![]() They may also want to know how they can get more attenuation out of their filter design in order to pass an emissions or immunity test. However, selecting the proper filter for a given situation may be confusing to some, especially if they are new to the EMC field or have not dealt with the subject in some time.ĮMC practitioners may be asking themselves what filter configuration is the best one to use for any given application or how to correctly choose the values of components given the frequency, circuit impedance, and other parameters of the circuit. The purpose of a filter is to establish either a low-impedance path for RF current to return back to the local source of energy, and/or to provide a high impedance to prevent RF currents from flowing on a cable. If you’ve got any EMC problems then give me a call, I’d be happy to help.One of the roles of the practicing EMC engineer or product designer is to be able to design filters to add to circuits in order to get them to pass various EMC immunity and emissions standards such as IEC 6 for ESD immunity, IEC 6 for Radiated RF immunity and IEC 6 for Electrical Fast Transient/Burst immunity and other various international standards covering Radiated Emissions (RE) or Conducted Emissions (CE).ĮMI filters are often used along with proper shielding in order to achieve EMC compliance. Much nicer to be able to recommend a low cost cable assembly than £$€ 20 worth of filter block. This was a fun half day project fixing this particular problem. Not just for the higher frequencies but lower ones too! Will this have the desired effect on emissions? Logically then, connecting the DC negative to the PE on the power supply will provide a shorter path for this common mode noise from the power supply. The AC mains line to each LISN contains a PE connection and, inside the LISN, this is connected directly to the floor of the chamber. Our common mode emissions measurements are being made in relation to the metalwork of our screened room test setup which is connected to the AC mains Protective Earth (PE). So where is this common mode current returning to? What is it’s reference? But, as I point out in one of my talks, current flows in a loop and always returns to the source. That’s much better! It looks like the problem was common mode noise after all.Ĭommon mode noise in this instance is current on both the DC output lines together. Let’s try a common mode mains filter on the output of the power supply to see if this is indeed the case. Most EMC problems are common mode in nature, I’m just used to thinking about LF conducted emissions as a differential mode problem. This sounds like a obvious question to ask in hindsight. Unfortunately it did nothing to the emissions! This pi filter was made up from two Panasonic FC series 470uF, 25V on either side of a Wurth 33uH iron powder inductor. I’ve got a little filter prototype board that I use in situations like this. So the conclusion here is that the industrial power supply DC output is very noisy, that this noise is propagating through the system, and manifesting as conducted emissions on the outputs via a variety of coupling paths.īecause conducted emissions noise in this lower frequency range tends to be differential in nature (+ve relative to -ve), my first thought was to implement a differential mode filter on the output of the power supply. The idea here was to eliminate the industrial power supply from my inquiries. I replaced the industrial power supply (often designed for Class A emissions performance) with my trusty Thandar TS3022S adjustable linear bench supply. The first thing I wanted to do was to simplify the test setup as much as possible. To my mind, this points towards a single component in the system causing the same noise to be seen everywhere. Notice how the shape or profile of the emissions is almost identical. The lessons from fixing this problem was that the first thought is not always the correct one, and that sometimes, all you need is a bit of green-and-yellow earth wire!Ī block diagram of the system is shown below with the major components shaded.Īn industrial power supply feeds power to the controller (a custom PCB connected to a Raspberry Pi) and to the power measurement board (measures the power consumed by the load).Ĭonducted emissions on both the Ethernet port and the AC mains port on the power measurement board were both dominated by a low frequency hump around 700kHz. The customer had some conducted emissions failures at another EMC lab and needed some help resolving them. Here is an interesting problem I had working on piece of industrial equipment.
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