've worked on hundreds of vehicles since the late 1950s and I've never seen one that had any sort of clamp, other than the vehicle's main battery. IIRC the Diamonte used something like 11 different microcontrollers in the various control systems and you can be pretty certain that all of them are powered through some kind of clamp or regulator.Note; and before someone asks, the "voltage regulator" in vehicles only controls the battery charging system. It does not directly control the DC voltage distribution system. If the alternator shorts, the regulator fails or if an excess external voltage is applied, the DC voltage system will exceed it's design potential. Been there, done that.

author: Stray Electron
author_rank: Super Contributor
timestamp: September 07, 2021, 05:09:15 pm
content: PS, there's no central load clamp in the BMW Z3 either.

author: bdunham7
author_rank: Super Contributor
timestamp: September 07, 2021, 05:31:49 pm
content: I have never seen (or at least recognized) such a device as a standalone unit. I don't think the standard load dump characteristics are as high as 100V, although I don't have any actual standards to read. And keep in mind that while very short duration pulses may not be absorbed by the battery due to inductance in the wiring harness, the many-millisecond sort of overvoltages simply cannot coexist with a functioning LA-battery in place. These sorts of things happen primarily if the battery fails open, is frozen or if there is a wiring problem. Otherwise, the LA-battery will absorb many thousands of amperes before it even allows the voltage to rise to double its nominal value.

author: richard.cs
author_rank: Super Contributor
timestamp: September 07, 2021, 05:33:06 pm
content: My understanding is that load dump clamping is handled by the alternator diodes having an intentionally low breakdown voltage of 60-100 V (the battery will normally clamp it to much less, maybe 16 V, but only if in good condition). I wasn't aware of any central load dump protection beyond that.Common automotive standards specify a load dump spec as you describe, this is the waveform that remainsafterthe other clamping in the system has taken effect.Whether you need to worry about it depends on a) is it a product where you need to follow a particular standard? b) can you live with a small probability of it being damaged?

author: TimNJ
author_rank: Super Contributor
timestamp: September 07, 2021, 06:11:07 pm
content: Quote from: Stray Electron on September 07, 2021, 05:07:20 pmQuote from: richard.cs on September 07, 2021, 05:33:06 pmYes...a substantially sized zener diode, but as richard.cs states, I think it is more likely to come in the form of a rectifier with zener breakdown characteristic. For example:https://www.semicon.sanken-ele.co.jp/sk_content/sg-c17vlz40r_ds_en.pdfHence, it may make sense that no one has seen a standalone clamp solution because the alternator rectifier diodes are "dual purpose" if you will.Taking a look at IEC60601-1-2 (which references ISO7637-2), there is a clause/note which allows testing w.r.t. to a clamped waveform if "central load dump protection is used". This (I think) matches your statement about the "waveform that remains after other clamping in the system has taken effect". But the decision as to whether this is acceptable is completely at the discretion of the designer. I'm working on a universal product that could be plugged into virtually any vehicle in the world. Given this, it would probably be logical to play it safe. But, on the other hand, since the DUT power consumption is >100W, series elements (other than low RDSON MOSFETs) are not desirable for normal operation, so the best approach (I guess) would probably be a large parallel element, relying on wiring inductance/resistance for the voltage drop. These are big and somewhat expensive of course.Thanks!

author: Ian.M
author_rank: Super Contributor
timestamp: September 07, 2021, 06:16:25 pm
content: If its going to be anywhere it would be in the alternator. Its already got the high current connections, is the most likely source of a voltage surge, and doesn't require an extra module to be mounted and connected up, so maybe you should research OEM alternator specs?

author: TimNJ
author_rank: Super Contributor
timestamp: September 07, 2021, 06:20:19 pm
content: Quote from: bdunham7 on September 07, 2021, 05:31:49 pmGood information. So the battery acts as an electro-chemical clamp of sorts? Trying to think back to electrochemistry in college to understand what happens in that moment, but I suppose the voltage source + series resistance model works okay too. As I read in some other threads, some people disagree with the philosophy behind load dump protection, implying that "you have other more serious problems to worry about" if you get a 200V load dump spike on the DC bus.

author: langwadt
author_rank: Super Contributor
timestamp: September 07, 2021, 06:37:10 pm
content: Quote from: TimNJ on September 07, 2021, 06:20:19 pmThe commonly used protected FETs to control loads like fuel injectors and various control solenoids clamp at ~40V

author: Ian.M
author_rank: Super Contributor
timestamp: September 07, 2021, 06:38:06 pm
content: Ground straps from the engine to the chassis or battery negative do fatigue. They are often poorly protected against corrosion so tend to do so faster than the positive connections. Therefore loss of battery connection while the engine is running in an older vehicle is not as rare as one would like.The question is: is your gadget likely to be the weakest link*and if so, is it likely to be far enough out of warranty that you wont have to worry about refusing free repairs damaging your company reputation? Alternatively, is it profitable enough you can afford to eat the cost of replacing the small number that will blow due to a load dump?* The end user is unlikely to get very far suing your company if most of the rest of the vehicle's electronics blew in the same incident.

author: TimNJ
author_rank: Super Contributor
timestamp: September 07, 2021, 06:42:52 pm
content: Quote from: Ian.M on September 07, 2021, 06:16:25 pmRight, this would be great if I knew where to look. Seems like the main people who own these specs are OEMs themselves and I assume they paid for them?For what it's worth, I thrashed around a bit on Google, found some replacement alternator rectifier plates. All parts I found used the term "avalanche" when talking about the diode types, which implies clamping functionality. But, seems replacing the rectifier plate by itself is not a very common thing to do...you'd probably just replace the whole alternator in most cases.

author: Ian.M
author_rank: Super Contributor
timestamp: September 07, 2021, 06:57:45 pm
content: Yep, its usual to replace the alternator, but there's often a 'core charge' on the old one that's a trade in discount baked into the 'new' one's list price. If you don't have one to trade in or its totally FUBARed, it costs more, so *someone* must be rebuilding them.

author: TimNJ
author_rank: Super Contributor
timestamp: September 07, 2021, 07:03:14 pm
content: Quote from: Ian.M on September 07, 2021, 06:38:06 pmThat's a good point...though hard to say without a ballpark probability of occurrence.I suppose a general disclaimer can be made upfront that clearly outlines the DUT's tolerance to load dump. This puts more responsibility on the end-use installer to check with each vehicle. But, in the event that DUTs start failing, each will require some autopsy (which takes engineering time) and then follow-up negotiations (arguments?) with the customer regarding who's at fault. That's not great either.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2021, 08:01:56 pm
content: If there was a separate load dump clamp, then someone would be selling replacements and I have never seen such a thing. The battery provides this function.Alternator diodes fail occasionally and used to be replaceable, but their voltage rating varies wildly. Manufacturers are just cheap. Nobody is grading alternator diodes for controlled breakdown, although they might be avalanche rated for reliability.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 07, 2021, 08:07:27 pm
content: Quote from: TimNJ on September 07, 2021, 06:20:19 pmIt's my understanding load dump doesn't really...happen? With a battery? I guess the same physics should apply, the regulator can only slew so fast regardless, it should experience a swell from any load-drop condition, switching lights, motors, etc. Though maybe none of those loads are strong enough to overpower the battery, in terms of height or duration of that swell (it might be shorter than the full load dump case, even if it's the same time constants in play, which might also not be the case).As for the battery itself, it behaves pretty much symmetrically in charge or discharge, up to the point where charging is complete, and where other reactions take over -- primarily water electrolysis. Electrolysis is a runaway condition, where due to bubble formation, the electrolyte cross-section is reduced, greatly increasing resistance, allowing terminal voltage to rise further. I would guess a car battery can do a good 20, maybe even 40 or 50A for brief charging in a low-state-of-charge condition (terminal voltage < 14.4V), but won't sustain such high currents for long; and in particular, as cells begin to fully charge (and this happens inhomogeneously across the plates: the facing surface has a direct line of sight; the pores behind, not so much), gas formation takes over and internal resistance rises. I would guess the current drops to ~20A steady-ish-state for terminal voltages in the 16-18V range. So there's a bit of a negative resistance characteristic, over a fairly long time scale (seconds).As to philosophy, heh, for anything noncritical, indeed, who cares. The point of automotive tests is, everything built-in should last the lifetime of the vehicle. So it's worth testing even fairly rare events like this. But for like plugin aftermarket stuff, who cares, really. For hard-wired aftermarket stuff, well, I suppose that's up to the user, but one might argue the user isn't exactly inexperienced with replacing such things (obviously, it's been done once!). Or will necessarily be irate about random on-road failures, should it happen at a very inopportune time...Speaking of aftermarket, it could well be that a replacement alternator might not be equipped with avalanche diodes, so the electronics should be protected even if everything is nominally to spec. One should hope such parts don't make it onto the approved replacement list, but who knows...Tim

