EMC Pre-Compliance Visit: What to Expect and Bring?  - Page 1
Forum section: electronics / projects

author: techy101
author_rank: Unknown
timestamp: April 10, 2019, 01:54:46 pm
content: First some background: I’m a young engineer (young in the field, not age) having returned to school then graduated at the end of 2016. I work at a small medical device company, am the only engineer here, and have spent the last year developing a new product. It’s a 4-cell NiMH powered iontophoretic stimulator that contains four 5VDC->120VDC flyback converters in an ABS enclosure. It has external cables that would be connected to a patient and is the first truly new product we’ve made in about eight years.I am taking this device to a testing lab tomorrow for IEC 60601-1-2 EMC pre-compliance testing, have never done this before, and was hoping to get some guidance as to what to expect and what to bring. We don’t have any in-house EMC test equipment (probes, spectrum analyzer, etc…) so I feel like I’m going in pretty blind. I’ve been told that most devices fail the first time and I’m prepared for that, especially since this is my first time designing for EMC and have what I imagine to be a noisy device. I have five prototype units that I’m bringing along and have sprayed the inside of one unit with MG 843AR EMI/RFI shielding to compare the difference between coated and non-coated enclosures. We weren’t able to get a factory-coated enclosure in time for testing.The engineer I talked to at the facility recommended that I do radiated and conducted emissions, ESD, and conducted immunity for sure. He said the radiated immunity takes a very long time, but I might be able to get in one pass. He also inquired if I was okay possibly destroying one of my devices (which I am); which tests might be expected to destroy the device?Do people have any suggestions on how to best make use of my time there and/or what tools I should bring? I realize that’s a pretty vague question, but nerves are getting the better of me right now and any guidance would be appreciated.Thanks!

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 10, 2019, 02:49:57 pm
content: ESD may damage your device.As for what to bring? Copper tape? Common mode clamps? If you want to do some on-site debugging maybe inductor kits, capacitor kits, solder gear,...

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 10, 2019, 02:52:06 pm
content: Do ESD last, as that might kill something.Take as many spare boards, options for filters, shielding etc. as you can, a good soldering iron & any other rework gear you may need. Assume nothing about what tools may be available there.

author: pix3l
author_rank: Regular Contributor
timestamp: April 10, 2019, 02:59:53 pm
content: If the device has external (USB) cables it's always useful to have some ferrite cable clamps on hand (EMC lab will likely also have some available but then you have to go with the ones they have there or retest once you've chosen a ferrite clamp).

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 10, 2019, 03:22:39 pm
content: Bring anything needed to make your device function in a nominal way and what's needed to show it does (that may include test tools, test programs, etc)Bring the schematics and be prepared to describe what your criteria of "functioning properly" in different use cases are, as this is how tests will be issued and judged. If your device can be used with cables and accessories, bring them.Usually, testing labs already have a lot of equipment including soldering stations and many lab instruments, so this is rarely needed to bring them (and if they do IEC 60601-1-2 compliance testing, which is for active medical devices, this is likely to be a decent lab), but ask just in case before going there so you don't get stuck. They usually also have lots of component kits including inductors, ferrites, resistors, capacitors, ... just bring anything extra that could be needed, such as smd fuses if your device has any, etc. But again, ask them.As already mentioned, bring several indentical units, as some tests may be destructive. Most likely ones are ESD tests, but some immunity tests could potentially be destructive as well if some part of your design is particularly sensitive to that. Also, if your device contains some MCU or the like, destruction could also occur as a consequence of some immunity tests getting the MCU in an abnormal state that would cause it to control the underlying hardware in an unexpected way (causing shorts or other issues...)A very important point as I mentioned first is to be able to make your device function without any direct user interaction, as you will usually not be able to enter the testing chambers while the test is running (for most emission and immunity testing). (This isn't the case for ESD testing which is usually done manually in the lab.) So, for instance, if your device required the user to press a button continuously to execute some function and then stopped once they release it, that would be a problem. You'd normally have to find a way to make the device execute that function continuously for testing.

author: techy101
author_rank: Unknown
timestamp: April 10, 2019, 03:52:28 pm
content: Thank you everyone for the responses!Quote from: SiliconWizard on April 10, 2019, 03:22:39 pmThe device delivers a constant current through a load and, for obvious reasons, we can't use a person in the chamber so we're using resistors as our loads. It monitors and displays the applied current via an ADC, but it seems to me this is not a reliable measurement method because it's dependent on the device operating properly in the first place. We currently use DMM's in line with the load resistor to verify the applied current, but I was cautioned that it's not the best idea to put a DMM in the chamber during immunity testing because we don't know how the test will affect the DMM itself. I'm at a bit of a loss then for how to verify proper device operation without the use of a DMM attached to the device. The cables used to connect to a patient are only six feet long so they won't reach out of the chamber.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 10, 2019, 05:56:16 pm
content: Quote from: techy101 on April 10, 2019, 03:52:28 pm

