Design Of Automotive Electronics - Page 1
Forum section: electronics / projects

author: Griffin
author_rank: Unknown
timestamp: December 23, 2014, 04:54:20 pm
content: Hi Every1Been thinking about designing my own Push-To-Start system for my old car (1999 Toyota Tazz). Obviously the design will include more features (RFID reader, Remote start, Indicator control, Boot Opener) but I was wondering if those of you who have worked on automotive electronics have any good resources I can reference regarding design tips.I'm looking for specific circuit considerations, layout/part recommendations or any general advice on designing something that needs to go into an vehicle. Any advice or references will be very much appreciated.

author: KJDS
author_rank: Super Contributor
timestamp: December 23, 2014, 05:25:05 pm
content: I've done very little for automotive, but the two key things areTemperature rangeSupply spikes.The temperature range it needs to work over is huge. The manufacturers work on -40 to +70 case temperatures for inside the cabin. A cold night in Canada and getting into a car that has been parked in the sun when it's +40 in the shade means that's necessary.The voltage supply normally sits at 13.8V, but there are horrible switching spikes and generally you need to ensure the design is robust enough to survive +/-100V spikes on the power rails.

author: max_torque
author_rank: Super Contributor
timestamp: December 23, 2014, 05:29:54 pm
content: Simplest way to learn is to copy someone else.Buy a load of cheap, (even broken) vehicle electronics modules off ebay or similar. You'll find the same sorts of components, layouts, design techniques and I/O connectors cropping up time and time again for good reason!

author: rx8pilot
author_rank: Super Contributor
timestamp: December 23, 2014, 06:06:43 pm
content: Quote from: max_torque on December 23, 2014, 05:29:54 pmFor input filtering/spike control - are TVS arrangements tasked with the heavy lifting of preventing damage?

author: Niklas
author_rank: Frequent Contributor
timestamp: December 23, 2014, 06:07:55 pm
content: Copy some other design is not as easy as it seems. Can you get the spec of the components?- X7R or better on caps- Use 1206 or smaller packages due to thermal expansion and risk of cracks- Apply good safety margins for both voltage and power rating. 12V on 16 V rated caps is not ok, use 25 V or more.- Add a common mode filter (51 uH is common) to the CAN bus.- Use low capacitance bidirectional TVS diodes on CAN to avoid problems if the GND connection is lost. 24 V rating- The power supply must handle 60V (400 ms clamped load dump) and probably also down to 6V (cranking).- Vibrations can cause a lot of strange failures. Wires can break without any signs on the insulating jacket.Consider what is happening if your electronics crash or fail to operate when you are driving. Will the key ignition signal go to off or remain on? Think about GM and the lock spring

author: Wilksey
author_rank: Super Contributor
timestamp: December 23, 2014, 06:20:05 pm
content: I wouldn't copy anything that came from Ebay, especially if it was cheap and from China.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 23, 2014, 07:03:46 pm
content: Automotive spikes are relatively benign. The magnitudes are in the 100s of V range, for ns to us. In comparison, spikes defined in IEC 61000-4 for ESD and AC line transients and surges are massive, in the kV.To handle the short spikes, any common TVS device will do. I'd suggest at least SMA or P6KE range. Or you might not need any at all, input filter caps will usually withstand that sort of treatment.The elephant in the room is load dump, where the supply voltage swells from 14-15V (alternator charging battery at full rev) to 60V or more (battery comes disconnected, alternator output freewheels, regulator can't respond fast enough to reduce bus voltage), with plenty of amperes in reserve. The swell starts in a few ms and lasts for 100-300ms, so the delivered energy is massive, enough to cook pretty much anything.If you have a high power application, shunting that surge may actually be reasonable. For pretty much everything else, you have little choice but to ride it out -- a 12V nominal (actually 13.8 to 15) input, 60V capable. There are switching converters available for this purpose, or you can use an LDO that's normally wired for a voltage slightly higher than nominal. An LDO is handy because it's compact, and although it still has to dissipate power (up to 45V drop?), the current magnitude is not the 10A+ from the load dump, but only the load current of your device (< 0.1A?), so it's much easier to design in the extra power / energy capacity.You'll still want the TVS in front of everything, so the transients/surges don't fry your load-dump-protection circuit.You also need to withstand double voltage (I think it's usually called jump starting, but I don't know why you'd be jumping a 12V system from a 24V system..), half voltage (cranking) and reverse voltage (accidental reverse battery, ground-bounce transients).Regarding CAN, beware: the CAN standard itself is very, very broad and loose. The exact signaling, grounding, shielding and isolation used by a manufacturer may vary significantly. It might be easiest to inspect (trace the circuit?) existing hardware on the bus. Without any reference to the signal condition, it's useless to try discussing things like common mode chokes (they only do anything if you can add loading capacitance) or shielding (do they use STP or is it really just flapping in the wind?).Tim

