ISO 7637-2 nonsensical transient tests - Page 1
Forum section: electronics / projects

author: trevortjes
author_rank: Contributor
timestamp: June 18, 2025, 06:22:19 am
content: Hi all,ISO7637 has a few transient pulses the device has to endure on its power input to pass for RED.But these tests make absolutely no sense.Pulse 1 emulates a transient when a power supply is disconnected from an inductive load.Leading to a bit of dead time, then a large negative sweep.The whole duration of this event is 200ms (T2).The negative portion is easily "protected against" with reverse polarity protection.But then you also have to keep your device running with ~200ms of dead time.Sure, some bulk capacitance could fix that, but this can get quite expensive, both in PCB space and components.The Analog website states this pulsehas to be repeated 500 times.Luckily some "on" time makes sure the bulk capacitance can charge back up for the next pulse.The transient isn't the problem here, it is the dead time.But then we run into pulse 2b, the most nonsensical of them all.This one emulates what happens when a vehicle gets turned off, and some inductive stuff like motors generate a bit of voltage that trails off to 0.T12, T6 and tr are all 1ms. Td is 200ms to 2s where ofcourse the former is easiest for the product.You'd expect a transient which is meant to emulate a power down sequence shouldn't have the need to be repeated but no, the standard wants you to repeat it 10 timesThe minimum repetition time however is 500ms.Weirdly enough the standard does not make clear if the remaining 297ms are 0v or Ua.Again the transient is peanuts, but a dead time of again at least 200ms can already be killer.To make matters worse, we heard from out testhouse that they utilize a dead time of 450ms from the end of td up to the next Ua!You'd need impractical amounts of bulk capacitance to bridge that gap!Any people here with experience with ISO 7637-2 transient pulses?Would love to hear your thoughts!

author: the Chris
author_rank: Regular Contributor
timestamp: June 18, 2025, 06:43:26 am
content: This all comes down to customer specification which usually define a state in which the system is allowed to be after applied ISO pulses. Usually, for pulses interrupting the power supply, the system "just" needs to be in a self-recovery mode without a functional failure.

author: Someone
author_rank: Super Contributor
timestamp: June 18, 2025, 07:24:23 am
content: Quote from: the Chris on June 18, 2025, 06:43:26 amMuch time has been wasted when intermediaries up-rate the required specifications.

author: trevortjes
author_rank: Contributor
timestamp: June 18, 2025, 08:21:31 am
content: Quote from: the Chris on June 18, 2025, 06:43:26 amWhat would functional failure mean in this context?Damage to the device (both in hardware as corrupt memory etc)?

author: the Chris
author_rank: Regular Contributor
timestamp: June 18, 2025, 10:23:36 am
content: Quote from: Someone on June 18, 2025, 07:24:23 amIndeed, a real nuisance causing lot of work for no benefit of the final product.Quote from: trevortjes on June 18, 2025, 08:21:31 amThe device must have performed a successful self-recovery and no abnormal operation must have been recorded, such as outputs changing their state unprompted.

author: trevortjes
author_rank: Contributor
timestamp: June 18, 2025, 11:16:15 am
content: You have a certain experience with the 7637?The device reboots with these tests and they tell us it is a fail.While in many other test reports I've seen that just specifies a certain class.But also EN 301 489-1 comes snooping around.They use that to tell us we fail because of "The application of the transient phenomena shall not result in a change of the mode of operation".They see a reboot as a change of the mode of operation.Fixing this means we have to add much more bulk capacitance to the point it seems like a costly joke.Or we'd have to add a UPS to overcome the power interruption, which is even costlier.

