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

[bdunham7]

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

Moving 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!

[Stray Electron]

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.

   The 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.

[langwadt]

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.

I've read of people using alternators as welders. Regulating the field to set current, ~100V open circuit
and that some need diodes replaced because they fail at ~30V

[TimNJ]

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.

Thank 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.

[joeqsmith]

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.

At 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.   

[T3sl4co1l]

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...

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.

And 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

[bdunham7]

Thank 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.

Well, 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. 

[NiHaoMike]

Nowadays, many newer cars use a high voltage alternator (often doubling as the starter) and a DC/DC converter to step it down.

[mikerj]

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.

ISO-16750-2 is the current standard

[David Hess]

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.

And 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!

When 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.

[richard.cs]

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.

In 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.

I've read of people using alternators as welders. Regulating the field to set current, ~100V open circuit

All 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.

[Miyuki]

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.

In 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.

It 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

[HighVoltage]

42 Volts is the highest limit I have seen on an internal standard in Germany.

[bdunham7]

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?

Yes, 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.

[Miyuki]

42 Volts is the highest limit I have seen on an internal standard in Germany.

It is for a dry environment I think
Cars are in wet/aggressive
But there are multiple standards with various levels

[richard.cs]

Yes, 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.

Really? 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.

[TimNJ]

Thank 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.

Well, 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.

Indeed, 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 seeking manufacturer'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!

[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.

ISO-16750-2 is the current standard

(Attachment Link)

Thanks. 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.

[TimNJ]

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...

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.

And 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

Ah. 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.

[TimNJ]

42 Volts is the highest limit I have seen on an internal standard in Germany.

Thanks. 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.

[richard.cs]

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.

This 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).

[TimNJ]

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.

This problem with this approach is that whilst the current surge is lesser, the power dissipated in the clamp element may be greater.

So, 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.

[richard.cs]

This problem with this approach is that whilst the current surge is lesser, the power dissipated in the clamp element may be greater.

So, 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.

An 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.

[bdunham7]

Really? 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.

I 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?

[bdunham7]

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.

This 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.

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....

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.

I 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.