author: bdunham7
author_rank: Super Contributor
timestamp: September 07, 2021, 08:35:21 pm
content: Quote from: T3sl4co1l on September 07, 2021, 08:07:27 pmThe main driver is the time constant of the field coil, which is typically a few hundred milliseconds. So yes, any sudden significiant change in loads (headlights, defroster, etc) will typically cause a sag or surge.QuoteWhere do you get that? Modern (as in the last 50 years, calcium cycle or not) automotive type batteries are designed for the purpose and certainly don't behave this way in my (extensive) experience. A typical medium-sized battery can take 40-50 amps of chargecontinuouslywithout exceeding 15-16 volts although it obviously will get a bit warm and smelly if you keep it up too long. The internal resistance goes down as they get warm and those bubbles actually circulate the electrolyte. I've seen the occasional full-field failure and the result is that a 100 amp alternator can bring a battery to a rolling boil without getting anywhere near 18 volts. If you keep it up obviously internal damage occurs, gas builds up and perhaps you start boiling it dry, but we're talking about short term surges here--and even a fully charged battery can absorb huge currents, certainly far more than any appropriately-sized alternator would put out.

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 07, 2021, 08:37:20 pm
content: Automotive power can be a messBattery can fail open, this makes voltage be crazy, but the car still runs (I experienced this once, lights was little uneven, but everything works, even AC)Also, you have plenty of high current DC motors switched with relays what can make huge spikes when run and stops

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 07, 2021, 08:48:25 pm
content: Well, I've measured it...Don't forget there's also gelled (sealed or vented) and AGM types out there. I don't remember which one I measured.Yes, good point, as temperature rises, ESR falls; conductivity rises and viscosity falls (so, bubbles have less effect). So over a time scale of some minutes (thermal time constant), this also takes effect.Tim

author: bdunham7
author_rank: Super Contributor
timestamp: September 07, 2021, 08:51:44 pm
content: Quote from: David Hess on September 07, 2021, 08:01:56 pmIt would make the most sense to simply use this type of rectifier in the alternator because they already have large, well heat-sinked diodes. They don't have to be very precise, just limit the surge to 40-50 volts maybe. Of course they can't do that for very long--just absorbing the full output for a fraction of a second will be in the kilojoule range of energy. So perhaps this method has caught on and if it allows them to use less protection elsewhere, that takes care of 'cheap'.However, I'm sure this wasn't done in at least some older alternators because some of those had quite high open-circuit voltages (>100V). There were even a few cars that would divert the alternator for a few minutes and operate it at a much higher voltage to power a heated windshield, IIRC @ 90-100V.

author: bdunham7
author_rank: Super Contributor
timestamp: September 07, 2021, 09:03:57 pm
content: Quote from: T3sl4co1l on September 07, 2021, 08:48:25 pmMe too!But yes, I'm certainly only talking about a conventional liquid-electrolyte automotive-specific starting battery, whether 'sealed (they really aren't) or otherwise. Something like the Mazda MX-5 battery or any non-automotive SLA will not tolerate such treatment, although most would still absorb a significant transient surge. Other than chemistry, the main feature of the automotive starting battery is the grid design that maximizes surface area above all else--which is how they manage their 10C+ maximum discharge rates. Interestingly (to me anyway) even my all-electric car has one of these for the 12V system, and it is in fact a starting-type battery, not a deep-discharge or AGM or anything like that.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2021, 09:31:57 pm
content: Quote from: T3sl4co1l on September 07, 2021, 08:07:27 pmA load dump happens when the battery gets disconnected while charging. Normally one would not expect a terminal to become disconnected however a common failure as batteries reach the end of their operating life is for a connection to a plate inside to crack making the battery high impedance.QuoteThe field winding of the alternator has a couple millihenries of inductance (1) so considerable energy is stored in the field and the high inductance limits the regulator's frequency response.(1) Personal measurement using an ESI 250DA impedance bridge. My neighbor spotted me and became very wary.

author: TimNJ
author_rank: Super Contributor
timestamp: September 07, 2021, 10:15:28 pm
content: Thank you all for the insightful discussion, some of the best I've seen on the topic!To me, it seems that the likelihood is low given that:1. The battery is replaced regularly and not allowed to work to complete failure ("plate crack")...maybe true of a fleet vehicle, though I've never managed a fleet to know.2a. The vehicle is relatively new, with ground strap and battery terminal clamps in good condition2b. The vehicle is not used in a corrosive environment (high humidity, salt water, etc.) which may speed up corrosion of the above conductorsIn the event that the battery does become disconnected for whatever reason, then it seems the avalanche rated alternator rectifiers probably have a high likelihood of clamping to some (reasonably safe) voltage like 40-50V. But, the question remains as to the prevalence of avalanche rated rectifiers in the vehicles on the road today.Here, Motorola from 1987 talking about "new" rectifiers which clamp alternator voltages:https://www.jstor.org/stable/44472894To me, it seems like paying once for centralized suppression would be better than X amount of times, per module onboard. Plus, you have to pay for alternator diodes anyway...so just use the avalanche type and you get built in suppression for (possibly) free. But again, no idea if the adoption of avalanche type diodes is widespread.

author: Ian.M
author_rank: Super Contributor
timestamp: September 07, 2021, 10:23:33 pm
content: 2b. Unfortunately 27 US states count as a "corrosive environment" in the winter. Seehttps://en.wikipedia.org/wiki/Salt_Belt

author: bdunham7
author_rank: Super Contributor
timestamp: September 07, 2021, 10:37:42 pm
content: Quote from: TimNJ on September 07, 2021, 07:03:14 pmIf you are making a gizmo to go into any and all cars, it becomes bit trickier--but if you are not required to meet a particular standard with a particular specification for this and it is not safety-related (like an airbag controller) or otherwise super-critical, then simply using an input regulator that can withstand 35-40V is probably enough to reduce failures to a vanishingly small number. And keep in mind that nothing can provide 100% protection--I've seen at least two vehicles that were struck by lightning and one that was jump-started from a large truck with a 24-volt system--in reverse polarity to boot.However (and this is the voice of experience...) if you are making a gizmo that can be put into any type of vehicle, be aware that some vehicles may have pretty horrific shorter-term noise and spikes, even some pretty nasty common mode stuff--so avoid multiple ground points and metal cases if you can. Automobiles have actually vastlyimprovedin this area over the past 4 or 5 decades, simply out of necessity. However, if your gizmo gets installed in older vehicles, vehicles with compromised (corroded) ground connections, commercial vehicles that have been retrofitted (RVs, buses, work trucks,vehicles with electric winches, or any number of similar electrical horror shows, it needs to be pretty robust noise wise. Oh, and add reverse polarity protection for sure, and always a fuse someplace just in case.

author: TimNJ
author_rank: Super Contributor
timestamp: September 07, 2021, 10:38:57 pm
content: Given that it's not really practical to review every alternator ever made, I think reviewing the performance standards for each auto manufacturer may give good insight into the situation. Only because I saw someone mention it in another thread, I am aware of LV124 for German manufacturers. For a 14V nominal system, the peak transient voltage stated is 27V (although I see a different reference stating the LV124 limit is 42V). Nonetheless, there's no mention of anything >100V, seemingly implying that some centralized suppression must be used for those vehicles.So, I am curious to know what standards other manufacturers design according to and if they are publicly available, i.e. for aftermarket equipment manufacturers, etc. Then, I can do a survey of the common auto manufacturers and see if any of them talk about the >100V load dump scenario that is commonly referenced in various ISO/IEC standards.

