Development of a Ferrite Material for Inductive Chargers
Y. Matsuo, S. Otobe, F. Nakao, H. Sakamoto*
FDK CORPORATION; 2281, Washizu, Kosai-shi, Shizuoka 431-0495, Japan
*Kumamoto Institute of Technology; 4-22-1, Ikeda, Kumamoto 860-0082, Japan
Tel: +81-53-575-2532, Fax:+81-53-575-1651, E-Mail Address: matsuo@fdk.co.jp
Abstract: A new ferrite material, 6H40, was developed and its performance
verified for use in inductive charging systems. Because the 6H40 surpasses
FDK's conventional ferrite materials in magnetic characteristics, it was
expected that 6H40 would be useful in automotive DC-DC converters and also
in coil products. Super-large pot cores were then manufactured from the
6H40 so that their possible use in the development of fully automated power
charging systems for future motor vehicles could be studied.
Key Words: "Charger", "EV (electric vehicle)", "Inductive
charger", "Materials".
1. Introduction
Automakers are
actively researching the development of electric vehicles (EV), hybrid electric
vehicles, and fuel cell vehicles for protecting the global environment in the
21st century. EVs are particularly desired for their energy source diversity and
environmental friendliness. There are two major methods of charging automobiles
-- the inductive charging system (ICS) and the conductive charging system. An
ICS incorporates a connector which divides the primary and secondary coils in a
transformer and adopts a high-frequency switching circuitry operated between 100
and 370 kHz1). MnZn-family ferrite cores are known to be most suitable for this
frequency range. However, ICS-use ferrite cores need to have a) a low-loss
characteristic to minimize heat generation during charging and b) high
saturation induction characteristic to cope with large input currents. We have
developed a MnZn ferrite material, 6H402), which exactly meet these
requirements.
2. Characteristics of Soft Ferrites
Efforts are being made by many researchers to develop transformer core
materials featuring higher saturation induction, higher initial permeability,
and lower core loss in relation to frequency. As shown in Figure 1, various
soft ferrites and a number of non-ferrite materials including Fe-group
amorphous alloys, Fe-Al-Si and nanocrystalline alloys are well-known candidates
for transformer core materials3),4). Where irregularly shaped and low-loss
cores are in demand as in the case of transformers for switching power
supplies, ferrite cores are generally selected. In the case of ICSs, MnZn
ferrite cores are selected for their high saturation induction and low-loss
performance in the 100-370 kHz frequency range.
3. Development of High-Performance
Ferrites
Behind the rapid progress made in electronics in recent years is the ready availability of soft ferrites and other electronic materials. Because of the increasing demand for smaller and flatter electronic equipment, more compact and higher-performance electronic parts are being sought. MnZn power ferrites, for example, are used mainly for compact transformers5),6), and MnZn high-permeability ferrites for digital communication pulse transformers7)¡Á11). Important factors in the production of high-performance ferrites include high material purity, appropriate chemical composition, appropriate additives, uniform material quality, strict production processing and precise microstructure control5),8),9). Generally, core loss can be divided into hysteresis loss, eddy current loss, and residual loss. At low switching frequencies, it is most advantageous to reduce hysteresis loss. At high switching frequencies, the most effective strategy is to minimize eddy current loss. Since hysteresis loss can be reduced by increasing the initial permeability, methods for reducing hysteresis loss are the control of impurities and pores in the ferrite phase and the enlargement of the crystal grain size. Because eddy current loss can be reduced by increasing the resistivity of the ferrite phase, it is most effective to size down the crystal grains and insulate the ferrite phase with high-resistance grain boundaries5). In addition to these methods, FDK utilizes its original computer-aided engineering (CAE) technique to develop new ferrite materials in a short period through simulation. The CAE is combined with an electromagnetic field analysis technique to develop ferrite cores featuring higher performance and smaller size14).
4. Manufacturing
Process
As with ceramic tableware, the production of ferrite cores starts with
the mixing of materials and is completed with the sintering of the shaped
mixture. The composition (combination of primary and secondary materials)
is determined according to operation frequencies, while microstructure
is controlled by selecting appropriate sintering conditions, additives
and so on2),5),7). Figure 2 shows the core manufacturing process. First,
prescribed quantities of iron oxide, manganese oxide, and zinc oxide are
wet-mixed in a ball mill for some hours. The mixture, after drying, is
calcined at 700¡Á1,000¡î in the atmosphere for another several hours and
is then wet-pulverized in a ball mill for 5¡Á10 hours. This dry ferrite
powder is mixed with prescribed amounts of silicon oxide, calcium oxide
or other additives and, following the addition of a PVA (polyvinyl alcohol)
solution, is granulated and formed into various shapes. These shapes, after
dimensional adjustment by grinding, are sintered to produce the final cores.
FDK currently uses the types of ferrite materials shown in Figure 3 in
its production of cores. The 5H-series ferrites are applied to flyback
transformer cores for TVs and display monitors which operate at no higher
than 100 kHz. The 7H-series ferrites are used to produce high-frequency
power supply cores for operation at a minimum of 500 kHz5). The 6H-series
ferrites, including 6H402), are used to produce ICS and other cores for
operation between 100 and 370 kHz.
5. Reduction of Core Loss
The essential step in reducing MnZn ferrite core loss is to control the
microstructure of the ferrite. Figure 4 shows the relationship between
core loss and grain size at various sintering temperatures. We found the
lowest core loss at an average grain size of about 15¦Ìm. Next, each of
the additives shown in Table 1 was mixed with ferrite materials, and the
resulting cores were tested to measure their core loss. We found that core
loss was reduced by additives as well as by the control of grain size.
Moreover, as shown in Figure 5, we were able to achieve a further reduction
in core loss by increasing the resistivity of crystal grain boundaries.
The 6H40 ferrites were developed applying the above techniques for core
loss reduction2). The resulting 6H40 showed an improvement in the saturation
flux density as well as in core loss performance, and therefore was therefore
considered to be suitable for ICSs.
6.
Manufacture of Large pot Cores
Figures 6 and 7 show the external appearances of the large pot cores that we manufactured from 6H40 ferrites12). Both pot cores were intended for use in rapid charging. While the largest outer diameters of conventional pot cores range between 100mm and 200 mm, the new pot cores had outer diameters of 370 mm and 520 mm, respectively, two to three times larger than the conventional size. Consequently, to preclude cracks in the new pot cores, the speed of sintering temperature rise, composition of the sintering atmosphere, and other sintering conditions had to be controlled more strictly. Research into the application of these pot cores is now continuing at the Kumamoto Institute of Technology13), and efforts will be made to determine optimal shapes, achieve further loss reduction, and realize commercial applications. We have great expectations that the pot cores will be used in fully automated power charging systems and charging stations for EVs.
7.
Conclusion
A new ferrite material, 6H40, was developed, and the
performance of 6H40 ferrite cores was verified. Because they have more favorable
magnetic characteristics than conventional cores, the 6H40 ferrite cores are
usable not only in charging systems but also in automotive DC-DC converters and
coil products. We plan to continue the development of high-performance and
low-cost ferrite materials to contribute to a cleaner global environment of the
21st century.
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