author: Hydron
author_rank: Super Contributor
timestamp: June 18, 2025, 11:54:11 am
content: Have dealt with similar BS with one of the Fire-alarm/CCTV immunity standards (can't recall which right now).The "change in mode of operation" (probably slightly different wording) thing meant that a battery backed alarm system couldn't show a mains loss status light or error when doing the (many seconds duration) power loss tests as that was a change caused by the loss of power. Apparently the workaround was to delay showing said status for longer than the power loss period specified, at least until sanity prevailed and the standard was updated to allow a power loss warning not to count as a change in operation.I was working on a CCTV system rather than fire, so thankfully there was a cop-out where you could put in the manual "requires use of a UPS system to meet the standard" - all the manufacturers went down this road as holdup for many seconds is obviously impractical without battery backup.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 18, 2025, 06:35:57 pm
content: Quote from: Hydron on June 18, 2025, 11:54:11 amDid the standardreallysay that indicating power loss during power loss (while retaining required functionality) counts as such "change of mode", or was this just an interpretation by some unnecessary people, who, in a sane world, would have just been ignored instead of being listened to? (Referring to the above comment by Someone.)

author: Hydron
author_rank: Super Contributor
timestamp: June 18, 2025, 06:40:34 pm
content: I only saw the updated version that had the note about indication of power loss being ok, it was the guy at the emc test house who told me it used to be an issue when you ran into someone who was pedantic about following the exact wording.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 18, 2025, 06:47:35 pm
content: Quote from: trevortjes on June 18, 2025, 11:16:15 amAny way to make the thing boot really fast and transparently? I have done stuff like retain relay position through reboot, by proving by calculation and verifying by measurement that the inductance of the relay coil prevents the relay coil current from falling below the minimum level required in the relay datasheet for maintaining contact ratings during the time from MCU reset signal changing the outputs to hi-Z, to the moment in init code where the IO is enabled again. Of course sometimes this isn't possible or easy, but sometimes a 1 millisecond boot to the same state is really possible. For example, maybe you can retain RAM even if some parts of the circuit shut down due to brown-out detection / UVLO. Or maybe you have "RTC" backup SRAM which you can power with a diode and small capacitor; maybe that domain can also power some other things like external ICs that are slow to initialize.My (very limited) experience of compliance testing is that actual, observable operation is what matters: that the actual featuresrequired by standards, and extra features you define as normal operation, are available; internal implementation details being irrelevant (such as what is called a "reboot"). IOW, just stop using the word "reboot" and invent a new word, while making sure it works according to requirements, job done, no?

author: Hydron
author_rank: Super Contributor
timestamp: June 18, 2025, 06:55:20 pm
content: Quote from: Siwastaja on June 18, 2025, 06:35:57 pmJust found it - was EN 50130-4:2011, the 2014 ammendment specifically adds: "Signalling a mains fault during the 100 % voltage reduction test is permitted." - clearly this had been an issue when encountering pedants at the test house to prompt a change!Btw the test in question is to turn mains power off for 250 cycles.Note that compliance criteria is as follows:"There shall be no damage, malfunction or change of status due to the conditioning. Flickering of anindicator during the conditioning is permissible, providing that there is no residual change in the EUT orany change in outputs, which could be interpreted by associated equipment as a change"I'm assuming that showing a mains fault indicator (or activating an associated fault state output) would fail it in the eyes of some jobsworths before the ammendment came along.

author: Niklas
author_rank: Frequent Contributor
timestamp: June 18, 2025, 07:12:04 pm
content: What are the requirements for your product?The reason I ask is that both ISO 7637-2 and ISO 16750-2 only specifies the outline of transients, but not always pass/fail criterion. We had several automotive OEMs as customers at my previous job and they had a list of requirements that referred to these standards and defined the test levels (1-4) and the expected performance class (A-E). In our case, Pulse 1 was tested with - 600V and 50 ohm, but the required performance class was C (reset with restart without user interaction).The requirements will be different depending on the use cases for the product. Our sensor was ok with class C for a pulse 4 (cold crank), but that would not be OK for an engine control module that would require class A.