author: bdunham7
author_rank: Super Contributor
timestamp: September 07, 2021, 10:45:52 pm
content: Quote from: Ian.M on September 07, 2021, 10:23:33 pmMoving from the Midwest to California was quite a change--cars can last forever here! (if you don't mind the no-paint look). However, even in the salt belt, OEMs have gotten pretty good at designing and manufacturing cars that functionally resist corrosion and while minor electrical issues do arise, the car is more likely to fall apart before it suffers a corroded-off ground or the like. With exceptions of course. However, vehicles that have been refitted--work trucks, ambulances, etc often have some pretty interesting problems because they aren't as carefully engineered and tested. Don't make splices or grounds under the vehicle!

author: Stray Electron
author_rank: Super Contributor
timestamp: September 07, 2021, 10:55:44 pm
content: Quote from: bdunham7 on September 07, 2021, 08:51:44 pmThe 94 Cadillac that we owned had an electrically heated windshield and it used a special alternator that had a second coil in it that produced roughly 100 VAC to heat the windshield. We never used that feature and when the alternator failed, I replaced it with a standard alternator.

author: langwadt
author_rank: Super Contributor
timestamp: September 07, 2021, 11:13:55 pm
content: Quote from: bdunham7 on September 07, 2021, 08:51:44 pmI've read of people using alternators as welders. Regulating the field to set current, ~100V open circuitand that some need diodes replaced because they fail at ~30V

author: TimNJ
author_rank: Super Contributor
timestamp: September 07, 2021, 11:22:14 pm
content: Quote from: bdunham7 on September 07, 2021, 10:37:42 pmThank you for your inputs. The DUT does in fact have to meet the requirements I mentioned above per ISO7637-2. The standard lets you test a load dump scenario according to either the "a" waveform (unclamped @ ~200V peak) or the "b" waveform (clamped, peak voltage to be defined by manufacturer). The particular project I am talking about is for a paying customer and they will ultimately make the call whether they think "a" or "b" is appropriate. (A little weight off my shoulders.)This thread is mainly for my own edification, and possibly to help answer any questions the customer may have if they are not fully educated on the matter. (Fine line to dance on there.) The end-use application is fairly critical, but it's still reasonably high volume with cost constraints, so I thought it reasonable to think twice before throwing $5-6 in chunky load dump TVS diodes at it.Good points about vehicles with miscellaneous electro-mechanical gizmos.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 08, 2021, 12:53:21 am
content: QuoteAt least back in the my younger years, this was correct. I think we tested for 40 then 60V peak. There were many automotive parts that came out that would call out specifically to handle the 60V load dump. There was a limited number of cycles they had to survive (MOVs were common back then, before the fires and moving to active protection). I may still have some of the early standards that called out how to construct much of the equipment used to run these tests. I want to say the fast transient was an AC clutch. Back then, I also think for inside the car, we had to test -40 to 85. Jump start was another. Seems like 24V for minutes....If you work in automotive, I wonder how you calculate the MTBF today? What standards are used? I started out, we were still using the military standards. Then PREL was going to be the next big thing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 08, 2021, 12:56:13 am
content: Yeah, also 7637-2 comes in different levels, corresponding to different ratings/sizes of alternators.Or I assume it's something like that; I've not actually seen an analysis of alternator output impedance, and how that varies with regulator design (not that that would be relevant since solid-state regulators took over, long ago), RPM, etc. A notable omission, for something so important to industry. But I suppose such background is really only relevant to the writers of such standards...Quote from: David Hess on September 07, 2021, 09:31:57 pmAnd not just that energy, but that energy multiplied by the engine's motive power -- it's an amplifier after all. It would seem reasonable for regulators to be designed to fly back, to allow faster dumping slew rate -- loading-up would still be limited by supply voltage -- but I'm guessing they don't, and just stick with clamped rails for driving it. So the slew rate is pretty symmetrical. Anyway, dumping the fractional joule there would be nice, but since that's not done, we have to deal with the hundreds of joules from the other side. Bah!Tim

author: bdunham7
author_rank: Super Contributor
timestamp: September 08, 2021, 02:20:47 am
content: Quote from: TimNJ on September 07, 2021, 11:22:14 pmWell, if you have to meet the standard you posted, then that's one less decision for you! I read the IEC60601 spec that you posted--it appears this is a medical-related device in a 24-volt system, like an ambulance or something? That's one of the particular electrical 'horror shows' that I mentioned. I don't know how much power it draws or the typical way of doing this, but it seems to me that for anything using significant power the safest bet would be a regulator that can simply withstand the overvoltage on a continuous-except-for-thermal basis. In addition to not blowing up your device, you also don't want it to damage anything else by being a huge current sink during the surge.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 08, 2021, 02:46:00 am
content: Nowadays, many newer cars use a high voltage alternator (often doubling as the starter) and a DC/DC converter to step it down.

author: mikerj
author_rank: Super Contributor
timestamp: September 08, 2021, 09:34:33 am
content: Quote from: TimNJ on September 07, 2021, 10:38:57 pmISO-16750-2 is the current standard

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2021, 11:51:32 am
content: Quote from: T3sl4co1l on September 08, 2021, 12:56:13 amWhen I designed a replacement regulator, I clamped the field winding with a big fast recovery diode so decay would have been pretty slow, but it was never a problem. Mechanical regulators relied on slow decay since they chopped the field current pretty slowly but thinking about it now, I do not remember any diode. The relay was SPDT so maybe they were shorting the field winding when off?Today if I did it I would drive the field with DC from a switching regulator. The field winding resistance is about 3 ohms.

author: richard.cs
author_rank: Super Contributor
timestamp: September 08, 2021, 01:02:22 pm
content: Quote from: bdunham7 on September 07, 2021, 09:03:57 pmIn a Zoe it's a key part of the safety case, because it runs the vacuum pump for the brakes, lights, power steering, etc. after failure of the traction battery or DC-DC. To the point where it is tested every startup with a big contactor and a dump resistor to validate that the battery is in good condition with adequately low ESR.Quote from: langwadt on September 07, 2021, 11:13:55 pmAll kind of fun things you can do with them, but the efficiency is pretty dire. At high RPm and field strength, as you need for getting high voltages without rewinding, the iron losses are huge. I did play with a design once with an alternator rewound for 400V d.c., and the field winding driven with a 50 Hz sine wave, (resonated with large capacitor). Follow it with a chopper to restore the a.c. and it's a cheap way to get mains-like, almost-sinusoidal power, though with pretty crappy regulation.Side note, on older cars with dynamos load dump is less brutal because they're pretty good voltage sources (automotive alternators are much more current source like) just with a bit of series inductance.

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 08, 2021, 01:30:53 pm
content: Quote from: richard.cs on September 08, 2021, 01:02:22 pmIt makes sense, you need "safe voltage" to power those devices, so you can choose 12 or 24V (everything higher is not a safe voltage so cannot be easily used)And things like airbags, abs and so takes a decade to develop cost millions to billions

author: HighVoltage
author_rank: Super Contributor
timestamp: September 08, 2021, 01:35:20 pm
content: 42 Volts is the highest limit I have seen on an internal standard in Germany.

author: bdunham7
author_rank: Super Contributor
timestamp: September 08, 2021, 01:41:59 pm
content: Quote from: David Hess on September 08, 2021, 11:51:32 amYes, they only switched a few times per second and grounded the field when not energized so that the field current wouldn't change too rapidly. Letting the field go open results in some pretty high voltages.

