ISO 7637-2 automotive pulse 2b timing definition - Page 1
Forum section: electronics / projects

author: trevortjes
author_rank: Contributor
timestamp: July 15, 2025, 09:48:38 am
content: Hi all,For the ISO 7637-2 the following test pulse is defined:However, nowhere is stated what the time is supposed to be between the trailing of the transient (after td) to going back to UA.We just know the transient has to be repeated every 0.5 to 5s.For tests, it is very beneficial if the dead time is as short as possible.The testhouse measured the curve as such:450ms dead time? Quite a lot.Anybody has experience with this pulse?

author: MihaiSs
author_rank: Contributor
timestamp: July 15, 2025, 03:09:52 pm
content: Hi,From what you are showing, the oscilloscope waveform shows a pretty standard 2b pulse for 24V system. Now, the time after td seems to be on the shorter side since you have a range of anywhere between 0.5 to 5 seconds.The pulse is basically defined until td, which in this case is 2V. In my experience, everything is turned off in the DUT @2V and below, therefore after td within 0.5s to 5s the pulse must be repeated.I'm not sure if your question is answered

author: trevortjes
author_rank: Contributor
timestamp: July 16, 2025, 10:21:54 am
content: Quote from: MihaiSs on July 15, 2025, 03:09:52 pmLet's say, if the pulse repetition time minimum is 0.5s, and 0.2s of that is the transient, why isn't it possible to go straight back to UAafter 0.2s?

author: MihaiSs
author_rank: Contributor
timestamp: July 16, 2025, 06:01:35 pm
content: I cannot answer that, but I'm pretty sure this standard was written based on some real world measurements. Usually, each big automotive player has a team that works on these standards and they are all commonly agreed upon.It can also be based on how fast someone can cycle the ignition (or whatever it's called nowadays).

author: floobydust
author_rank: Super Contributor
timestamp: July 16, 2025, 06:28:58 pm
content: My understanding of 2b is it's a key-off event. So UAisn't coming back fast unless you can key-on and start up the engine quick..."The first part of the test pulse is a step drop due to the ignition being switched off, while the second part is a pulse from the motor regeneration energy. This motor regeneration pulse is referred to as a double exponential waveform. This is a characteristic shape of many types of pulses, where stored energy is dumped onto, and then subsequently dissipated by a load. Other test pulses in ISO-7637-2 have this characteristic shape, but are of much shorter rise and fall times."source:https://incompliancemag.com/mathematically-defining-test-pulse-2b-of-the-iso-7637-2-section-5-6-2-automotive-test-standardADI LTspice: Models of ISO 7637-2 & ISO 16750-2 Transientsedit: a bit more info about 2b:"Pulse 2b simulates transients from d.c. motors acting as generators after the ignition is switched off. (see ISO7637-2:2004 Annex F)""Pulse 2b is only applicable to switched battery lines"sourcehttps://www.flexautomotive.net/EMCFLEXQUOTE/FLEXDATA/PROCEDURES/705304B_113_CTI_ISO7637_2_PULSE_2B.pdf

author: trevortjes
author_rank: Contributor
timestamp: July 17, 2025, 07:05:56 am
content: I know why the pulse looks like it does and does what it does.What I do not understand is why this obvious "power vehicle down" transient has to be repeated as if that happens in the real world.And when it is repeated, the values are unknown or free to interpret.The problem lies in that there is a big dead time in the transient of 0.2s + 0.45s = 0.65s where the device relies on your bulk capacitance to keep everything alive.Why would I need to keep a device alive during a pulse that literally mimics the turn off behavior of a vehicle.Where does this 450ms come from, it is defined nowhere so I rather have them use 0s.