author: Kurets
author_rank: Regular Contributor
timestamp: June 19, 2025, 05:18:16 am
content: This is the critical point. For each transient the EUT is required to perform according to a defined function class, which depends on device and requirements. And requirements are not defined by the test house, but by your customer.So for any safety critical system the requirement is likely to be ride-through of all transients with no noticeable effect. While for entertainment systems it may be sufficient that it suffers no damage but recovers after a power cycle.The decay to zero transient is a typical case where the duration of Ua should be sufficient that the EUT enters nominal mode of operation, and then the decay is applied. And most equipment is expected to restart when exposed to that profile, but shall suffer no damage. Damage could be caused by improper handling of brownout conditions etc.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 19, 2025, 07:07:40 am
content: Quote from: Kurets on June 19, 2025, 05:18:16 am... or, if this is your own product, yourself, as we noticed with a generic building automation product, where the relevant standards stated the test conditions (like field strengths and pulse shapes/levels/durations for A and B criteria), but we were free to define what B criteria eventually means to us, and in our case it would be for example dropping messages but recovering normal communications after the surge is over.But some safety-critical, medical etc. standards obviously are less hand-wavy and give exact operational requirements. So I agree that the most important question for the OP indeed isn't what the standard says about the transient shapes and durations, but what it says (or does not say) about functional requirements during those transients.

author: trevortjes
author_rank: Contributor
timestamp: June 20, 2025, 09:59:26 am
content: Quote from: Niklas on June 18, 2025, 07:12:04 pmI indeed seen the classification for ISO 7637-2 where even damage to the device would be ok but have the lowest classification.Sometimes it can be a bit confusing what is decided by the manufacturer and what by the testhouse.What if we are okay with the device turning off and rebooting during a pulse that mimics a power off?But we are also dealing with EN 301 489-1.They indeed use the "change of mode of operation" to give us a fail because our device reboots.The literal text from the standard is:QuoteBut I had a hard time finding the actual definition of "change of the mode of operation".We are wondering if we can just state in the manual that the use of a UPS to bridge the dead time is recommended to meet the standard.But I didn't get the idea from the testhouse that would justify everything and they'd tick off the ISO pulses as passed.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 20, 2025, 11:42:53 am
content: This all sounds similar to the IEC 61000-4-11 testing we do for medical devices. IEC 61000-4-11 states:QuoteThe "technical specification" being something produced by the manufacturer.I don't know if it contains similar language forbidding a "change of mode of operation". But "mode of operation" is something you are expected to define as you wish. I'm guessing what the standard really cares about is that the transient not cause the operating mode to changeunintentionally.For example, let's say your device was a UPS. Your documentation states that it has three operating modes: pass-through mode (when the mains are present) and backup mode (when the AC is not present) and idle mode (no output, regardless of the mains). Well obviously your device is going to change between pass-through and backup modes during a long mains interruption, that's the point! But this is described by your technical specifications, so all is good. On the other hand, if a mains transient causes it to switch to idle mode (probably not according to your technical specification) then you would fail the test.I don't have any direct experience with your specific standard, but I'd be surprised if the interpretation is very different.

author: trevortjes
author_rank: Contributor
timestamp: June 20, 2025, 11:57:02 am
content: Makes sense honestly.We tend to think the testhouse is in charge of everything but I often become skeptical about what is and what isn't our responsibility.If we say (=state in the technical documentation) that the device completely shuts down after (just an example) 100ms after power down, this is intended behavior and the device can never survive pulses with a 0v time longer than 100ms, this should justify the fails?At the moment we have nothing like that specified in any documentation, which quickly makes any behavior other than "running" a fail I assume.We're engineers, not jurists.But it seems like you really have to play around with the words and interpretations to pass these tests without overengineering the EUT.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 20, 2025, 12:06:33 pm
content: Yeah I think a lot of disputes with NBs is just a difference in vocabulary. Every company should have someone familiar with the lawyerspeak, it makes everyone's life a lot easier. Thankfully I'm not that person.