In this article, we'll go over some of the ways ferrites should not be used in a PCB, as well as how they actually operate in terms of their filtering behavior. Ferrite beads are sometimes applied for EMI in two ways in attempts to minimize EMI, but the designer ends up creating a new EMI problem if these components are not used correctly. You're probably reading this article on a laptop that uses a ferrite to filter out conducted EMI from the supply line.Ī problem begins to arise when you try to apply the same logic to other areas of a PCB. These components are basically filters, and they do perform a useful function on power cords in many electronics. One of these areas relating to EMC in PCB design is the use of ferrite beads. We may need to do a lot more.EMI and EMC can be a tricky subjects, and it's often tempting to mis-apply design guidelines to try and reduce EMI. We may, in some awkward designs, need to use two cascaded $\pi$ filters and apply a broadband assault on interference. In other words, we can only go so far with the physical realities of a single $\pi$ filter stage. You realistically can't rely on a $\pi$ filter to deliver the goods across more than 2 decades of spectrum.Ĭapacitor self-resonance is certainly a consideration Hopefully you can see that just a few nano henries of self inductance for C2 makes a big potential problem around 20 MHz or 30 MHz and, above 100 MHz we might as well not have this $\pi$ filter at all. Capacitors are usually at least an order of magnitude better than inductors for filter applications but, they are still prone to problems and can resonate with leakage inductance significantly:. There is also self-resonance for capacitor C2 to consider. Inductor self-resonance is certainly a consideration We are still getting 60 dB attenuation above 3 MHz but, sometimes, we need more. With L1 at 47 uH and C2 at 100 nF, the point where $F_C$ occurs is now at 73.4 kHz but, from about 3 MHz, we get no deeper attenuation. The parasitic (aka self) resonance of the inductor occurs at just over 2 MHz and although this is produces impressive attenuation (a big "notch") there is a down-side. So, if I make L1 into 47 uH and added 100 pF of parasitic capacitance, I would get this AC response:. This self-resonance is due to internal parasitic capacitance. Initially I've considered the self-resonant frequency of a practical inductor. In other words, I don't want to overload the images with too many responses because it might confuse things. In the examples below I've constrained the load resistance to be fixed at 20 ohms for reasons of simplicity. To get an inductor of 47 uH also comes with another hidden cost and that is self resonance. This is a potential cost and size constraint placed on any "volume-product". For instance, to drop from 734 kHz to 73.4 kHz requires that L1 increases to 47 uH and C2 increases to 100 nF. Ideally, you would want $F_C$ to be as low as possible but there are constraints on how low you can go. The one above begins at this frequency:. How you use and load the $\pi$ filter is certainly one main consideration.Īnother consideration is where the attenuating slope of the filter's response begins. However, to use a $\pi$ filter on it's own or connected to a load that has high impedance is asking for trouble because, at the resonant point of the circuit, it can seriously amplify interference: -Īs you can see, if the loading is too light, the resonance of the inductor with load-side capacitor causes significant problems. The regulations dealing with EMC and EMI are largely interested in the prevention (or significant reduction) of high frequencies and the $\pi$ filter achieves this because it is a low-pass filter it allows low frequencies to pass unhindered (such as AC mains power voltages and current) but attenuates the higher frequencies progressively. Applying a $\pi$ filter like the one below can significantly attenuate those unwanted high frequencies:. Using the filter correctly and understanding its limitationsĮMC or EMI (electromagnetic interference) is noted for its high frequency energy content. What would be the main considerations to use Pi-filters for EMC?
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