Elecro-magnetic Interference (EMI) Shielding requirements for electric vehicles
The company I work for, makes materials that can be utilized in a composite (fiber reinforced plastics) enclosure that effectively shield from EMI. Prime application would be a battery box. We have data (astm d 4935) with frequency on the x-axis and shielding effectiveness on the y-axis. What I want to know is this.. what region on this graph is important to the electrical engineers designing the parts of the vehicle? And why? What systems are interfered with? How much signal needs to be attenuated for the system to continue to work?
In the past, these types of parts would be metallic and would inherently shield from unwanted signals in both directions. Moving to composite structures creates a new challenge.
Posted Jun 30, 2021, 4:20 AM UTC
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A battery box sounds a little odd for analyzing EMC effects. A battery doesn't generate EMI, although I suppose that conducted emissions could cause structures inside the battery to act as an antenna, and true in reverse. However, I'd thing the very low impedance to ground (since a battery only has two terminals, one of which is ground and it's filled with things that conduct well and have a large ground area) that it wouldn't act like much of an antenna. But I'm not sure. I do know that my vehicles now do not have a battery box really of any kind.
There is not graph attached, but the short answer is lots of frequencies. The lowest I remember is 9Khz (and I'm not even sure if that's correct) and can extend into the GHz.
All electrical systems are subject to EMC requirements for both transmitted and received directions, and both conducted and radiated. You of course are only interested in radiated effects.
As for attenuation, that's like asking how long a piece of string is. There's no simple answer and there never will be. It depends on circuitry design and layout, how much noise the is produced, or that it's susceptible to. I used to work in LED lighting. Since those assemblies had to be visible, there was no enclosing structure, and we did not bother shielding the electronics because the LEDs themselves could not be shielded. SO the design were all done without the need for any shielding materials, and I suspect some other designs would be the same since there is often a transfer between radiated and conducted, so even metal boxes around some parts could see issues that come and go through wiring and reradiate inside.
I actually found that conductive metalized reflectors were the bane of my existence at times because they would act like dandy antennas is not ground, and they were sometimes really tricky to ground. That that is an indication they your conductive composite materials will need to provide a way of getting a good bonding to ground. It's not enough to shield, the shield have to be ground otherwise it easily becomes a pretty decent antenna itself.
Very well put! I haven't done vehicular emc in years but for some reason 3GHz is standing out as an upper limit... But that could have been just for the head units alone and not the car as a whole.