author: SeanB
author_rank: Super Contributor
timestamp: December 23, 2014, 07:09:17 pm
content: Boot opening the best is to use central locking actuators, which are cheap and simple to drive, as you do not need the one motion, so just having a pull is fine. A good thing there is to disable the open if the ignition is on, and possibly use the central locking unlock to trigger a timer to only enable it to be used for a period of 10 seconds after unlocking.I use an old Ford central locking motor, from an old Sierra, to do the open. I modified it slightly to run a full cycle on trigger, and simply added a timer such that the button has to be pressed and held for 5 seconds before the motor will do a cycle to open the door.

author: Andreas
author_rank: Super Contributor
timestamp: December 23, 2014, 08:25:03 pm
content: Quote from: Griffin on December 23, 2014, 04:54:20 pmHello,usually you do not have the infrastructure on old cars to support push to start on older cars.Starter is safety relevant: risk of unintended movement of the car.(might not be funny in case of your children playing around the car).A single failure (broken or shortened sensor/wire) must not lead to a wrong decision of the starter logic.So the missing features on older cars are usual:- redundant clutch switch (upper + lower)- on very old cars (without ETC): redundant brake switches- gear neutral sensor (redundant)- 2 instead of 1 starter relays (highside + lowside) in case of a defective (always on) relay- full diagnosis of all sensors and actuators so that a single failure is detected and lights the error lamp.further you will need a engine speed sensor and a vehicle speed sensor to prevent automated start in case of driving.you should also learn about FMEA and FTA method to find additional points for security measures.Quote from: Niklas on December 23, 2014, 06:07:55 pmI would not recommend 25V voltage rating for components in a system which has up to 40-60V under "normal" operation conditions. By the way the 40-60V are only valid on newer cars. Older cars which have no central protection against load dump may have up to 100V for a 12V supply system. (in 24V systems even more).with best regardsAndreas

author: Griffin
author_rank: Unknown
timestamp: December 23, 2014, 09:09:30 pm
content: Thanks everyone for the very valuable information. I am not looking to make this into a product just something custom for my own car. The input power problems where something I did not even consider but will have a look at that in more depth. Given that I am the only person using the vehicle I will take note of some of the safety options and consider them om a case by case basis.Now to the fun stuff:1. Would using the brake signal (when depressed) as the safety before starting be a valid option? (considering implementing a fail safe/spare brake switch)2. Starting the vehicle will happen only if the RFID tag is present on the reader in a compartment in the center console.3. Implementation of dual starter relays4. Proper power supply design/part choices to handle voltage spikes as mentioned.5. Will wire the system in conjunction with the existing key ignition wires for Accessory, Ignition and Starter to insure I can still start the vehicle in case of failure6. Use of solid state relays for closing the necessary circuitsThis is my list of recommendations I am working on so far from the info provided. Please let me know if there are any more considerations I can take into account when designing this.Also what would be the best way to simulate the over voltage conditions as mentioned given only a 30V 5A supply?

author: theoldwizard1
author_rank: Regular Contributor
timestamp: December 23, 2014, 09:14:42 pm
content: Quote from: Griffin on December 23, 2014, 04:54:20 pmI literally sat across the aisle for the guy who wrote the software for "One Touch Start"(internal nickname, "Otis")for one of the Detroit 3 probably over 10 years ago. This version still used the key to initiate the starting sequence. There was no electrical connection between the ignition lock cylinder/switch and the starter relay/solenoid. Turning the key from OFF to ON powered up the PCM. Turning from ON to START effectively sent a signal to the PCM that the driver had "requested" that the engine be started !The "hard" part was determining EXACTLY when the engine was "running" (or close enough to running) that the starter could be disengaged.When the PCM started up, it pulled in a relay that kept the module powered up until the PCM was "done" doing ts shutdown sequence. So yes, the the ignition key/switch really did NOT start up or shutdown the engine !This first implementation had no input from the key fob. The "chip" in the "fat" end of the key was validated via a wireless connection before the "request to start" was sent to the PCM. A few years later, when the key fob signal was added, things got a lot more "complicated".I doubt you will find a much more detailed explanation than that.