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 08, 2021, 01:48:31 pm
content: Quote from: HighVoltage on September 08, 2021, 01:35:20 pmIt is for a dry environment I thinkCars are in wet/aggressiveBut there are multiple standards with various levels

author: richard.cs
author_rank: Super Contributor
timestamp: September 08, 2021, 02:15:20 pm
content: Quote from: bdunham7 on September 08, 2021, 01:41:59 pmReally? The mechanical regulators for dynamos (which are more common, as there was only a short overlap between alternators and mechanical regulators) buzz at a few hundred hertz.

author: TimNJ
author_rank: Super Contributor
timestamp: September 08, 2021, 02:34:47 pm
content: Quote from: bdunham7 on September 08, 2021, 02:20:47 amIndeed, it is for ambulance use. The power level is 100-120W. The DUT already uses a LT4356 protection IC, which allows ride-through up to 80V for ~100ms, 60V for ~300ms, and will cut the power completely if the fault/surge lasts longer than those durations. In this way, I believe it to already be fairly robust and will not act as a huge current sink for a large part of the load dump waveform. But due to the Vds(max) rating of the pass MOSFET (100V), can't guarantee it will survive a surge >100V. Of course, it seems simple: Just use a 200-250V MOSFET! The issue is that the additional losses incurred by the increase in Rds(on), due to increase in Vds(max), will not be acceptable during normal operation.So, to recap, the input protection circuit can handle the bottom "half" (24 - 100V) of a load dump surge waveform via linear regulation, but not the top "half" (100 - 200V). To handle the top half, it seemed reasonable to clamp that via parallel element. Yes, it will cause a current surge, but not as bad as if the entire waveform was clamped via TVS. Still, it costs money, and if vehicles are not really making these kinds of transients, then that is probably over-engineering. The first revision of ISO7637-2 is from 1990, and it is conceivable that the levels were selected based on the types of vehicles that were on the road at the time. But that's only a hunch, which is why I am seekingmanufacturer's standards, and not the ISO type standards.There are standards for construction of ambulances, see EN1789, but I don't think it is terribly explicit on all details, and who knows about adherence country to country.As I mentioned, the decision is ultimately in the customer's hands, but I want to be informed on the subject enough to know if their decision ultimately feels right, or if it will be a regret later.Thanks!

author: TimNJ
author_rank: Super Contributor
timestamp: September 08, 2021, 02:41:24 pm
content: Quote from: mikerj on September 08, 2021, 09:34:33 amThanks. Yes, as I understand it ISO-16750 supersedes ISO7637-2, although the medical EMC standard IEC60601-1-2 still references ISO7637-2 for automotive transients. But, same as the old standard, ISO16750-2 leaves the designer with the decision of decided whether the DUT needs to handle an unclamped (~200V) or clamped (~60V) load dump transient.

author: TimNJ
author_rank: Super Contributor
timestamp: September 08, 2021, 02:45:17 pm
content: Quote from: T3sl4co1l on September 08, 2021, 12:56:13 amAh. That explains the range of peak voltages and series resistances given in the standard. It's meant to represent a particular alternator..Obviously, in an ideal world, you could test to the highest voltage and lowest series resistance. This is what LTSPICE defaults to if you use their built in transient waveform models.Thanks.

author: TimNJ
author_rank: Super Contributor
timestamp: September 08, 2021, 02:49:49 pm
content: Quote from: HighVoltage on September 08, 2021, 01:35:20 pmThanks. That is what I saw too. Now, if I could only find what US, Japanese, etc. manufacturers design to, then I could probably get a reasonable picture of reality. Are they using SAE, JIS? Or do they design to some internal GM, Ford, or Toyota standard?Of course, as others have mentioned, once you start tacking on aftermarket equipment, as in an ambulance or RV, it is debatable whether these manufacturer standards are still relevant, as the vehicle starts to deviate quite heavily from the stock version.

author: richard.cs
author_rank: Super Contributor
timestamp: September 08, 2021, 03:06:52 pm
content: Quote from: TimNJ on September 08, 2021, 02:34:47 pmThis problem with this approach is that whilst the current surge is lesser, the power dissipated in the clamp element may be greater. Minimal clamp element dissipation is achieved with a crowbar, e.g. overvoltage triggers a shunt SCR, but then you have to rely on a series fuse opening to prevent damage (and then being replaced by the user).

author: TimNJ
author_rank: Super Contributor
timestamp: September 08, 2021, 03:17:35 pm
content: Quote from: richard.cs on September 08, 2021, 03:06:52 pmSo, you mean to say: Vz * Iz may be greater for higher values of Vz (for instance, ~90V, compared to ~40V)? I actually find that somewhat non-intuitive. I'll try some simulations.This is a sealed enclosure, no service is really possible, unfortunately.

author: richard.cs
author_rank: Super Contributor
timestamp: September 08, 2021, 03:28:34 pm
content: Quote from: TimNJ on September 08, 2021, 03:17:35 pmAn alternator at high output current and high engine rpm (and hence frequency) is a voltage source (the e.m.f. induced across the windings) of perhaps 100 V, in series with several Ohms of winding reactance. Hence my earlier comment about them being current-source-like because they are high-impedance with an open circuit voltage >> the regulated voltage. What this implies is that for transient events outside of the regulator bandwidth, the maximum power point occurs at about 100 V / 2 = 50 V.

author: bdunham7
author_rank: Super Contributor
timestamp: September 08, 2021, 04:21:04 pm
content: Quote from: richard.cs on September 08, 2021, 02:15:20 pmI don't recall the Delco generator regulators buzzing quite that fast, but perhaps other models did. However, yes, the GM/Delco alternator regulators would snap back and forth with a quiet but distinctive rhythm and you could directly (visually) observe the duty cycle! It's been a long time, but I'd guess 5Hz?

author: bdunham7
author_rank: Super Contributor
timestamp: September 08, 2021, 04:37:51 pm
content: Quote from: TimNJ on September 08, 2021, 02:34:47 pmThis is the sort of thing that would drive me crazy! Am I correct in assuming that since the current draw is only ~5A, the objection to the higher RDS-on of the higher-voltage MOSFET is a budget/thermal/space on PCB type concern and not a voltage drop issue which could be addressed with larger/multiple devices? That said, you probably are really unlikely to see >100V extended surges, so if you design it to withstand 100V/surge and 60V/forever, and then add on something to meet the standard you'll likely be fine. However, keep in mind the other hash you may run into.QuoteI think this situation has improved with the greater availability of medium-duty cab-and-chassis vehicles, but it was fairly common a few decades back for the upfitter to remove the factory alternator and install a larger, often self-contained or separately regulated, alternator. These alternators could exceed 300 amps, and in many cases, dual aftermarket alternators were installed. In addition to being huge, these monsters and their regulators were quite crude compared to the OEM setup. So, if your product will be widely installed into unknown ambulance configurations, some paranoia is probably warranted.

author: richard.cs
author_rank: Super Contributor
timestamp: September 08, 2021, 04:52:09 pm
content: Quote from: bdunham7 on September 08, 2021, 04:21:04 pmMine (Lucas) buzzes gently with that looks like a continuous arc. I am kind-of guessing frequency from the sound, but you certainly can't see individual cycles.

author: HighVoltage
author_rank: Super Contributor
timestamp: September 08, 2021, 05:50:42 pm
content: The old BOSCH alternator regulators had an over current protection that arced only if high current got pulled. But the voltage regulator contacts arced repeatedly.

author: richard.cs
author_rank: Super Contributor
timestamp: September 08, 2021, 05:57:51 pm
content: Yes, the Lucas ones are the same. The voltage and current regulators have tungsten contacts to support continuous arcing of the field current. The "cutout" (prevents reverse current flow) has something else as it carries the main armature current and should switch rarely. Probably silver - cadmium oxide or similar.

