# Microstructure, Magnetic Properties, and Application of FINEMET-Type Alloys with Co Addition
## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Input Data of the Designed Choke**} \\ \hline Nominal voltage & 900 V DC \\ Insulation voltage against earth & 4 kV DC \\ Inductivity & 500 uH \\ Nominal current & 100 A DC \\ Magnetic linearity/overload & 150 A DC \\  & 8 kHz—25 Arms \\  & 16 kHz—0.7 Arms \\ Ripple Current & 24 kHz—2.6 Arms \\  & 32 kHz—0.7 Arms \\  & 40 kHz—1.0 Arms \\ Duty cycle & S1—continuous \\ Cooling & AN—air, natural \\ Max ambient temperature & 45 \({}^{\circ}\)C \\ Insulations class & Class F (T\({}_{\rm max}\) = 155 \({}^{\circ}\)C) \\ Noise level & \textless{}50 dB \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Technical specification of parameters and operating conditions of chokes. Figure 7: Electrical diagram of a DC/DC converter in the interleaved topology. Explanation for variables: CU3 DC—DC circuit capacity, CoU3—output filter capacity, LoU3—phase inductances of filter chokes, iau3, ibu3, icu3—choke phase currents, S11–S13 and S21–S23—transistor switches. operating temperatures are permissible, but they are short in duration. For this reason, it is extremely important that the magnetic core maintains the temperature stability of parameters over a wide temperature range. The temperature stability of losses of the core material is particularly important. Figure 6 shows the temperature dependence of losses of the tested magnetic alloys in the temperature range up to 650 K. The best temperature stability of losses is shown for alloys with 25 % and 6 % cobalt contents. Choke's operating at medium frequencies can be a source of nuisance acoustic fields, which are mainly due to magnetostriction effects. The noise level generated by the choke is also one of the criteria for selecting the magnetic core material. In rail converters, the noise level must be particularly limited for the sake of passengers' comfort. Although some frequencies are close to the limits of human hearing, they can, nonetheless, cause annoyance. This noise is also audible to animals and can cause them additional stress and irritation. Nanocrystalline materials with cobalt content have very small magnetostriction coefficients, which are close to zero [4]. The cores of chokes are made with multi-gap technology. The use of multiple narrow gaps in the core helps to reduce dispersion losses. The material for the cores is a thin film with a thickness of about 25 \(\upmu\)m of nanocrystalline material with a suitable alloy composition. Nanocrystalline tape is bonded with high-temperature epoxy in blocks from which the core is built [29]. Designed chokes with a column-type core have two independent winding coils connected in series. Since the main component of the choke current is the DC component, choke windings are designed from a thin aluminum sheet. Figure 8: Single phase 2 RTF type choke with nanocrystalline block core made with CoreECO multi-gap technology: dimensional draft of the choke (**a**); dimensional draft of the core (**b**). A and L, B, C and H—length, depth and height of the chokes, respectively. \begin{table}
\begin{tabular}{c c c c} \hline
**Core Materials** & **Fe\({}_{\textbf{83}}\)Nb\({}_{\textbf{3}}\)B\({}_{\textbf{13}}\)Cu\({}_{\textbf{1}}\)** & **Fe\({}_{\textbf{77}}\)Co\({}_{\textbf{6}}\)Nb\({}_{\textbf{3}}\)B\({}_{\textbf{13}}\)Cu\({}_{\textbf{1}}\)** & **Fe\({}_{\textbf{58}}\)Co\({}_{\textbf{25}}\)Nb\({}_{\textbf{3}}\)B\({}_{\textbf{13}}\)Cu\({}_{\textbf{1}}\)** \\ \hline Losses \({}^{\star}\), P (W/kg) & 21.9 \(\pm\) 0.1 & 20.8 \(\pm\) 0.1 & 16.3 \(\pm\) 0.1 \\ \hline Saturation magnetic flux density B\({}_{\textbf{3}}\) (T) & 1.21 \(\pm\) 0.02 & 0.84 \(\pm\) 0.02 & 0.87 \(\pm\) 0.02 \\ \hline \end{tabular} \({}^{\star}\) Determined by the transformer method for a frequency of 1 kHz.  
\end{table} 
Table 3: Choke cores magnetic materials parameters. This solution makes it possible to simultaneously achieve optimal current density in the winding, reduce eddy current losses, and keep the choke mass low.