author: f4eru
author_rank: Super Contributor
timestamp: December 23, 2014, 10:49:53 pm
content: HiI worked in this field.QuoteA typical modern passive start/passive opening system is incredibly complicated.When triggering the system(start button, lock or unlock on the external handle) , the car wakes up, and sends a trigger along with a crypto challenge and localization signals on 125 kHz LF (unidir!). This takes place with about 5 Tx antennas in the car, two in the front handles, 3 arrenged in clever places in the car. The key has 3 receiving coils ( one per axis), and a very clever low power wakeup capable of triggering on these 3 coils.The key wakes up, then locates itself in or around the vehicle based on the different antenna RSSIs ( must discern zones in trunk, in car, sideways, etc...) aim is not to allow to lock the trunk with the key inside, not to start when the key is outside, not to close when the key is inside, not to allow unlock if the key is not on the side of the triggered handle, etc...Then the key calculates the crypto response, sends it via RF (434 or 315 MHz ISM) to a receiver in the car, the car unlocks.This whole sequence is 40-200ms typically, plus actuator motor time.Extremely stringent and optimized, considering the uC in the key has a very limited battery (often CR2032 with a lifetime of 4 years continously listening to 3 LF receivers). You probably cannot build this kind of key without the specialized asic(s)The car system has also limited background power, often needs to listen to capa sensors in the handles, and to wakeup very fast many ECUs.The LF transmitters in the car often have a lot of power ( up to 500W peak), coz it needs to wakeup a kew at 2m distance, with metal in the surrounding.The most advanced systems even poll continuously for keys in the vincinity. This is a even more optimized kind of beast, any mistake in system design, and it sucks empty the vehicle and/or key battery...The RF return path is the same as for the RKE (open/close buttons on the remote).Then, there is another function called immo, on old systems eparate 125kHz RFID, today integrated in the same chipset of the key, using one of the 3 LF antennas. it works only on a few cm as an unpowered 125 kHz RFID, works even if the key has no power, usually only to start the vehicle putting the key on a specific spot (ignition switch, slot, whatever...)The crypto normally is distributed between the engine ECU, body controller, key, etc....Crypto is optimized (made less secure?) to fit in the stringent constraints of the key.There are two crypto stacks : one for RKE (hash of salted counter on both sides ), one for passive actions (challenge-response)

author: f4eru
author_rank: Super Contributor
timestamp: December 23, 2014, 11:20:52 pm
content: a good place to start, with perhaps available ICs:http://ww1.microchip.com/downloads/en/DeviceDoc/21981B.pdfhttp://ww1.microchip.com/downloads/en/DeviceDoc/41099D.pdfNot so available, RKE+immo only, but shows the actual level of integration:http://www.nxp.com/documents/leaflet/75017233.pdfgenerally :http://en.wikipedia.org/wiki/Smart_keyhttp://eprint.iacr.org/2010/332.pdf

author: Niklas
author_rank: Frequent Contributor
timestamp: December 23, 2014, 11:25:34 pm
content: 12V on 16 V rated caps was perhaps a bit more general example with respect to temperature derating. For use on unfiltered supply inputs, it is not enough and it could be wise to consider two caps in series.Common mode filter for CAN bus will not only filter noise from the outside. Some transceivers can be quite noisy due to unsymmetric drive performance/timing on CANH and CANL.