author: TimNJ
author_rank: Super Contributor
timestamp: September 08, 2021, 06:00:36 pm
content: Quote from: bdunham7 on September 08, 2021, 04:37:51 pmNailed it.Yes, board space and thermal constraints mainly. Cost is tied in there too. No additional locations on the heatsink for a parallel MOSFET and we are already at the limit of Rds(on) vs Vds(max). (1.9mΩ @ 100Vds). If we use a higher Rds(on) MOSFET @ 200/250Vds, will fail strict a customer requirement on temperature. A fun time for all.Thanks for the information on these types of vehicles...DUT is intended for worldwide deployment, so indeed there could be a wide array of vehicles it makes its way into.I checked Ford for instance, and they offer ambulance chassis that seem like they are fully fitted and ready to go (minus the actual medical equipment). I would take a guess and say that these are relatively robust and well thought out.https://www.fleet.ford.com/showroom/specialty-vehicles/ambulance/But again, some countries, and probably even parts of the US, may not have the latest greatest stuff.

author: bdunham7
author_rank: Super Contributor
timestamp: September 08, 2021, 06:27:36 pm
content: Quote from: TimNJ on September 08, 2021, 06:00:36 pmIs this a 24-volt device only? Or are there two flavors, or perhaps it is dual-voltage? A lot of ambulances, as in all of the light-duty, non-military ones that I recall in the US, are actually 12-volt systems with a separate 120VAC power bus for medical equipment. Clearly your 100V MOSFET works there.QuoteYes, the newer Ford setups are probably pretty good from the factory before the upfitter hacks get hold of them, especially the F-series diesel ones. The E-series gasoline ones were notorious for very high underhood temperatures and fires. The Transit-series ones are likely OK but pretty light duty. And I think all of those are still 12-volt.

author: TimNJ
author_rank: Super Contributor
timestamp: September 08, 2021, 06:29:21 pm
content: Quote from: richard.cs on September 08, 2021, 03:28:34 pmOh okay, got it. Thanks.On that note, I ran some simulations for a few TVS diodes: 15KPA40, 15KPA60, 15KPA78 and 15KPA100.NOTE: 15KPA** devices are actually not suitable for the power and duration required. I just used the SPICE models to form a general relationship. The exact clamp voltage values (vs. current) will be a little different with a more suitable part (i.e. Vishay SM6S**AT).I suppose total energy (Joules) absorbed during the transient is probably the best gauge on TVS stress/sizing?? Indeed, the peak and average power dissipation did not go down monotonically with increasing TVS voltage, as I was thinking before. There exists some maximum point.I was also surprised to see how moving from 15KPA40 to a 15KPA78 only resulted into a ~20% reduction in energy absorbed by the TVS. So, we have an expensive Linear Tech "surge stopper" up-frontandwe'd still need a TVS that can handle 80% of the surge energy of a solutionwithoutthe fancy LT IC. If pulling a huge clamp current was not an issue, I'd say, to hell with the fancy IC and just clamp it with the 20% larger TVS. I guess that's a bit of a selfish approach, i.e. "who cares about the other equipment as long as mine survives". To get an idea about I2t, I calculated the I2t for a bunch of different conditions. Depending on what type of fuse you are using, could be problematic, but picking a fuse with ample I2t is not terribly difficult, unless you have a whacky space constraint or otherwise.I do doubt myself a little bit because I'd read this Maxim article:https://www.maximintegrated.com/en/design/technical-documents/app-notes/7/7084.html...and they showed a similar approach whereby they were able to reduce the energy in the TVS from 160J to 35J by moving from a 38V to 120V TVS clamp voltage. But they also used a 151V source voltage, and not 202V like I did...even though their diagrams show 202V. Might change the numbers in their favor.

author: TimNJ
author_rank: Super Contributor
timestamp: September 08, 2021, 06:44:31 pm
content: Quote from: bdunham7 on September 08, 2021, 06:27:36 pmIt is "universal DC input", i.e. 12 - 24V nominal, but can work down to 9V and up to 36V continuously. My guess is that the vast majority of installations will be 12V systems, which raises another point on likelihood of ~200V transients. The standards imply that the peak load dump voltage is proportional to nominal system voltage. As far as I know, only heavy trucks like 18-wheelers and heavy construction use 24V systems though?

author: bdunham7
author_rank: Super Contributor
timestamp: September 08, 2021, 07:00:33 pm
content: Quote from: TimNJ on September 08, 2021, 06:44:31 pmYuck! I suppose they want the 'dual voltage' because then they only need one SKU, right?I suppose if they expect that you can deliver multi-voltage operation with transient protection for the highest level for $5 per unit or less, it makes sense.The potential load-dump transients are lower and less likely on a 12V ambulance system, less likely because they typically havedual batteries in parallelwhich practically eliminates battery failure and greatly reduces electrical disconnection as potential causes of 'the dump'. Does the standard not specify a lower test voltage for 12V systems?In the US, most civilian on-the-road equipment up to the Class 5 and probably most Class 6 trucks are 12 volt. Class 7/8, off-road and military vehicles are mostly 24 volt. Aircraft are commonly 24/28 volt with additional power bus options--in case this might go in an air ambulance or helicopter.On those TVS diodes, am I wrong or is their pulse energy absorption about 15-20J--a lot less than you need them to be by your charts?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 08, 2021, 07:28:05 pm
content: Bunch of years ago, did a 6-80V(!) input for equipment that went on a forklift... in the end I think they dropped the survival requirement for load dump, the TVS was just too big/expensive for them.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: September 08, 2021, 07:42:54 pm
content: Quote from: bdunham7 on September 08, 2021, 07:00:33 pmNailed it again. The temptation of single SKU cannot be resisted! That's an interesting point about 12V batteries in parallel. And thanks for the info on the different Classes...That's all new information to me.Ah, I just happened to use 15KPA** because I had the SPICE models imported already. I guess that's a bit misleading..I will amend my post. Thanks. Really, something like SM6S**AT is more appropriate, but didn't have a model:https://www.vishay.com/docs/87735/sm6s10atthrusm6s43at.pdfI think the numbers may change a little with a different TVS device (i.e. higher or lower power/energy ratings), but I think the general relationships in the table are still true.

author: TimNJ
author_rank: Super Contributor
timestamp: September 08, 2021, 07:48:49 pm
content: Quote from: T3sl4co1l on September 08, 2021, 07:28:05 pmDamn. 6 - 80V...what is the basis of that?

author: richard.cs
author_rank: Super Contributor
timestamp: September 08, 2021, 09:35:51 pm
content: Quote from: TimNJ on September 08, 2021, 07:48:49 pmOne of my customers wants 14.4 to 154 V operating (at about 100 W) with transient survival up to 4 kV from 2 Ohms. We managed it but at a push, this is to cover a range of systems from 24 to 110 V nominal, +/- 40%. Splitting it into two product families to cover the range would have been more sensible really.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 08, 2021, 10:38:27 pm
content: I forget what exactly, I think it was something like: operational on 12V systems including cranking, so 6V; and operational/surviving up to 36V doubled jump start so 72V, plus some slop?Tim

author: bdunham7
author_rank: Super Contributor
timestamp: September 08, 2021, 11:37:13 pm
content: Quote from: T3sl4co1l on September 08, 2021, 10:38:27 pmI've seen at least one ancient Continental-powered forklift with a 6V electrical system and battery forklifts can go up to 48V nominal or 59.2V on a charger, so perhaps the one-size fits all theory was all there was to it. I would have omitted the 6V part, that seems a bit much and likely made it all that much harder to accomplish.

