Page 8 - EE Times Europe September 2021

8 EE|Times EUROPE

The Magnetics of Silence — Advanced Materials for New Challenges

Ferromagnetic materials for cable shielding and CMC cores are
typically ceramics, metals, or composite materials. Manganese-zinc
(MnZn) and nickel-zinc (NiZn) ferrites are widely used and are in
the category of ceramic materials. Nanocrystalline (NC) cores are a
new class of materials that are classified as metals and are becoming
widely used in applications such as energy storage, in which low-
frequency electromagnetic noise can become a problem. The mate-
rials’ intrinsic properties permit cable ferrites and choke cores to be
smaller than is possible using conventional ceramic ferrites, while
delivering superior magnetic performance, and so present a solution
for dealing with the automotive e-NVH challenges now emerging.
NC cable ferrites are produced as a rolled metal film, as seen in
Figure 2. The figure shows a cross-section depicting the metal film
and the external protective coating needed to prevent damage to
the metal film during handling and assembly as well as during use,
thereby maintaining electrical continuity.
The NC material is created using a high-temperature melting process
(1,300°C) and quickly cooled to promote formation of the amorphous
structure. Annealing then follows, during which the material is heated
between about 500°C and 600°C to achieve the nanocrystalline state. Figure 3: Impedance versus frequency comparison of ceramic and
During this time, the material is submitted to magnetic fields to NC cable ferrites
optimize its magnetic properties. The resulting NC material exhibits
ultra-fine grain sizes of typically 10–15 nm, which compares with grain
sizes in the order of tens of microns for ceramic materials. inductive impedance. Figure 3 compares various ferrite materials of
The properties of NC cable ferrite materials can be effectively com- identical dimensions under equivalent test conditions. The results
pared with MnZn and NiZn ceramics by examining the permeability show a recommended impedance-frequency behavior that acts as a
guideline for implementation in noise-suppression applications.
Würth Elektronik eiSos has several families of suppressor ferrites
for all types of EMI in electronic systems, including automotive appli-
cations, comprising both NC and ceramic materials. Among these, the
WE AENA series contains axial cable ferrites that feature the latest
NC core material technology to provide noise suppression across a
very wide frequency range. The core material is protected with a plas-
tic case to assist with physically assembling the cable and to enhance
the robustness of the completed assembly.
In addition, the WE TEMA series of MnZn toroidal cores have high
permeability and are mainly directed at applications in the low- and
middle-frequency ranges. The WE AEFA axial series is specially
Figure 2: Cross-section of NC cable ferrite showing internal designed for the high-frequency spectrum to address a wide variety
layers and protective outer coating of applications and provide high noise suppression between data and
power supply lines. Finally, the WE TEFA toroidal family is mainly
intended for use in the high-frequency range. This special material is
of each type. Ceramic materials tend to have a high saturation and composed to be wider than standard materials in the market.
a permeability value between 100 and 15,000 microns. Historically,
MnZn cable ferrite has been an important soft magnetic material due NC CMCs
to its combination of high initial magnetic permeability, saturation Common-mode chokes comprise a coil wrapped around the NC core.
magnetization, electrical resistivity, and low power losses. Its practi- CMCs based on NC magnetic cores combine excellent attenuation
cal range of frequencies covers the medium range from some tens of properties with small size. The magnetic properties of the NC core
kilohertz to a few megahertz. allow a low number of coil turns, making this a powerful technology
NiZn ferrites are one of the most versatile soft magnetic materials. for use in EMC-critical automotive applications. With reduced DC
With their high magnetic permeability and higher cutoff frequency resistance, owing to having fewer coil turns, the chokes can handle
than MnZn, they are suited to high-frequency applications from tens high-rated currents. In addition, lower capacitance ensures better per-
to hundreds of megahertz. formance and stability across a wide frequency range. These qualities
On the other hand, NC benefits from a much higher initial permea- make NC CMCs well-suited to noise suppression in equipment such as
bility than ceramic materials, due to the annealing and the effects of high-current DC/DC converters, inverters, on-board–charging systems,
the magnetization process. This improvement of magnetic properties and electric-power–steering systems.
results in permeability values up to about 100,000 microns.
CONCLUSION
PERFORMANCE OF NC MATERIALS NVH engineers must adapt their skills to encompass aspects of electri-
Although permeability is one of the most important parameters cal design that can help mitigate EMI effects. On their side, the latest
that define the performance of a cable ferrite, the magnitude of NC cable ferrites and chokes enable EMI mitigation solutions that are
the impedance versus the frequency is another important means of compact, robust, and effective over a wide range of frequencies, as well
characterizing these components. The magnitude of the impedance as meet the automotive industry’s demands from the production line
depends on the equivalent resistance of the cable ferrite and its to the road. ■

SEPTEMBER 2021 | www.eetimes.eu