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 24, 2014, 10:45:15 am
content: I also plead to keep your fingers away fom the ignition key circuit, due to safety reasons!Remember, General Motors currently has an essential poblem with failing mechanical keys.. at least 13 people, or up to 153 are assumed to have died, because a faulty spring in the keyhole let the car de-energize during the ride, which also caused loss of brake and steering power, and airbag circuit, and so on.http://www.npr.org/2014/03/31/297158876/timeline-a-history-of-gms-ignition-switch-defectThe circuit therefore would need several double check mechanisms so that the engine and central electronic won't be interrupted in any case, during the car is moving.That means, the circuit would have to be designed to strict automotive safety electronics rules, with the appropriate automotive components, under environmental automotive conditions (-40..+85°C..125°C, humidity, vibration, etc.).The ECU would need to be programmed to cross check any faults in the ignition circuit.And so on and so forth...And at last, as already described, you would have to change the immobilizer circuit completely, from the existing near-field type coil/transponder to the complex system, which f4eru has outlined.For sure, no amateur has ever tried to replace these two circuits..OK, just a few additional automotive hints:Electronic components for automotive usage have to fulfill the qualification rules defined in the AEC-Q 100, 101 and 200 for semiconductors and passives:http://www.aecouncil.com/AECDocuments.htmlThe ambient temperature range for Interior applications is usually -40..85°C, and the main functions have to be fully operational.For Power Train applications, i.e everything under the engine hood, it's -40..125°C, fully operational.Therefore, a careful selection of components and suppliers, and also the PCB and soldering technology has to be adapted to these requirements, to guarantee at least 10 years of flawless operation..Jump Start:When cars are shipped from manufacturing site to other continents, which may take several weeks on the ship, the starter battery may be exhausted on arrival.To quickly load the cars 12V battery with sufficient energy, i.e. within up to 2 minutes, or so, they use 24V from an external truck battery to Jump Start the battery and the car. Therefore all electronics, which are directly attached to the battery (Kl. 30), like for example the Instrument Cluster, have to be designed for about 28V in the input regulator.. The electrolytic caps on the input of the regulators have to be 16V ... 25V types, at least, and the voltage regulators also have to withstand that voltage.Besides the overvoltage spikes from load dump pulses, the starting pulse behaviour of the vehicle energy system requires a sophisticated under voltage sustainment circuit.. The battery voltage may break down to 4..6V for several 100ms, which has to be buffered by big caps, especially in such a starting circuit.Conclusion: All these requirements and safety rules make a replacement of the ignition circuit useless.Frank

author: Psi
author_rank: Super Contributor
timestamp: December 24, 2014, 12:31:21 pm
content: Quote from: KJDS on December 23, 2014, 05:25:05 pmAlso expect to get a constant DC voltage of significant more voltage than 13.8V for prolonged periods.It usually happens when the car battery totally dies and it's internal resistance becomes terrible.Without proper battery loading you can get quite high voltages out of the alternator, especially when they have faulty diodes.The 12V automotive design guidelines say the device should be able to withstand a constant 24V.I sell a automotive product as a side business and have had around 3 units out of 200 returned with exploded TVS diodes. Started with a 18V TVS, then moved to 20V and now are using 24V TVS diodes.So it just goes to show you, it's not that uncommon to get a constant Vbat of 20+VDC !I've not had any issues with temp range yet, we're using the -40°C to 85°C parts and not the ultra wide range -40 to 105C parts.

author: f4eru
author_rank: Super Contributor
timestamp: February 13, 2015, 08:01:45 pm
content: HelloDont put TVS diodes on automotive ECUS. DON'T. Don't do it.Before, people put TVS or zeners on every small ECU.That made some sense. That was expensive. That was even dangerous. Why ? because you have many different zeners in paralell. The one having the smallest voltage could suck a lot of current over wires and connections not dimensionned for that. the zeners could smoke, the wires could smoke.Today, the alternator provides one central zener or TVS for the whole car. I's dimensionned appropriately, it has big wires.Usually it's around 36V.Why 36V and not, say, 18V would you ask ? Funny question. Because, believe it or not, in some (cold?) places it's usual to jump start a car with a 24V battery when the car battery is flat. This means a 26V/200A source connected to your poor 18 or 24V zener for two minutes. It just smokes.So usually you just put capacitors at 25V rated (but tested for 30V/2min) or 35V caps, and you choose your regulators for the highest possible Vin (usually 40-80V) why so high ? because of load dumps. a load dump is a big and very nasty transient (100-150V / 100ms) that happens when youz switch off big electrical consumers and the battery is old. The alternator then must react to compensate, and this reaction is very slow. a big 100V plateau can happen then, usually clamped to 35/40V by the zener in the alternator.

