Column: EMC
Suppressing radio frequency noise with ferrite cores
By Dr Min Zhang, EMC consultant at Mach One Design, and design engineers at REO UK U
ndesired high-frequency electrical noise can cause electrical equipment to malfunction. One way to combat this is by using ferrite
cores, which suppress electromagnetic emissions by blocking low-frequency noise whilst absorbing high-frequency noise to avoid electromagnetic interference (EMI). Ferrite materials like manganese-zinc
(MnZn) or nickel-zinc (NiZn) are oſten found in an inductor’s core material. Tey are iron-based magnetic materials in the form of a ceramic, meaning they have high magnetic permeability and high electrical resistivity, making them ideal for inductors. Tey are also used to create a range of inductive and resistive components called ferrite cores. Ferrite cores are extremely useful in
suppressing RF noise on cables and, during the product development stage, they are oſten useful for quick troubleshooting and product fixes. For a product close to market launch, with board re-design no longer feasible, putting a ferrite core on cables is sometimes the only cost-effective way of getting the product to pass the EMC requirements. Ferrite cores can be used on either
a single wire or a bundle of them. A single-turn feedthrough configuration can sometimes provide sufficient attenuation. However, most of the time you might need to put multiple turns of a cable through a ferrite core to increase its impedance – the resistance of an electrical circuit or component to an alternating current. Te impedance value of a ferrite core is proportional to the square of the number of turns. Engineers should be aware that although
the core materials are oſten the same, different cores work in different frequency ranges, depending on their particular features. Manufacturers will oſten make specific cores for a specific frequency
Figure 1: Ferrite used on the ouput of a static frequency inverter
range, so it’s important to use the right core for the right job. For example, when there’s medium frequency range noise between a few MHz and 30MHz that must be suppressed, it is important to find a ferrite core with impedance peak in that frequency range.
Challenges with ferrite cores Te way ferrite cores are used depends on their required application. In industries like automotive or aerospace, multiple- turn ferrite cores are oſten not allowed, due to limitations on the bending radius of a cable. Engineers can also face issues with
multiple-turn configurations of a ferrite core. Firstly, as the number of turns increases, so does the turn-to-turn capacitance, or the capability to store electric charge in an object or device. Whilst this may not be a problem at lower frequencies, it does have an impact at higher frequencies. For example, a three- turn configuration may start dropping once it reaches 40MHz. Worse still, the impedance of a three-turn configuration may drop lower than that of a one-turn configuration should the frequency exceed 200MHz.
10 December/January 2023
www.electronicsworld.com
Other considerations Another consideration that engineers must take into account is the location of the ferrite core. It is known that ferrite cores have the greatest effect where the RF current on the cable is highest. So, positioning a ferrite core adjacent to a low-impedance connection is a good approach. An example of this would be the cable entry point of a chassis. Tere are rare situations where a
ferrite core could lead to increased emissions at a certain frequency. In those scenarios, the most practical approach is to try positioning the cores in a few locations and compare the results. Overall, although ferrite cores are
useful in suppressing RF noise on a cable, they can’t replace a properly designed inductor. In environments where vibration and shocks are prevalent, ferrite cores need to be secured by cable ties or other means. While a well-designed inductor is preferred, ferrite cores are useful as a last resort in the design and development stage or when the production volume of the product is very small.
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