author: jonpaul
author_rank: Super Contributor
timestamp: September 09, 2021, 06:38:10 am
content: Bonjour a Tous, just recovering from terrible Chi Vi, and reading this long thread.We faced this issue in 1980s in 24V Avionic PSU design and manufacture. A few notes please:1/ An alternator or generator has a large iron core that stores energy according to 1/2 LI sq, which can be many joules.2/ There is no limit to the voltage on a charged inductor, suddenly open circuited, except the classic 2ond order RLC ringing and external limiters.3/ The load dump transient test is specified by various standards, in Avionics it was DO-160 and for automotive I am sure there is an IEEE or SAE standard.4/ Am unaware of a "centralized transient protector" in a vehicle. Even if it existed, every connected electronic device could still be exposed to transients due to the series inductance between the load dump source, the protector and the susceptible device.5/ In 1980s..1990s Motorola Semi made a very large area transient protection diode with heave gauge leads and terminations specifically for this purpose. We used two MR2525 in series on 24-28 V nominal bus, but various other similar diodes existed.6/ Motorola long exited this market but modern fabrication may come from Central Semi, and others.7/ We added a series LC filter on the incoming bus before the diode.8/ The standards had recommended simulator circuits.Just the ramblings of an old retired EE,Bon Chance,Jon

author: richard.cs
author_rank: Super Contributor
timestamp: September 09, 2021, 08:46:17 am
content: Just to be clear, there are two separate effects relating to load-dump with a normal automotive alternator:The inductance of the alternator stator stores energy as jonpaul states, which means if a load is removed unlimited voltage is available to keep that current flowing. This is a short duration effect, maybe a hundred microseconds or so? Energy limited to½I2LThe control bandwidth of the alternator regulator is limited to perhaps 10 Hz by the inductance of the field winding and the regulator design. If a load is suddenly removed the field current cannot be reduced quickly, and the alternator continues to drive the same current as before for 100 ms or more, with a compliance voltage dependent on engine rpm and initial load, but around 100 V or so. This is not stored energy, rather mechanical power continues to be extracted from the engine for that time, so the available energy is higher.The first effect provides higher voltages if not clamped by something, and higher dV/dt so the inductance of clamping arrangements becomes important and the battery is less-effective as a clamp. The second effect, the slow one, delivers more energy.

author: Renate
author_rank: Super Contributor
timestamp: September 10, 2021, 12:47:50 am
content: Quote from: bdunham7 on September 08, 2021, 07:00:33 pmMany diesel passenger trucks have dual batteries in parallel for starting.Do we know if any/some/all ambulances have split systems (like RVs) with truck battery(s) and coach battery(s)?There's also series batteries. I run two 6V "golf cart" batteries in series for coach.And what about LiFePO4? You're running 4X (or 8X) 3.2V in series.

author: floobydust
author_rank: Super Contributor
timestamp: September 10, 2021, 04:31:48 am
content: The field decay time is made much worse due to the back-EMF diode. GM has a 2003 patentUS 6,876,177 B2adding a series mosfet for fast field collapse. The patent includes load dump waveforms with comparisons.I use around 2010 as the point when all manufacturers implemented zener diode avalanche rectifier diodes, with the corresponding clamped smaller load dump energy.I thought Motorola came up with them in their button-diode line up 1981 patentWO1982002797A1i.e. MR2500 series.Zetex BZP61/62but it took a long time for the industry to use zeners for some reason, either cost or royalties. You'll still see press-fit diodes with no (low voltage) avalanche rating.It is important to know what load dump specs need to be met. ISO-16750-2 gives a range and suggested parameters, and each automaker uses their own test values. It's a standard mainly for the test and measurement setup. Decay time, series resistance, # repetitions etc. varies widely in the parameter tables.Emergency vehicles are standard chassis fromGM(Delco) orFord(Bosch), you can get datasheets on the alternator offerings looks like to 200A. That load dump energy is much greater than a car. I don't bother designing for it (ride through) anymore because of the added cost and PCB real estate and a blown fuse is fine. Load dump is uncommon and only occurring in old derelict vehicles where the battery terminals are corroded, the battery has corroded interconnects/sulphated and going high resistance. This is just my opinion.Ambulances have in-cab PC's, radio and medical electronics which is all fragile, so I would look at what they are spec'd for. And visit a company that services or puts the vehicles together, to see the systems and wiring and issues they have.The question is if OP is going to actually perform the test suites. Renting a transient generator is expensive and much worse if the design fails and you need the rental for a longer time period to do design changes and re-evaluate. The LT4356 was not a success despite good intentions. It gets killed by -ve transients (unless you add diode/2nd mosfet) and the mosfet(s) fail because it's in a linear-region exceeding SOA and quickly getting hot during ride-through. If it shorts no one notices. The noob engineers designed it in but it did not work out, product failures. I think the datasheet is mostly a theoretical starting point if you're going to use the IC, and it's say $6 plus mosfets. So a technical solution, while interesting is too much design time and high risk of failure. I'll use honkin' DO-218 TVS fromVishay SM8orDiodes Inc.and there are smaller SM5 3,600W parts and be done with the issue.

author: f4eru
author_rank: Super Contributor
timestamp: September 10, 2021, 08:38:43 am
content: Yep. Load dump is absorbed by rectifier diodes avalanching in the alternator, inallnew ICE vehicles.Around 2005, manufacturers got tired of having to spend a single huge TVS built into each device, when a single centralized one inside the alternator is the better solution.Basically, suppliers were forced by spec to protect their devices against load dump, and often had to include a huge TVS.Load dump transients happens in two cases :1) disconnection of the battery while charging at high current: corroded connections, improper maintenance, vibration failure of ground strap, etc...2) switching off huge loads (headlight, defogger, etc...) when the battery is old and failing (has high//very high internal resistance)

author: TimNJ
author_rank: Super Contributor
timestamp: September 10, 2021, 04:52:01 pm
content: Quote from: floobydust on September 10, 2021, 04:31:48 amThanks for the information. Indeed, in researching this, I've learned that it certainly can vary widely between vehicle type, alternator type, on-board equipment, etc. For this application, the standard that must be met is IEC60601-1-2 which defines the load dump parameters for the test setup given by ISO7637-2. In this case, IEC60601-1-2 selects, for a 24V system, 174Vpk, 2Ω source impedance, 350ms decay time, single shot. I assume these numbers were not completely pulled out of a hat andshouldrepresent some nominal (or maybe worst case?) ambulance installation.Regarding "ride-through" and LT4356 et. al. I think its not fair to say that failure due to violation of MOSFET SOA is really the fault of LT4356. Seems more like poor selection of MOSFET, and not much rigorous qualification, either empirically and analytically. It still does raise the question of whether LT4356 brings any value at all, apart from lowering current during the fault/surge condition. But if it can't cover the entire range of load dump situations (due to 100V IC limitation), then doubling up on TVS and IC seems pretty dumb too. You could clamp the IC's voltage sense pin to <100V via R + zener to keep the IC safe at >100V, but this potentially endangers violating the SOA as the IC uses the sensed Vds to adjust the allowable on-time in linear operation. If you clamp the voltage sense pin to 100V, then the IC will maintain the same on-time at a 100V surge as it will with a 200V surge.If one or two DO-218 load dump suppressors solves the issue, why does the Linear Tech chips exist? For suckers like me? As long as the parts can handle it, and additional protections are built in, it does provide a fairly "clean" solution, but who cares?Thanks.

author: bdunham7
author_rank: Super Contributor
timestamp: September 10, 2021, 05:23:49 pm
content: Quote from: f4eru on September 10, 2021, 08:38:43 amMy old diesel pickup with large dual batteries had a 140 amp alternator and also had an intake air heater that was designed to turn on intermittently after a cold start, say -20C or so. It had to cycle intermittently because it drew 175A @ 12V, so it overwhelmed the alternator even at full-field and would discharge the batteries a bit. During the cycling process, when it would turn off, not only was there no load dump spike, the voltage would actually rise gradually over a 5-10 second period because the batteries could absorb the entire 140 amps without even rising to the target charging voltage for at least a few seconds, even when they were pretty cold.So it takes a real problem to cause a serious load dump surge, although the capability is always there lurking, ready to strike when that one in a million set of circumstances occurs.

