Automotive Load Dump: What % of Vehicles Have a Centralized Clamp Diode?

[TimNJ]

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

[Stray Electron]

  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.

[Stray Electron]

  PS, there's no central load clamp in the BMW Z3 either.

[bdunham7]

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.

[richard.cs]

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 remains after the 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?

[TimNJ]

  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.

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 remains after the 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?

Yes...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.pdf

Hence, 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!

[Ian.M]

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?

[TimNJ]

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.

Good 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.

[langwadt]

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.

Good 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.

The commonly used protected FETs to control loads like fuel injectors and various control solenoids clamp at ~40V

[Ian.M]

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.

[TimNJ]

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?

Right, 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.

[Ian.M]

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.

[TimNJ]

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?

That'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.

[David Hess]

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.

[T3sl4co1l]

Good 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.

It'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

[bdunham7]

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).

The 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.

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).

Where 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 charge continuously without 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.

[Miyuki]

Automotive power can be a mess
Battery 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

[T3sl4co1l]

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

[bdunham7]

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.

It 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.

[bdunham7]

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.

Me 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.

[David Hess]

It's my understanding load dump doesn't really...happen? With a battery?

A 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.

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.

The 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.

[TimNJ]

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 condition
2b. The vehicle is not used in a corrosive environment (high humidity, salt water, etc.) which may speed up corrosion of the above conductors

In 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/44472894

To 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.

[Ian.M]

2b. Unfortunately 27 US states count as a "corrosive environment" in the winter. See https://en.wikipedia.org/wiki/Salt_Belt

[bdunham7]

That'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.

If 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 vastly improved in 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.

[TimNJ]

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.