Characterization of customized ferrite cores for a compact six-phase coupled inductor

Abstract

Demands for circuits with higher density in modern electronics have imposed the necessity for electronic components with compact size, high frequency operational range, high efficiency and cost-effectiveness. This requires the new design of electronic components, such as multi-phase coupled inductor with ferrite Ni-Zn core which is studied in this paper. Printed circuit board (PCB) technology was used to develop six symmetrical conductive copper coils, while specially designed innovative ferrite core has been mounted through the PCB, to create a compact component. For ferrite core, two Ni-Zn materials denoted with C2025 and CN20 were used. Testing of the component has been performed using the following characterization methods: (a) structural characterization using scanning electron microscope; (b) electrical characterization using Impedance analyzer; and (c) mechanical characterization using Nanoindenter. It can be concluded from tests carried out (load-displacement curves and hardness module) that designed and manufactured component can be exposed to mechanical stress and vibrations and can be used in applications such as DC/DC convertors.

Keywords

Ferrite inductor SEM nanoindentation hardness test

1. Introduction

Typical applications of DC/DC convertors include voltage regulation for modern microprocessors, graphics processing units and other complex digital circuits. Using coupled inductors as the magnetic component in multiphase buck DC/DC converters can decrease current ripple in every phases, consequently reducing switching losses and improving transient response [1, 2, 3]. With miniaturization, higher operational frequency and improvement of electronic circuit and device performances, power electronic systems have been developed to the situation that active components are no longer the main influencing factor to the size and cost of the electronic devices. Actually, passive electronic components have represented most of the size and price of power electronics systems [4, 5]. Because of that it is a very important task to design of compact coupled inductors with small size magnetic (ferrite) core. Thin planar core can be positioned directly on printed circuit board (PCB) with printed inductive structures [6]. The magnetic core need to be thin to meet low-volume requests and to reduce quantity of magnetic materials which will be used. In addition to this, a proper selection and characterization of magnetic materials for this use is one of the crucial steps in a complete process of realization of multi-phased coupled inductors for DC/DC convertors [7]. Furthermore, a novel DC resistance current sensing methods of multiphase coupled-inductor buck converter were reported in thesis [8]. Coupling inductors between channels with the idea to improve the steady-state and dynamic characteristics of voltage regulator circuits were presented in [9]. For multiphase coupling, authors introduced in paper [10] a new topology which provide much stronger coupling and thus enable reducing ripple and improving transient response. For high-quality inductors, nickel-zinc (Ni-Zn) ferrites are ideal materials for realization of the magnetic composites [11]. The core loss measurement technique for random excitation with a cancellation concept was presented in [12] and experimentally tested up to 10 MHz. Microstructural, mechanical and magnetic properties of nickel-zinc ferrites fabricated by powder injection molding were analyzed in [13]. Ferrite cores for coupled planar inductors can be exposed to mechanical stress or vibrations either during handling/mounting procedure or during operational process in some electronic device. Because of that it is critical point to analyze mechanical behavior of the magnetic component. Nanoindentation is a widely used method for investigation of mechanical integrity of several materials [14]. For instance, nanoindentation was used for studying: (a) behavior of nano BiFeO ${}_{3}$ [15]; (b) mechanical properties of yttria-stabilized tetragonal zirconia [16]; (c) correlation between microstructure and mechanical characteristics of acicular ferrite and upper bainite [17]; (d) nanohardness experimental results on martensite, lower bainite, upper bainite, granular bainite and ferrite [18], etc. Moreover, effects of incorporating of different materials in ferrite structures on their magnetic and mechanical properties [19, 20, 21] as well as on the stress insensitivity [22] were studied. Furthermore, by controlling the calcination temperature, after-calcination milling time as well as sintering conditions, it is possible to create ferrite cores with increased strength and lower losses [23].

With respect to the needs for a symmetric (balanced) structure, power delivery and space requests a new compact electronic component has been designed, fabricated, characterized and described in this article. The idea was to create a novel concept of multiphase component (in this paper – six phases) with a circular planar structure and symmetrical arrangement of inductive coils, which are mutually coupled on PCB. The design of ferrite core is innovative, unconventional, composed of two parts and six pillars going through the PCB. Two types of Ni-Zn ferrite materials were used and tested, one with low permeability (around 175) – C2025, and the other one with high permeability (around 925) – CN20. Their complete structural, electrical and particularly mechanical characterization (through nanoindentation) were performed, because these information can not be found in data sheets and are very important for deeply understanding of operational regimes of this significant electronic component in modern DC/DC convertors.