author: floobydust
author_rank: Super Contributor
timestamp: September 10, 2021, 08:29:46 pm
content: OP I would very carefully check the requirements 60601 is calling for. Are they cut'n'pasting old test values from 7637-2 or just calling it out, "to meet 7637-2" and how old is all of this?ISO 7637-2:2004"The test levels reflect the situation of load dump at generator rated speed. If a central load dump protection is used, apply test pulse 5b as defined in Figure 12 and use the values in Table 10." {customer specified}ISO 7637-2:2011"The test pulses 4, 5a, and 5b have been removed from this edition of this part of ISO 7637, since they are specified in ISO 16750-2 and ISO 21848."It's a 24V system? Either 151-202V pulse OR 65V with alternators having zeners. 174V is a brutal single pulse like 13kW TVS if you hard-clamp it. Test Pulse 1 is -600V, Test Pulse 3 is -200V for severity IV.The LT4356 is for a clamped alternator 80V max. and milder transients. Not suitable for road vehicles and big trucks 24V systems in my experience, even with decent design enhancements. The-3datasheet has more design details, and there is newLTC4381datasheet does better discuss the mosfet requirements (even though the part has a smaller internal mosfet). But dropping 150V while flowing a few ampsIs it permissible for the product to reboot or blow a fuse with load dump, or does it need to operate like nothing happened?I would not jump to solution just yet (engineers love to do that) but instead look at the detailed requirements.The trap trifecta is operating during cranking (4V/12 power), taking high voltage -ve spikes, and either ride-through or switch off during a load dump.If you design an SMPS with high input voltage say 75V, then clamping transients up there is much less costly. Or an intermediate DC bus that can be dual-fed from a extra boost-converter to keep the product running during cranking. You also need big capacitance for hold-time to cover the other pulse tests for switching transients.Motorized fire engine siren Federal SignalQ-Sirenoperating current is 100A/12V, so imagine that much load switching on and off but it's less than that heater. Federal also makes all the light bars and solid-state sirens for cop cars emergency vehicles etc. which look reasonable at 20A Class-D and LED's only a few amps. Note Federal says nothing about transient rating or protection in their vehicle products specs.

author: TimNJ
author_rank: Super Contributor
timestamp: September 11, 2021, 04:35:19 pm
content: Quote from: floobydust on September 10, 2021, 08:29:46 pmThis is 60601-1-2 4th edition (2014). They reference the test setup/procedure of ISO7637-2, but with specific values (the ones I mentioned above, 174V/350ms for 24V system), if you are to meet unclamped load dump. As you note, in the case that centralized load dump suppression is implemented, the clamp value is (I believe) at the discretion of the installer/designer. Even though ISO16750-2 supersedes ISO7637-2, the medical standard has not been updated to reference the new standard.As was mentioned above, the customer wanted a one-size-fits all solution. The original specification provided implied full compatibility with 12V and 24V automotive systems and 14 and 28V aerospace systems. For aerospace, the relevant standard is DO-160G and going back 1+ year, the DUT was primarily designed to meet the 80V/100ms transient given in DO-160 for 28V systems (no source impedance given in the standard). The solution for this is LT4356 + AOT66914L (pass transistor) + 60V N-FET (reverse polarity) + SMB75A (positive) / SMB36A (negative) for some of the other transients (i.e. those modeled with higher source impedance like 50Ω. It works well for repetitive surges, even at elevated ambient and with the unit hot. As you know, the main goal is to keep Tj < Tj(max).Since then, questions around automotive transients have arisen.Based on some feedback above, it seems unlikely that the DUT will be in a 24V automotive environment, but with a nameplate rating that includes 24Vdc, that might be a little tricky to get around. Customer wants the DUT to act like nothing happened. But, I can advise on the practicality of such a requirement.If 24V load dump protection is truly needed, I will probably advise against the LT4356 type solution and push for a brute force DO-218 solution. Customer had specified "real" over-voltage protection, i.e. indefinitely tolerant to over-voltage beyond X voltage...but I am not sure the point of that. LT4356 met this requirement by at least allowing the unit to non-destructively cycle on/off until fault removed.Thanks for your inputs!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 12, 2021, 12:38:34 am
content: Probably better not to put 24V on the nameplate. If it's made for 12V-nominal systems, put 12V on the plate -- it's a type rating not an exact voltage. You don't see 400VDC or 2.5kV peak on commercial equipment (heh, well sometimes you might), the 120 / 240 / 85-265 / etc. rating is a type rating that assumes all the other things that ride on the mains.It's not really specified that way, it's specified in multiple disparate standards, with a callout for them all from the customer (or whatever the exact circumstances are); but it might be useful to think of it that way.Tim

author: David Hess
author_rank: Super Contributor
timestamp: September 12, 2021, 04:30:08 am
content: I have implemented the LT4356 form of overvoltage protection using discrete parts. Having it shutoff can be considered a form of foldback current limiting where the current limit decreases as the voltage across the pass element increases. This prevents excessive power dissipation in the pass transistor.

author: TimNJ
author_rank: Super Contributor
timestamp: September 13, 2021, 03:57:25 pm
content: Quote from: T3sl4co1l on September 12, 2021, 12:38:34 amThing is, the original specified input voltage range *implied* use with 12V + 24V automotive systems and 14V + 28V aerospace systems. Interestingly enough(?), the 28V aerospace transient requirements (per DO-160G) are not nearly as brutal as for 24V automotive, so as is, the DUT can handle 12V (auto), 14 (aero) and 28V (aero). It is unclear as to whether the customer actually needs 24V automotive or they just threw it in there because it seemed logical that it should work (since 28V aero is okay). But, that's for me to figure out.I need to check ISO nameplate conventions but I think you can probably do two separate lines, one for automotive and one for aerospace, similar to how you might handle separate input voltage ranges for 50/60Hz and 400Hz on the same unit.Thanks!

author: TimNJ
author_rank: Super Contributor
timestamp: September 13, 2021, 04:19:21 pm
content: Quote from: David Hess on September 12, 2021, 04:30:08 amThought about a discrete approach too. But, probably like a lot of other people strapped for time, decided for the IC solution. The discrete approach has the advantage of using easier to get jellybean parts, which is totally valid reason these days. Cost-wise, I assume discrete is cheaper, although depends on how many parts you wind up needing, and how closely really want to match the LT4356 functionality. I imagine a few op-amps, comparators, timers, and flip-flops should do it. Even if you needed 4-5 ICs, if they are all jellybean, probably not more than $1-2 in quantity, compared to $4-5 for LT. Of course, if you want to sleep at night, probably will need to spend a lot more time evaluating edge-cases than you might care to do with an ASIC solution.

author: floobydust
author_rank: Super Contributor
timestamp: September 13, 2021, 05:36:50 pm
content: I find a discrete solution has an issue during cranking, the standard for a 12V vehicle are only 4V so a mosfet partially on can be an issue. Otherwise it works fine or just use a BJT. The LT4356-3 has a charge pump for extra gate drive.If you're switching off the pass element during over voltage events, works well- if the load has the hold time of say 350msec and can keep running during the blip.edit: did not yet look at MaximMAX16128/9A bit more herehttp://www.flexautomotive.net/EMCFLEXBLOG/post/2016/09/15/automotive-centralized-load-dump-test-requirments

author: 16bitanalogue
author_rank: Regular Contributor
timestamp: October 09, 2021, 08:26:25 pm
content: Apologies if this has been answered, but directly to the OP's question about a "central clamp", not that I have seen from any of the OEMs that manufacture modules; i.e., Bosch, Aptiv, etc. Each module will have EMI and suppression circuits. The instrument cluster PCB will have its own protection circuitry separate from the infotainment module.

author: Hydron
author_rank: Super Contributor
timestamp: October 10, 2021, 02:56:40 pm
content: Quote from: floobydust on September 13, 2021, 05:36:50 pmWould be interested in where the 4V minimum comes from, that sounds rather pessimistic. Or is it assuming cranking on an iffy battery in the extreme frozen north somewhere?

author: floobydust
author_rank: Super Contributor
timestamp: October 10, 2021, 04:58:11 pm
content: The starting voltage spec is complicated. ECU manufacturers are using SEPIC or a boost-converter to comply. It's funny old Lucas used to advise you'd burn out your headlights if they were on during cranking due to the surges... Anyhow, this is what I have for ISO-16750 values for cranking and again it's a loose standard with four severity levels and car makers have their own standard levels as well. If your product is off and can reboot during cranking, not so much of problem, the low voltages.Below -20°C starting ICE is not fun, you have seconds to stop if the engine has flooded and shut off the fuel, before your battery craps out. Canadian cars usually come with a bigger battery (except Honda, theirs is the smallest car battery Group 51R lol). Gasoline does not evaporate much at those low temperatures so you are literally pouring in fuel for cold start enrichment.Just a note for OP's question- "centralized" means in the alternator: "alternator with/without centralized load dump suppression" is the exact term.

