Carbon fibre enclosure for electronics...radiated emissions - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: July 20, 2021, 10:46:55 am
content: Hi,Do you know what carbon fibre is like compared to metals as a shield for the enclosure of electronics?...in relation to passing EN55032 radiated emissions with equipment comprising SMPS's and various cables coming off the PCBs

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 20, 2021, 10:54:26 am
content: Seems the answer is that it is very effective :https://www.sciencedirect.com/science/article/abs/pii/S1359836818343762https://www.sciencedirect.com/science/article/abs/pii/S1359836898000651It may even be better than metal, as I suspect absorbtion (vs. reflection) may be better due to the higher resistivity

author: beanflying
author_rank: Super Contributor
timestamp: July 20, 2021, 11:41:22 am
content: As someone who has been on the wrong end of Carbon shielding in R/C Gliders yep can be very effective. We learned way back when to route Aerials at 90 degrees to the Carbon Fuselages or run dual length with a solid wire outside the airframes.If you use prepreg and multiple layers will give best results as otherwise a hand layup with one or two layers will have effectively holes in the matrix as far as shielding goes as it is a woven cloth (so peaks and troughs). Also if you go down the path of Carbon sheets to cut up be very very aware of the dust and preferably cut it wet or at minimum wear a good mask and cut it outside.

author: fcb
author_rank: Super Contributor
timestamp: July 20, 2021, 12:08:20 pm
content: Metal enclosures are rubbish for radiated emisions if designed and implemented poorly. Same goes for CF.That said, you can probably make a better enclosure than with metal as things like seams/gaps can be more easily gotten rid off. We did some stuff that had an alloy (7075) puck, with electronics on it and a CF cover - it measured better than we expected, absorption might well have been a factor as well as the irregular shape.

author: Faringdon
author_rank: Super Contributor
timestamp: July 20, 2021, 05:10:11 pm
content: Thanks, when you have your offline 20W power PCB (LS15-13B12SS) mounted on your PCB, its awkward to cheaply arrange for an earthed metal enclosure to totally surround the module (above below and all sides), because the PCB its mounted on is in the way.Do you have any tips for easily doing this, a cheap to assemble method?https://www.mornsun-power.com/index/sitesearch/partlink/keyword/LS15-13B12SS.html

author: David Hess
author_rank: Super Contributor
timestamp: July 20, 2021, 06:12:49 pm
content: Besides carbon fiber, some black plastics are very conductive as well.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 20, 2021, 06:55:50 pm
content: Quote from: Faringdon on July 20, 2021, 05:10:11 pmPCB itself can be one side. A shielding can for the other sides - one way of doing this is photochemical etching, where you can make score-lines to allow teh flat sheet to be cut and easily folded into shape.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 20, 2021, 07:05:08 pm
content: Yeah, custom shield cans aren't that expensive.Plastic can also be metallized on the inside, whether as a vacuum process, electrochemical, or conductive paint.Have used such a box before, one of those "EMC rated" IP67 enclosures. The gasket looks funny, and yep, it is indeed conductive while sealing the thing air tight. Resistivity and shielding effectiveness about as stated. Downside, they're not cheap...Tim

author: SMdude
author_rank: Frequent Contributor
timestamp: July 21, 2021, 02:15:02 am
content: The best way to treat EMC issues is circuit design and layout.Shielding has little effect at the lower end of the frequency spectrum and this is where your device is likely to fail(between 60-300mhz).Shielding is more to prevent static discharge from damaging your device or external capacitance from effecting sensitive measurement circuits, much like shielded cables.If you are designing a product that is intended to be sold, you just need to bite the bullet and get emc and electrical safety testing done where applicable. Most labs will also offer a pre-compliance test that will be much cheaper and if the device will not obviously pass emc, then you have saved some money and depending how nice/useful they are, may even provide some suggestions on how to improve your emissions. Rise and fall times of mosfet gates in SMPS play a big role in radiated emissions, along with trace length's and ground plane routing. Make sure you have allowed for ferrite beads to be mounted on the input and outputs along with filter caps. You may also need to put ferrite rings on your wires to and from the power supplies. The EMC lab will generally have samples of these or make suggestions as to what types to use depending on the test results.The devices I design and build generally cost me about $1200aud each for emc testing. I don't sell that many, but it is just something that I have to do. The more I do it, the less stressful it is as you learn along the way what makes things fail and what you can do on the design level to help mitigate this.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 21, 2021, 04:35:27 am
content: Well... as always, It Depends.For a lot of things, a flux band around the transformer, or an electrostatic shield around the circuit (often including heatsinks, metallized enclosure, etc.), handles low frequency, near-field issues.Most commercial stuff doesn't seem to need much electrostatic shielding, so that's nice. Have seen a few electric field tests, that show up in some automotive and other standards, that are awfully sensitive to the sort of thing, and had to put an RF can over a piddly little flyback converter just to knock it down a few dB (10 was needed, the can gave us more than 40).Tim

author: Faringdon
author_rank: Super Contributor
timestamp: July 21, 2021, 06:48:51 am
content: QuoteThanks, this is interesting, regarding 60-300MHz, i have definetely seen big improvements in this frequency range when using shielded cable instead of unshielded cable, so i would have thought that metal shielding boxes for offline switch mode power supplies were also useful at 60-300MHz, i mean, am i wrong? In fact, i have tested Offline SMPS's that are totally enclosed in metal enclosures, (radiated emissions pre-compliance) and they are EMC silent....virtually no noise at all.....so i find it hard to understand that a metal box round an offline SMPS is not useful for 60-300MHz. This has not been my experience.----------------------........................Also,We have three low voltage, SMD, Buck converters for overall 12W that we want to shield (block radiated emissions) with the following through hole metal PCB shield.BMI-S-205-F (on page 11 of the following.....)https://www.laird.com/sites/default/files/board-level-shields-catalog-download.pdfDoes the Frame bit have to be soldered in to the PCB ?(ie presumably the PCB slots are plated through for soldering purposes.Also, regarding the PCB shield two posts back(BMI-S-205-F and BMI-S-205-C).....is there any reason that our mechanical engineer can't copy these, so that we can make our own?