author: Scrts
author_rank: Frequent Contributor
timestamp: February 13, 2015, 08:35:48 pm
content: A load-dump is a situation where alternator is working and the battery gets disconnected. You'll get a huge jump then.There are a couple of rules. E.g. major ones are:- Ceramic caps connected to the battery must be connected two in series to ground. They must be placed 90deg one to other on the PCB. This doesn't apply for flexiterm caps.- Your device must be protected from reverse voltage and withstand 28V for 2 minutes minimum.- You must be very careful about EMC. Especially in 500kHz - 1.7MHz band. AM radio is there.There are lots of requirements for pin spacing. You should avoid putting smaller than 0603 components on the battery connected traces because of humidity.We still leave placeholders for TVS, but don't use them if not required. CAN common mode chokes are usually 100uH.

author: free_electron
author_rank: Super Contributor
timestamp: February 13, 2015, 08:47:39 pm
content: 1206 or other long body parts are no-no as they crack too easily.signal input should be shunted for esd by a small cap , followed by a input resisotr of about 1 k and THEN a TVS diode. that way the garbage coming in through wiring harnesses still ggets clamper internally but the current into the tvs diodes is limited so you don't play shunt for the entire car.each line has its own protection. bav199 diodes are great for that usagefor can there are special tvs diodes and common mode chokes.

author: kolbep
author_rank: Frequent Contributor
timestamp: February 13, 2015, 09:30:03 pm
content: Don't forget to have the key in the ignition, and in the in position. Or else steering lock will get you in trouble when you can't turn.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 14, 2015, 04:19:24 am
content: Quote from: Scrts on February 13, 2015, 08:35:48 pmWhat nonsense is this? What agency administers such a requirement?IPC however recommends ceramic chip components (resistors, capacitors, inductors, etc.) be mounted away from stress raisers (mounting holes, heavy connectors, inside corners, slot ends, etc.), and that they be mounted crosswise to the bending direction (so it acts to flex the part sideways, in the shorter dimension, rather than lengthwise, which encourages fracture of the end terminals).Quote from: free_electron on February 13, 2015, 08:47:39 pmFor general manufacturing and reliability purposes (I see no reason that automotive should be any different), 1210 is considered an acceptable maximum (ref: IPC SMT standard). Bigger caps are made, but if you're expecting high reliability, they should probably be avoided. For modest reliability, I would consider 1812s still acceptable. There are leaded parts (dipped with leads, or chips with lead frames, often incorporating many chips at once for high density), but they're always expensive and/or bulky, and rarely worth going for.Tim

author: free_electron
author_rank: Super Contributor
timestamp: February 14, 2015, 04:30:09 am
content: Quote from: T3sl4co1l on February 14, 2015, 04:19:24 amthe agency of smart engineering.here is the background : ceramic caps can suffer from electrode cracking under mechanical stress.In almost all cases this results in a shorted capacitor. Cracked electrodes occur during the manufacturing process. : depanelising of assembled board (that is the same reason that it is forbidden to use mouse bites or v-scoring on boards for automotive applications. you either populate board by board , or if a panel is used the depanelisation happens using a router that mills the retaining tabs to get the board out ) , screwing down the board in the casing etc. The short may not immediately be there , but it will . Due to humidity a whisker will form and turn the capacitor in a short.so the rule in the automotive world is : any ceramic capacitor that connects to the vehicles power rail directly, without current limiting element must be placed as two capacitors 90 degrees in respect to each other. in case of mechanical a mechanical force large enough to crack a cap the force vector will only kill one of the two.so the ceramic input filter capacitor before the regulator of a subsystem meets that criteria. once you are after the regulator that rule is void.if you have an on board fuse in the subsystem and the cap sits after the fuse the rule is void. most of the time caps are placed close to connectors to dampen the EMC being emitted from the system back into the wiring harness. so you can't always hit a fuse first.so , once again : anything that has a DIRECT connection without current limiting into the vehicles main power grid : 2 caps in series 90 degrees out.if the damaged cap finally shorts you don't end up with a fireball.most vehicle subsystems do not have on board fuses. the fuses are located in the fusebox in a central place. oh, and there is no such thing as an automotive grade polyfuse... the ambient temperature shift is simply too large to guarantee a stable trip point... so its non resettable fuse or bust... most car electronics themselves draw little current, but the motor or mechanics they controls does. do the fuse is rated for electronics + attached loads. see where we are going with this ? fuse is strong becasue of the loads. so a short on the electronics spells immediate disaster. that fuse will not blow or blow waaay to slow.so there you have it.same for super small components directly on power rails. there are very strict rules for surface contamination post-process. super small components (0603 and velow) are difficult to clean 'under' . remnants of flux or solvents can contaminate the area under the part. combine this with themperture and humidit and you hav a breeding ground for CAF Conductive anodic elements ( think of a whisker , but one that creeps over the surface of the pcb underneath the part. if such a filament shorts the main rail disaster strikes. car batteries pack a serious punch in terms of shorting current...

