# Ferrites and Nanocrystalline Alloys Applied to DC-DC Converters for Renewable Energies
## 1 Introduction
### Literature Review  
The design of MFTs (600 to 20 kHz) [9; 10; 16; 17] always involve a core material in the procedure. The core materials can be nanocrystalline alloys [16; 17], ferrites [18], or silicon steel [9; 10]. Each of these materials is mainly selected for a specific frequency range. Silicon steel is used between 600 Hz [9], and 1 kHz [10]. Nanocrystalline alloys are used between 1 kHz [19] and 5 kHz [16; 17] and ferrites from 20 kHz and up [18]. At its respective frequency, each material has high efficiency (>98%). Besides efficiency, each material has additional interesting characteristics. For instance, silicon steel is a low-cost material, it is highly available in the market, and there is plenty of information available for designers for it. In the case of ferrite, this is also a low-cost material, but also, it has high power density at high frequencies. Different from the previous materials, nanocrystalline alloys are a high-cost material (considering per unit of volume) compared to ferrites and silicon steel. However, it has higher permeability, lower core losses, and a higher design flux density in the 1 to 5 kHz range [16; 17; 19].  
MFT designers rely on the latest information from the most recent research works in order to select the most suitable core material for obtaining the best behavior at given frequency and power levels of MFTs [20].  
Related to cutting-edge information, nanocrystalline alloys and silicon steel MFTs are compared at 1 kHz [19]. In this situation, the MFT with nanocrystalline core shows higher power density and 7.8 times less weight. Nanocrystalline alloys cores are designed for 0.9 T flux density and silicon steel for 0.1 T. Although the cost per cm\({}^{3}\) of the nanocrystalline material (87.62 USD/kg) is higher than silicon steel (11.71 USD/kg), the final total cost  
Figure 1: MFTs applications.  
of both MFTs is similar because of smaller size of the MFT obtained with nanocrystalline alloys, USD 395 for the silicon steel MFT, and USD 375 for the nanocrystalline one.  
Silicon steel is not commonly considered for MFT designs at frequencies higher than 1 kHz because its core losses sharply increase beyond this frequency. As a consequence, the power density decreases, and the size of the MFT increases due a lower flux density. On the other hand, in the case of nanocrystalline cores, MFTs are mostly designed at medium frequency range (5 kHz) [16; 17]. At this range, both high-efficiency and high power densities are obtained. To the best of the Author's knowledge, currently, there are no research works presenting any kind of information about physical prototypes of nanocrystalline alloys MFTs designed and operating at 20 kHz. Table 1 shows a comparison of transformers with a nanocrystalline core in the medium frequency range. Therefore, it is important to compare the MFT prototypes of nanocrystalline alloys and ferrites at 20 kHz, in order to find a high power density in MFTs with high efficiency. This article presents the design, the experimental results of prototypes, and the comparison of results between these two MFTs, as well as a discussion about the power density of the MFT obtained in this document and the power density obtained in previous works.  
Ferrites can manage flux densities of 0.2-0.3 T. Although these are low flux densities values compared to those of nanocrystalline alloys, ferrites have generally been used at 10 kHz and up while nanocrystalline alloys usually at 5 kHz with power density 1.5 times higher [16]. Therefore, from the point of view of research, the design and construction of nanocrystalline alloys MFTs at 20 kHz, as well as comparing the performance of this MFT with that of the ferrite MFT, at such frequency, are of great interest. The strong and weak points of the nanocrystalline MFTs at such a frequency is unknown.