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

[richard.cs]

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?

Mine (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.

[HighVoltage]

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.

[richard.cs]

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.

[TimNJ]

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.

Nailed 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.

[bdunham7]

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.

Is 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.

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/

Yes, 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. 

[TimNJ]

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.

Oh 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-front and we'd still need a TVS that can handle 80% of the surge energy of a solution without the 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.

[TimNJ]

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.

Is 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.

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/

Yes, 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.

It 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?

[bdunham7]

It 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?

Yuck!  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 have dual batteries in parallel which 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?

[T3sl4co1l]

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

[TimNJ]

It 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?

Yuck!  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 have dual batteries in parallel which 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?

Nailed 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.pdf

I 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.

[TimNJ]

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

Damn. 6 - 80V...what is the basis of that?

[richard.cs]

Damn. 6 - 80V...what is the basis of that?

One 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.

[T3sl4co1l]

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

[bdunham7]

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?

I'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.

[jonpaul]

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

[richard.cs]

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 to
  ½I²L
• The 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.

[Renate]

... less likely on a 12V ambulance system, less likely because they typically have dual batteries in parallel

Many 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.

[floobydust]

The field decay time is made much worse due to the back-EMF diode. GM has a 2003 patent US 6,876,177 B2 adding 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 patent WO1982002797A1 i.e. MR2500 series. Zetex BZP61/62 but 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 from GM (Delco) or Ford (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 from Vishay SM8 or Diodes Inc. and there are smaller SM5 3,600W parts and be done with the issue.

[f4eru]

Yep. Load dump is absorbed by rectifier diodes avalanching in the alternator, in all new 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)

[TimNJ]

The field decay time is made much worse due to the back-EMF diode. GM has a 2003 patent US 6,876,177 B2 adding 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 patent WO1982002797A1 i.e. MR2500 series. Zetex BZP61/62 but 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 from GM (Delco) or Ford (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 from Vishay SM8 or Diodes Inc. and there are smaller SM5 3,600W parts and be done with the issue.

Thanks 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 and should represent 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.

[bdunham7]

2) switching off huge loads (headlight, defogger, etc...) when the battery is old and failing (has high//very high internal resistance)

My 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.

[floobydust]

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 -3 datasheet has more design details, and there is new LTC4381 datasheet does better discuss the mosfet requirements (even though the part has a smaller internal mosfet). But dropping 150V while flowing a few amps 
Is 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 Signal Q-Siren operating 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.

[TimNJ]

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 -3 datasheet has more design details, and there is new LTC4381 datasheet does better discuss the mosfet requirements (even though the part has a smaller internal mosfet). But dropping 150V while flowing a few amps 
Is 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 Signal Q-Siren operating 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.

This 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!

[T3sl4co1l]

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

[David Hess]

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.