author: max_torque
author_rank: Super Contributor
timestamp: February 14, 2015, 02:35:39 pm
content: Quote from: free_electron on February 14, 2015, 04:30:09 amIt is clear that this^^^ is "best practice", but i'm struggling to think of a single production case where i have seen such measures applied?

author: free_electron
author_rank: Super Contributor
timestamp: February 14, 2015, 03:31:32 pm
content: Quote from: max_torque on February 14, 2015, 02:35:39 pmthink outside of the group of 'fossils' that only make vehicles that require a distillate of dinosaur remains.I did a board a couple of months ago for the drive invertor for what is now the worlds fastest accelerating , 7 passenger , sedan. Those, and other rules did apply.

author: SeanB
author_rank: Super Contributor
timestamp: February 14, 2015, 03:41:19 pm
content: I have a board Vince had a ahand in designing, and so far the power on time count is at 36172 hours, and it has slowly accumulated 14 reallocated sectors during this time over 81 power cycles. Not bad for a ST3500418AS.

author: max_torque
author_rank: Super Contributor
timestamp: February 14, 2015, 05:27:49 pm
content: Quote from: free_electron on February 14, 2015, 03:31:32 pmOn the 12vdc bus or the HV traction bus??

author: SeanB
author_rank: Super Contributor
timestamp: February 14, 2015, 05:38:48 pm
content: I would guess both buses are used on the board, though one of them is more likely to result in a very spectacular fireball if a ceramic capacitor fails.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2015, 01:42:32 am
content: Quote from: free_electron on February 14, 2015, 03:31:32 pmI asked for standards; who administers them?

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 15, 2015, 02:09:35 am
content: i also did a little, 2 most important thing to mind:1) spike management: isolate that as much as you can, ensure stable Vinput to your circuit.2) failsafe management: when a component fails, make sure it will not harm a life, such as pressing the fuel throttle full, or rendering the car unstartable in the middle of nowhere.i still have the version1 diy circuit working for about 2 years in my car that failed criteria 2. i have version 2 that handle criteria 2 but havent got time to produce it.

author: SL4P
author_rank: Super Contributor
timestamp: February 15, 2015, 06:23:15 am
content: OK, it's a 15 year old car - even older design... but when push comes to shove, you will not have ANY insurance cover if the modifications are spotted during a claim.The idea is 'simple' enough with the caveats noted above, but the police, insurers and court will not see or understand any of that.You may even have the liability shifted to your side --- driving an 'illegally modified' vehicle. It could cost you far more than the car is worth to defend even if the actual cause was clear as day to blind Freddie.

author: Niklas
author_rank: Frequent Contributor
timestamp: February 15, 2015, 11:47:24 am
content: Quote from: T3sl4co1l on February 15, 2015, 01:42:32 amIn the commercial vehicles (trucks, buses) and construction equipment segments there is a great variety of customer specific specifications. It is very common to have OEM specific specifications where tables are copied from ISO documents and pasted into the OEMs' document. Off course they add some of their own additional requirements on top of that, but much of the baseline can still be recognized. Take the EMC requirements and all the required pulses and start comparing the details from ISO-16750-2 with the OEM's spec.There are also specific design requirements where things like the series connected capacitors can be listed together with blacklisted components, for instance CAN transceivers that are not releasing the bus when unpowered. It is almost as if they took an FMEA or a reported failures list and translated parts of it into a design requirements document.I have seen the demand for two caps in series or resistor + cap in at least one of those requirements we have.

author: Scrts
author_rank: Frequent Contributor
timestamp: March 23, 2015, 04:46:41 pm
content: EMC/EMI requirements vary sooo much from OEM to OEM...You can't rely on single OEM requirements.BTW, do anyone know any automotive based startups? I wonder if it's easy to get into that market.