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 10, 2019, 06:01:06 pm
content: I would try connecting a shielded cable in parallel to your resistive load, long enough to get out of the chamber via one of its feedthroughs (probably something like 3 or 4 meters). I'd suggest preparing this cable, attachment to your dummy load and the measurement setup in your own lab, checking that you can correctly visualize the generated current with an oscilloscope for instance. Experiment thoroughly before going to the testing lab. Ask them for a recommended cable length adapted to their chamber.

author: techy101
author_rank: Unknown
timestamp: April 10, 2019, 06:31:40 pm
content: Quote from: mikeselectricstuff on April 10, 2019, 05:56:16 pmI had to do some digging, but I found an analog meter buried away! Only the negative range works, but that's good enough for me.

author: jbb
author_rank: Super Contributor
timestamp: April 10, 2019, 08:00:05 pm
content: I would add screwdrivers (there are so many sizes!), multimeter, and a set of PCB prints. The PCB prints should show component placements and track placements. Multiple pages may be required.If you need a PC or laptop, bring it. Trying to runanythingone one of their lab laptops is asking for trouble simply because you might have forgotten something (device driver, terminal program, application software library etc.).Also make sure your device has all the boots, feet, stickers etc. fitted. This is relevant to ESD testing, because if they see a metal mounting screw they will hit it with a contact discharge. If that screw is covered with something (often a label) then they will just do air discharge nearby which is much less severe. Also, having a real serial number on each device is a good plan (not just the good 'ol all zeros).Optimising the test procedure is also a really good idea. The lab people don't want to spend hours and hours staring at the on-screen display to see if it misbehaves; we try to provide our external labs with a laptop that has test software on it to go with the device under test. Our test software generally does a test loop,logs outputs, and maintains a running display the loop count, warning count and error count. It's not so relevant when you're on site, but test logs can be extremely helpful when you're not on site and something's gone wrong.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 10, 2019, 10:13:19 pm
content: Quote from: techy101 on April 10, 2019, 03:52:28 pmAre your immunity results actually going to be representative? Without the ~100pF bag of mostly water at the end of those cables, you can't expect the same RF currents as in practice.Human analogs exist, which I imagine you could rent for a test; or if that's rather over-the-top, it might be okay to use a box of about the right size, with some resistors to it to model the ESR of a body. The DMM could even be shielded within, where it could be monitored with a camera if you like.Tim

author: joeqsmith
author_rank: Super Contributor
timestamp: April 11, 2019, 12:34:59 am
content: Quote from: techy101 on April 10, 2019, 03:52:28 pmSounds like fun. They should document everything for you but it may not hurt to take a few pictures of the setup.You are right that meters can be a problem. Gossen was saying they test at 3V/m and call it an industrial. Here I am having some fun with a few of them.https://youtu.be/wYuzFtoHMqg?list=PLZSS2ajxhiQBTCU8Mq_i9jidT024A0dV6&t=829Sometimes the equipment radiates and needs some help. Aluminum foil is my weapon of choice. Always good to carry a new roll....Have fun.

author: coppercone2
author_rank: Super Contributor
timestamp: April 11, 2019, 01:08:50 am
content: king james bible

author: pix3l
author_rank: Regular Contributor
timestamp: April 11, 2019, 06:50:06 am
content: Quote from: blueskull on April 10, 2019, 06:18:18 pmTrue that

author: Berni
author_rank: Super Contributor
timestamp: April 11, 2019, 09:23:28 am
content: I can already tell you that the radiated emissions are going to be your new nightmare when i see it contains "four 5VDC->120VDC flyback converters"There are generally two things that cause a lot of radiated emissions, those are fast parallel data buses and switchmode power supplies. From the various switchmode typologies flyback tends to be particularly noisy and the high output voltage makes the already high inductor dv/dt even higher.The ESD test is mostly just a matter of putting protection diodes in the right places and the immunity test is mostly a matter of keeping your ports filtered and ground return paths near by.You can still do quite a bit of EMI precomplicance with a scope and FFT since most issues are visible under 100MHz

author: techy101
author_rank: Unknown
timestamp: April 11, 2019, 12:03:38 pm
content: Well, ready or not here I go! Thank you again for the comments and opinions.Quote from: T3sl4co1l on April 10, 2019, 10:13:19 pmThis is going to end up being something I might make a separate post about. I'm not sure how exactly to test this properly since the skin impedance changes over the duration of a treatment. It's also dependent on the person, their skin, location on the body, etc...Quote from: Berni on April 11, 2019, 09:23:28 amYup. This is exactly what has been making me so nervous about these tests.Quote from: coppercone2 on April 11, 2019, 01:08:50 am

author: coppercone2
author_rank: Super Contributor
timestamp: April 12, 2019, 09:31:32 pm
content: yes, this needs to meet class B emissions because its medical for iec 60601. it's considered tough. If you never did it before you will be suprized by how chaotic it is (i.e. you might think you can disconnect certain subassemblies to get a clear idea).. it probably won't end up being very clear.

