When Common-mode Chokes Fail to Work

by Dr. Min Zhang, the EMC Consultant

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The basic structure of an inductor is simple. Wind an enamelled wire around some magnetic core material will give you an inductor. But there are many different types of magnetic core material such as ferrite, powdered iron. The shape of the core can be toroid, E-shape and many more. The winding can be a single strand conductor, or multi-strand (rope-type) winding, or even a Litz wire. Engineers should select the right choice of inductor for their specific applications.

For example, nanocrystalline materials have become popular for inductor/common mode choke due to their performance in the broadband spectrum. But for motor drive application or high power switched mode power supply (SMPS) application, where EMI issue often starts with a few kHz, Manganese-Zinc or iron-powdered core is a better choice.

If you increase the number of turns of an inductor’s winding, you would expect the inductance value increase. In fact, you would expect the inductance value increase a lot since the relationship between the inductance and the number of turns is defined in Eq. 1. As it can be seen, the inductance is proportional to the n2.

L=n2Akμ0 (Eq.1)

where n is the number of turns of a winding, A is the cross-section area, k relates to the geometry of the coil of an inductor, μ0 is the permeability of free space.

In reality, however, this is often not the case. As the number of turns increases, so is the turn-to-turn capacitance of the winding. Increased turn-to-turn capacitance shifts the resonant frequency of an inductor to a lower frequency point, meaning the capacitance part of an inductor starts to dominate. In fact, this is one of the main reasons that engineers sometimes find an inductor/common mode choke (CMC) shows little impact in a design.

A case study is presented here to demonstrate the point we made. In Figure 1, a two-stage filter featuring two CMCs was designed to suppress noise in the frequency range of 20 to 30 MHz. The datasheet of the CMCs suggests good attenuation in the frequency range of interest, however, when the circuit was tested, engineers found the filter didn’t suppress the noise as they had hoped. The CMC is a nanocrystalline core with a ‘rope’ type winding structure. The first suggestion we made is to remove the two CMCs from the circuit and re-tested the board EMC performance.

Figure 1 two CMCs used in a two-stage filter for a DC-DC converter

To the engineers’ surprise, removing the CMCs improved the noise performance in the frequency range between 20 and 30 MHz by at least 6 dB. In the lower frequency range between 150kHz and 1 MHz, however, the noise performance was getting worse. We didn’t have the test result in hand, but for demonstration purposes, see below

Figure 2 The effect of the CMCs in this circuit

It is not a surprise that the CMCs didn’t work in the designed frequency range, as from 20 MHz, the winding capacitance due to the structure of this CMC dominates. In the lower frequency range, the leakage inductance of the CMC has an impact, this explains why removing the CMCs, the lower frequency EMC performance was getting worse. 

Changing the two CMCs to a ferrite core with less turns of winding solved the problem. 

Skin effect, eddy current and proximity effect are all related to frequency. As frequency increases, RF current tends to travel on the very outer thin layer of a conductor, hence the name ‘skin effect’. Engineers should be aware that these effects not only significantly increase the loss of an inductor, they also have an impact on the EMC performance. 

Ferrite Cores Basics

By Dr. Min Zhang, the EMC Consultant

Mach One Design Ltd

Ferrite materials such as Manganese-Zinc (MnZn) or Nickel-zinc (NiZn) are often found in the core material of an inductor. They are also popular materials for a range of inductive components called ferrite cores (as shown in Figure 1). Ferrite cores are extremely useful in suppressing RF noise on cables. During the product development stage, they are often used for quick troubleshooting and problem fixing. For a product that is close to the market launch deadline where iteration of the board design is impossible, putting a ferrite core on cables sometimes is the only cost-effective way of getting the product pass the EMC limit.

Ferrite cores can be used on single wire (as a differential-mode impedance) or a bundle of wires (as a common-mode impedance). A single-turn feedthrough configuration sometimes provides sufficient attenuation on the line. But most of the time, you might need to put multiple turns of a cable through a ferrite core so as to increase the impedance, as the impedance (inductance) value of a ferrite core is proportional to the square of the number of turns.

Figure 1 Ferrite cores for round cables

Engineers should be aware that although the core materials are often the same, different cores work in different frequency ranges depending on the manufacturing of these cores. Manufacturers often have specific cores for a specific frequency range. Make sure to use the right cores for the right job. For instance, if it is the medium frequency range noise between a few MHz and 30 MHz that you want to suppress, find a ferrite core whose impedance peaks in this frequency range. Figure 2 demonstrates ferrite cores on a cable (DC side) inside the cabinet of a three-phase uninterrupted power supply (UPS) system. In this case, noise level between 10 MHz and 30 MHz is quite high in the system, therefore a 31 material which works best in the same frequency range is selected to suppress the noise.

Figure 2 Demonstration of using ferrite cores on cables

Although ferrite cores are useful for suppressing the RF noise on the cable, they cannot 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. In applications such as automotive products, using multiple turn ferrite cores are not allowed because of the limit of the bending radius of a cable. In general, a well-designed inductor is preferred. Ferrite cores are useful as a last resort in the design and development stage or the production volume of the products is very small.