author: Psi
author_rank: Super Contributor
timestamp: March 24, 2015, 06:27:13 am
content: Did we come to a conclusion about using TVS diodes on the power rail to protect an automotive device from large spikes?In my product the battery voltage connects to two things. Both automotive rated parts- Linear 5V regTL720M05Input range -42 to +45V- High side driverVN5E010Input range -16V to +41VNegative voltage is only really for reverse polarity protection so -16V is fine.For positive spikes i am ok up to 41V without any TVS.So should i have a TVS diode that clamps Vbat to 41V? or no TVS at all?

author: luky315
author_rank: Regular Contributor
timestamp: March 24, 2015, 11:35:07 am
content: I would suggest using a TVS with the highest possible clamping voltage (Maximum clamping voltage=absolute maximum input voltage rating of the voltage regulator).A small (1 to 4.7Ohm) pulse proof resistor (MELF is not a bad idea) directly after the input can help a lot (if possible...)

author: Gribo
author_rank: Frequent Contributor
timestamp: March 24, 2015, 12:04:23 pm
content: Here is a result of a load dump test I did yesterday. The blackened part at the top is the DC input area. The damage so far are blown varistor, TVS shorted and input trace is gone, However, the regulator works like a champ

author: Corporate666
author_rank: Unknown
timestamp: March 24, 2015, 06:32:40 pm
content: Quote from: f4eru on February 13, 2015, 08:01:45 pmI have several ECU's from several different brands of modern cars and they all have TVS's on every input.The vast majority of the PCB's are flex, often folded over on itself and the whole housing filled with potting compound.

author: Corporate666
author_rank: Unknown
timestamp: March 24, 2015, 06:36:05 pm
content: Quote from: Psi on March 24, 2015, 06:27:13 amI put a TVS on everything.Everything.Because overall, it's the single most common reason for failure.However, I've found that - when I can afford it - I prefer to use a device specifically designed for automotive protection. Like the ST-Micro RBO40 which protects against transients and load dumps. There are others out there too, or you can roll your own - but I don't like relying on a linear regulator to protect, I've had devices fail in this way.

author: eneuro
author_rank: Super Contributor
timestamp: March 24, 2015, 07:25:45 pm
content: Quote from: Corporate666 on March 24, 2015, 06:32:40 pmIsn't they have low value power resistors before those TVS's ?It should help balance those TVS's clamp voltage when a few similar TVS's are put in parallel or I miss something?Anyway, even zener diodes with additional resistor on DC input should help a lot when we put in parallel many similar circuits, so if is it possible add input resistor probably it should help clamp voltages on different TVS's in parallel to battery.Isn't lack of this input resistor before TVS's the main issue to don't put TVS's diodes in parallel on automotive ECU's?

author: Niklas
author_rank: Frequent Contributor
timestamp: March 24, 2015, 08:09:23 pm
content: Quote from: eneuro on March 24, 2015, 07:25:45 pmResistors won't help much and can also create other issues with operation during cranking due to their voltage drop. Also the amount of energy that they would have to dissipate is not healthy for smaller package sizes. The demand for thermo shock, vibrations and wide operating temperature also limits the usable package to somewhere around 1206.TVS devices are mainly designed for fast transients with limited amount of energy, hence the 5/20us pulse load specification. Connecting them in parallel is not a good idea as the breakdown and clamping voltages can change with temperature. Dependent on the demands for operation during disturbances (No impact, ok to reset etc), the load dump protection can be designed either to disconnect or to clamp the output voltage and dissipate the excess energy as heat.A "standard" bidirectional TVS device could be useful to handle fast transients, such as pulses 1, 3a and 3b, if selected so it will have a breakdown voltage above the peak load dump voltage. Pulse 1 could be +-600V and kill MOSFETs and capacitors.I have tested a slightly modified version of the circuit described in the link below with 180V unclamped load dump with good results for a sensor application.http://www.eetasia.com/ARTICLES/2007AUG/C/EEOL_2007AUG14_POW_EMD_NP_01.JPGTVS, filter, diode, load dump protection, stepdown regulator to +5V, LDO 3.3V worked just fine from just below 6V and up to 50V input voltage for unlimited time.