author: mayor
author_rank: Regular Contributor
timestamp: April 13, 2019, 10:31:29 am
content: How'd it go?

author: techy101
author_rank: Unknown
timestamp: April 18, 2019, 05:12:21 pm
content: Quote from: mayor on April 13, 2019, 10:31:29 amSorry for the delay, I went on vacation right after my visit to Intertek and ignored work completelyOverall the testing went well. It was nice to actually see the test setups and go through the process. The unknown made it much more stressful and now that that’s gone I’m a lot more confident moving forward. I went out to PCB West last fall and took a few sessions by Rick Hartley and Daniel Beeker which talked about techniques and tips for reducing emissions, etc.. I implemented some of these and would like to think they helped out, but I suppose you never really know.Anyway, I passed radiated emissions by about 20dB, which was a huge surprise. I also passed conducted immunity and most of radiated immunity. One frequency, 870MHz, however caused a major failure. The output current shot up and hit one of our safety cutoffs (set at +- 0.1mA), triggering the device to log the fault and partially power-down, as it’s designed to do. The delay on the analog meter we had in the chamber, combined with some image vibration in the camera they use, makes it impossible to tell if the current actually increased or it caused some sort of fault in the ADC. My money, however, is on the current actually increasing. This only occurred when the antenna was in the horizontal orientation, not the vertical.I haven’t had much time to dig into this yet but did notice an awfully big coincidence; the wire length inside the device, between the external connector and the board header, is darn close to the 870MHz wavelength of 344mm (see attached image). I’m hesitant to make any large jumps based on this alone because there is a lot more than just the wire itself involved, but I do find it interesting. Ferrite clamps on the cables didn't make a difference and the issue occurred whether or not out long applicator cables were attached to the device or a small dummy load, so I see that as pointing to something internal to the device itself.The other failures I had were pretty bad on the ESD front. The good part was that it passed 15kV air discharge on the keypad. 15kV air discharge on the graphic LCD did cause some arcs which caused some pixels to go black, but they returned to normal within 15-20 seconds and there was no change in device behavior.Now the bad. The connectors on the back of the device (the ones shown in the photo) failed by arcing to the outside of the connector at just 4kV. The bodies of the connectors aren’t tied to anything and are mounted to an ABS enclosure. When the system was not under a heavy load (stimulating) this caused the LCD backlight to drop out then come back on. When the device was under heavy load it caused the whole system to shut down. To me, this is indicative of a brown-out because the microcontroller is used to hold the system 3.3V power supply on or shut itself down. I think the ESD strike is causing a power blip that, when not stimulating, the onboard capacitance enough to prevent the microcontroller from losing power and shutting the system down. But, when stimulation is happening, the large power draw from the flybacks is enough to cause the microcontroller to lose power, shutting down the 3.3V system power supply.At this point, I’ve not had time to dig into it, but we have tossed around a couple of thoughts. The device has four channels, three of which are isolated from the fourth channel which shares a ground with the rest of the system. However, there is no difference in behavior between ESD strikes on any of the four connectors. The isolated nature of this device has caused headaches all throughout the process and has me scratching my head at how to approach the problem. I’ll be digging into it over the next few days and we have placed a rental order for an ESD gun and IEC 61000-4-2 spec test table setup (I wanted to build one, but the boss went for rental instead). That should be here the end of next week and will be very interesting to play around with. On the one hand, I’m happy that the device failed “safe” by shutting down, but I also kind of wish it had failed catastrophically so Imightbe able to see the path it took.Finally, there were failures at 15kV through the gap between the battery door and the body of the enclosure that, I think, went to the battery terminals. This will require further investigation with the device opened up.Overall though, for my very first design, I’m quite pleased. I was worried everything would fail all over the place, but it passed many of the tests, especially the radiated emissions. With my lack of experience, I’ve spent a lot of time researching and focusing on design for EMC for this project and I’d like to think it paid off.Now, on to problem solving!

