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?
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.QuoteThere 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.
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/
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.
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.QuoteI 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?
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?
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?
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?
... less likely on a 12V ambulance system, less likely because they typically have dual batteries in parallel
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.
2) switching off huge loads (headlight, defogger, etc...) when the battery is old and failing (has high//very high internal resistance)
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.