In my previous post I mentioned Electro-Magnetic Interference. This got me thinking about the topic itself.
One of the things I was most worried about when I started my job as Field Applications Engineer for power components was how to help customers who phoned up from the EMI test lab where the equipment was failing the limits. Let me give you an example.
I received an e-mail from an establishment that was testing a piece of equipment. It was failing conducted EMI and they sent me the frequency plots to prove it. “Probably a layout issue, ” I mused in reply and asked them to send me the layout for review.
Now, as it turned out, this establishment wasn’t the manufacturer of the equipment and a few days later, I received a slightly embarrassing phone call from the manufacturer telling me I told their end customer that their layout was terrible. They actually took it in good humour and invited me down to review the layout with their engineers and PCB designers, an offer I eagerly accepted.
Well, it was me in a room with the design engineer, the PCB layout engineer, the project manager, the production manager and the MD of the company, all looking to me for answers. I loosened my necktie, rolled up my sleeves and got to work.
Well, on reviewing their layout, there were no big mistakes. They hadn’t filled a plane under the filter module, so the switching frequency of the converters wasn’t getting past the filter by capacitive coupling. It all looked rather well done actually.
The fact remained though that the switching noise from our converter was getting out of the box somehow.
I felt somewhat deflated (because I couldn’t help them rather than because I had been proved wrong in my assertion that it was a layout issue) but promised to write up the one or two minor tweaks that could be done in a report and added that some cooper shielding around the regulators may help reduce the noise.
Almost as an afterthought, they asked me whether I would like to see the unit and I graciously accepted (more because I love electronics than anything else) and I’m glad I did! I spotted that they had a cable that ran from the one side of the enclosure, all the way to the other where the inlet connectors were and guess what, it passed right over the top of our switching regulator.
The smoking gun I had been looking for!
I suggested re-routing the cable, away from the regulator and as far as I can tell, this has solved the problem and the device is now passing its EMI tests very well.
The moral of the story is that in all my years as an FAE there has never been an EMI problem that cannot be solved. The solution may involve re-spinning the PCB; it may not be easy, quick or cheap to implement the solution; but there is always a solution and sometimes, just sometimes, it’s a really simple one.
One thought on “Will it meet EMI?”
Your final comment is definetely correct.
The EMI problem / issue can come out also with good layouts depending on the loads that you have to manage.
The layout of the power board is for sure important but it’s also necessary to assure that all the loads you have to manage can be correctly filtered accordingly.
This is a matter of experience.
[Power Supply Designer]