author: 16bitanalogue
author_rank: Regular Contributor
timestamp: October 10, 2021, 06:05:11 pm
content: Quote from: floobydust on October 10, 2021, 04:58:11 pmInteresting. Then the internal clamps of the alternator must still be a higher voltage than what is allowed for downstream modules. That would explain why there would continue to be separate suppression circuits downstream.

author: floobydust
author_rank: Super Contributor
timestamp: October 10, 2021, 06:21:01 pm
content: More modern, newer alternators using avalanche rectifiers (for at least 10 years now) still have load dump- it's just much lower in voltage.Compare Load Dump amplitude Test B is 35V (clamped) verses 79-101V (unclamped) which is a huge difference energy-wise.

author: Hydron
author_rank: Super Contributor
timestamp: October 10, 2021, 07:52:11 pm
content: @floobydust, thanks for the info - I've recently designed something for 12V vehicles which has a UVLO (falling) of 6V, and wondered if I was going to have an issue with it.I've got a housekeeping micro on it (before the UVLO) which should be able to try and avoid the device (which takes a few seconds to turn off/on) being on during the majority of cranking events, but it's not guaranteed. Shouldn't be going anywhere near -20C thankfully, and is realistically a prototype (though I'm seriously worried if I need to change anything that we haven't already stocked up on given the current semiconductor supply nightmare - one part I'm using is already showing 2023 if I want more!).

author: floobydust
author_rank: Super Contributor
timestamp: October 10, 2021, 09:54:26 pm
content: The big dip during cranking is ~70msec so if you've got the capacitance and a vehicle that is not too cold or an old battery, it might not be a problem for you. A 5V MCU and LDO and reverse-diode is cutting it close.New is the Reset Test where you drop the voltage in 5% increments "Reset is permitted upon agreement" yet no real mention of test limits.

author: Hydron
author_rank: Super Contributor
timestamp: October 10, 2021, 11:00:21 pm
content: Can't hold up for 70ms, but I can handle a 6V minimum (just) - got a FET doing reverse protection duty (be dropping more in the wiring harness than in that) and the 5V DC-DC has under 0.5V of dropout at a larger load than I'll be using. UVLO is likely actually the ultimate limit, which could be pushed a little lower if needed. Housekeeping MCU is on a 3V3 linear which is OK down below 5Vin.Thanks for the reply; looks like I should be reading up on ISO-16750 - I was aware this was one of the auto standards to check out, but it was one of those projects where doing it quick was far more important than figuring out which standards to use and meeting them exactly, and designing for automotive is pretty new to me. Still designed the device to handle at least 40V sustained (though if it stays too long over 35V it'll shut down), 70V (probably more) transient and reverse polarity protection because it just seemed sensible to do so, so I think I'll be good given we have some latitude to dictate the conditions they'll be run in.I gotta say though, it'd be nicer if these things were clearer, and "12V" auto supplies turn out to be a massive pain if you actually want 12V as a rail voltage without excessive pissing about. Other industries have their own PITA stuff to deal with though - CCTV's 24V AC psuedo-standard, USB PD, or some of the PoE stuff come to mind (and god forbid you need to read the standards for the latter two - CCTV basically not having any may be a blessing!).

author: EPAIII
author_rank: Super Contributor
timestamp: October 11, 2021, 02:05:11 am
content: I have not read every response and I do know that there are members here from many different countries, encompassing many different languages so perhaps there is room for different viewpoints on this. But it strikes me that the term "load dump" seems to be a very poor choice for what is being discussed. "Load dump" seems to imply the dumping or disconnecting of the load.What is being discussed is over-Voltage protection. Perhaps a search for that would yield a better return on the subject.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2021, 03:41:52 am
content: Well, it's due to the sudden disconnection (dumping) of a heavy load -- the battery most of all given its charging capacity, and then also exacerbated by other changes in load, say turning off the headlights or whatever. One of those very context-dependent terms; it says what it is, if you know everything about the situation (automotive, conventional regulated alternator, battery disconnect), but if you're missing those pieces and take it at face value, yeah, it's not too helpful really. Anyway, for better or worse, this is the accepted terminology in the English speaking world. English being what it is, well, this is just par for the course honestly...Also, it's... not strictly overvoltage protection, as one might argue that -- well, if it's overvoltage, it's simply, whatever the applied voltage is, there you have it; you'll just footgun yourself trying to shunt it, voltage is voltage. So you still need to see the definition. And as it happens, this is defined to have some source resistance, so shunting isn't actually futile, just difficult. Example: jump starting 2x voltage is another common automotive standard, and that happens with very low source resistance; you have no choice but to ride through or shut off (including by fuse) under such a condition.And "surge" maybe isn't a great term, it's a general word but the most common kind of surge (without qualifiers) is understood as a lightning-induced type, of the 1.5/50 to 8/20 to 10/1000 µs waveforms; these do define source impedances, and the general waveform isn't very different (sharp rise, slow fall), so, to be fair it's not terrible, there's just the particularity about how the DC is wired in (it's diode-OR'd), and it would just be a little odd to say a 80V 1,000/100,000 surge. Though honestly, I'm kind of liking the generality, just make sure you don't leave off the units so it's very obvious it is in fact 1/100 ms or whatever.Tim

author: 16bitanalogue
author_rank: Regular Contributor
timestamp: October 11, 2021, 06:48:15 pm
content: I'm still struggling with the end design from a compliance perspective.Given that central load dump suppression is a function of the alternator, I presume it still cannot cover all use cases since a vehicles wiring harness branches off from the main charging system. If a portion of the harness breaks (corrosion, loose) then at least that path's downstream modules will still suffer from a load dump pulse. Which would explain why the designs I have seen still have module level load dump suppression circuits. Therefore one would still compliance test to the 5a waveform.

author: floobydust
author_rank: Super Contributor
timestamp: October 11, 2021, 07:33:53 pm
content: No, if the wiring harness breaks at the alternator's output, there will be no charging and no load dump. Downstream will just be running off the battery. The alternator is the exclusive source of load dump transients, and since the avalanche rectifier diodes are integral to it, no broken wire in the vehicle will allow full strength load dumps to start happening.Module-level transient protection is for the other transients. Recall the tests from ISO 7637-2:Test Pulse 1 - disconnection of inductive loadsTest Pulse 2a - disconnection of loads; wiring inductanceTest Pulse 2b - DC motors acting as generators when ignition is switched offTest Pulse 3a - switching transients -ve; incl. wiring harness inductance and capacitanceTest Pulse 3b - switching transients +ve; incl. wiring harness inductance and capacitanceTest Pulse 4 - crankingTest Pulse 5a - load dump (no centralized suppression)Test Pulse 5b - load dump (centralized suppression)

author: 16bitanalogue
author_rank: Regular Contributor
timestamp: October 12, 2021, 08:59:11 pm
content: Quote from: floobydust on October 11, 2021, 07:33:53 pmI'm presuming the vehicle is already running and either a break occurs at the battery (happened on an old car of mine), the battery clamp was just loose, or intentionally pulled off (old shade tree mechanic trick). If the break happens at the alternator the battery should help to absorb the transient. If the wiring harness breaks downstream then true, the interruption of current due to the inductance of the harness to that specific module would experience some transient, but not the 'load dump' as defined.I tried to find an alternator from Bosch or Denso with integrated clamps, but I have come up empty.

author: 16bitanalogue
author_rank: Regular Contributor
timestamp: October 13, 2021, 09:04:05 pm
content: If anyone is interested, it appears that Motorcraft makes alternators with central load dump suppression. No, schematics have yet to be found.https://www.motorcraft.com/us/en_us/home/our-products/batteries-starting-and-charging/alternators-gl.htmlLate edit:According to Denso North America, they do not have alternators with central load dump suppression.