author: Gyro
author_rank: Super Contributor
timestamp: April 18, 2019, 07:18:22 pm
content: It sounds like you did really well there. Yes, a trip out to the test house can make a fun outing (unless things go really badly). When CE was first being introduced, our test house used to run night slots due to the demand - the incentive was free pizza at 11pm! 20dB below the line on emissions isveryrespectable, especially in an ABS case.For the susceptibility and ESD, it's worth checking that the requirements for your product are 'no effect', 'disruption but recovers without power cycling', or 'must recover after power cycle'. Floating the shells of connectors is always a bad one - you can get solder tags for those connectors to allow them to be connected together and to therightgrounding point.For the 870MHz susceptibility, very low value capacitors from I/O connections to ground can make a difference. I once had a case where a 10pF capacitor in the right place solved a problem in that frequency range. Larger values had too much self-inductance to work effectively! The ferrite beads failing to cure it probably indicates that the cables weren't the way it was getting in (case again?). Shunting with a capacitors will hopefully be more successful.

author: techy101
author_rank: Unknown
timestamp: April 18, 2019, 07:39:57 pm
content: Quote from: Gyro on April 18, 2019, 07:18:22 pmI actually asked the guy to show me a run with nothing in the chamber because I didn't believe him. The board is probably going overkill on ground floods and ground transition vias, but it worked so I'm not going to try and get fancy with it. It's not a product that needs to be value engineered to the point of reducing drill hits.Quote from: Gyro on April 18, 2019, 07:18:22 pmThat was going to be the starting point, but I'm struggling with the "right" grounding point since there are four "grounds" on the device. We'll see how things shake out next week when the tools come in and I might end up making a dedicated post on the topic.As for the 870MHz immunity, am I correct that there's no good and cheap way to test this other than going back to the test house? I also forgot to mention that it happened only in the horizontal antenna orientation, not vertical.

author: Gyro
author_rank: Super Contributor
timestamp: April 18, 2019, 07:56:11 pm
content: Yes, the "right" ground is tricky - but you've taken the right action, hiring an ESD gun.Well 870MHz is around the UHF TV transmission band (don't know if you have that in the US), you're unlikely to have a TV transmitter but an antenna might be handy. Useful to know the horizontal polarization, I'm assuming that's the plane of the PCB too. In that case you're probably looking for a feature on the PCB that acts as a dipole at that specific frequency. For the fix, you may have to rely on going back to the re-test with a bunch of ideas (possible suspects), some low value caps and copper foil.

author: jmelson
author_rank: Super Contributor
timestamp: April 18, 2019, 09:21:08 pm
content: Quote from: techy101 on April 18, 2019, 07:39:57 pmYou could get an oscillator, likely need a small power amp, and a hand-made dipole around that frequency. Probably you can rent this gear, too.I'd strongly recommend getting or renting the gear and setting up the immunity test in YOUR shop. emissions testing is difficult due to the noise everywhere, but immunity should be simple to do in-house. As long as you aren't doing it all day, nobody is likely to complain, unless it takes KW of RF to cause the effect.Jon

author: Berni
author_rank: Super Contributor
timestamp: April 19, 2019, 05:32:04 am
content: Yeah it should only take a few watts of RF power to reproduce any of those immunity issues if you stick the antenna close enough. Not too difficult to find a RF amplifier for that power and make your own antenna. Tho some gear for tuning the antenna to the exact freqency is helpful (Like an antenna tuner or network analyzer)But i do recommend getting a small EMC chamber for your diy pre compliance testing. You won't get busted for blasting out RF a few times here or there, but you should generally try to avoid doing this as you have no idea what RF communications you could jam with your signal.The EMI tents are the best way to go. They are simply a tent made out of conductive stainless steel fabric. They don't cost too much, are easy to ship when packed up and provide a surprisingly good amount of RF shielding. You can get them in pretty much any size you want. They make EMI emissions testing much much easier because there is way too much RF in urban enivorments to do a proper test and for EMI immunity it allows you to pump in as much RF as you want without jamming anyone and getting a visit from the authorities.Solving EMI problems can be a very iterative process where you make fixes and test them. Some fixes do nothing even tho one would think they should, while other fixes that you think wont help at all actually help a lot. So having a way to do a crude test in your lab helps a lot and gives you confidence for when you go the real tests at the certification facility.Oh and a poor mans ESD gun is a piezo igniter from a gas grill, they produce a pretty consistent high voltage pulse. If you can't kill it with one of those then it will likely also survive a ESD gun.

author: Gribo
author_rank: Frequent Contributor
timestamp: April 19, 2019, 12:38:22 pm
content: 870 MHz is within the Cellular bands in north America. You can get a cellphone as an interference source.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2019, 01:08:05 pm
content: Any one that produces a "blip-bip-beep-bip" (GSM radio) in a nearby powered speaker (poorly filtered analog system) will do.