2. Design of six-phase coupled inductor with ferrite core

2.1 Design principles and materials for ferrite core

During design process of magnetic components for power electronics circuits, selection of proper ferromagnetic materials is a very important task. Based on our previous experience [24, 25], nickel-zinc (Ni-Zn) ferrites have been chosen for ferrite core materials which can be incorporated on a compact way in printed circuit board with conductive segments of the proposed inductor. As a first step, a complex permeability characteristics ( $\mu^{\prime}$ and $\mu^{\prime\prime}$ ) of different Ni-Zn materials were analyzed. Real part of the complex permeability has influence on the inductance behavior in frequency domain, whereas imaginary part determines resistance characteristic or losses in the material. We selected Ni-Zn ferrites which have almost flat characteristic of $\mu^{\prime}$ as a function of frequency, in a wide frequency range. Two materials, denoted with C2025 and CN20, were selected for manufacturing ferrite core for six-phased coupled inductor. These two materials, C2025 and CN20 (from Ceramic magnetics, http://www.cmi-ferrite.com/Materials/NiZn.htm) were selected, because they cover range from low initial permeability ( $\mu_{\text{C2025}}=$ 175) up to high permeability at 10 kHz ( $\mu_{\text{CN20}}=$ 925). Additionally, these materials had the best mechanical characteristics to support compact and complex design as well as selected tailor-made manufacturing process. Design of ferrite core in circular shape is depicted in Fig. 1. Diameter of this core is 20 mm. One part of the core has 6 vertical pillars, with height of 2 mm, equally distributed. Thickness of upper circular plate is also 2 mm, thus the complete structure has a thickness of 6 mm (including PCB thickness). There is no air gap in the proposed multi-phased inductive structure. The idea was to develop compact structure which can be embedded in PCB technology process.

Figure 1.

Ferrite core consisted of two parts, with its dimensions.

2.2 Design of inductor’s phase coils

Printed circuit board technology has been used for manufacturing of inductive structure. We designed the structure with 6 windings in one metal (copper) layer with distance of 200 $\mu$ m between neighboring conductive segments. The technical drawing of conductive segments, along with appropriate dimensions in mm is shown in Fig. 2a. The fabricated conductive part of the structure is presented in Fig. 2b, together with wires, as terminals for measurement points. The compact inductive structure – ferrite core together with conductive segments on PCB are displayed in Fig. 2c.

Figure 2.

(a) Design and dimensions of conductive windings, (b) fabricated windings in PCB technology, (c) ferrite core together with conductive segments.

2.3 Experimental

The following instruments have been used for characterization of C2025 and CN20 ferrite core materials: (1) for structural characterization – scanning electron microscope (SEM), JOEL JSM 6460 LV scanning microscope with EDS; (2) for electrical characterization – impedance spectroscopy, Impedance analyzer HP4194A, and (3) for mechanical characterization – nanoindentation, Agilent Nanoindenter G200.

3. Results and discussion

3.1 Structural characterization

First, we performed structural characterization of two analyzed materials using scanning electron microscopy. SEM micrographs are depicted in Fig. 3. These results reveal that material CN20 possesses internal structure with higher grain sizes (domains) comparing to the C2025 material. From SEM micrographs, it can be seen that grain sizes of material C2025 were in the range from 0.8 to 1.8 $\mu$ m, whereas for material CN20 the grain sizes were in the range 1–2 $\mu$ m. Consequently, we can expect that material CN20 has higher permeability and lower resonant frequency ( $f_{\textit{res}}$ ). Larger grain sizes in the CN20 structure gives major influence of moving domain walls which ensure lower value of $f_{\textit{res}}$ . Having small grain size, material C2025 has pronounced spin rotation effect which gives the higher resonant frequency. Moreover, it can be seen that material CN20 has less porous structure between grain boundaries which results in improved magnetic properties.

Figure 3.

SEM micrographs for NiZn ferrites (a) sample C2025, (b) sample CN20.

3.2 Electrical characterization

Electrical properties of compact inductive structure with ferrite core (made of both C2025 and CN20) have been determined by means of impedance spectroscopy. Inductance and quality factor were measured in the frequency range of 50 kHz–40 MHz and obtained results are depicted in Fig. 4.

Figure 4.

(a) Inductance as a function of frequency, (b) Q-factor as a function of frequency, for inductor with both ferrite cores made of C2025 and CN20.

The inductance and Q-factor are presented for one phase and they are the same for other six phases, bearing in mind the complete symmetry of fabricated inductive structure. It can be concluded that inductor made from CN20 material with high permeability has had inductance around 650 nH almost up to 1 MHz. The inductor with C2025 material has had inductance around 230 nH, but in a wider frequency range, up to 10 MHz and after that this curve drops suddenly, following original frequency dependence of real part of the permeability for this material. This wider frequency range for material C2025 can be also noticed in Fig. 4b. Additionally, inductor with ferrite core made of C2025 has higher peak of Q-factor, value of 41 at 2.5 MHz, whereas one made from CN20 has peak of the quality factor equal to 36 at 0.3 MHz. Table 1 shows the coupling coefficient matrix between two phases of the 6-phase coupled inductor. The core structure is symmetrical and we obtained: $k_{12}=k_{23}=k_{34}=k_{45}=k_{56}=k_{61}$ ; $k_{13}=k_{24}=k_{35}=k_{46}=k_{51}=k_{62}$ ; $k_{14}=k_{25}=k_{36}=k_{41}=k_{52}=k_{62}$ .

Table 1

Values of coupling coefficients for C2025 and CN20 samples

Core material	C2025	CN20
Coupling coefficient $k_{12}$	0.27	0.3
Coupling coefficient $k_{13}$	0.125	0.13
Coupling coefficient $k_{14}$	0.045	0.085

Moreover, some of important characteristics of the inductor made by two proposed magnetic materials, obtained by measurement, are presented in Table 2.

Table 2

The measured parameters of one-layer phase inductor

Core material	C2025 ${}^{*}$	CN20 ${}^{*}$
Phase inductance $L_{\text{s}}$ [nH]	230	650
DC resistance $R_{\text{DC}}$ [m $\Omega$ ]	4.5	4.5
AC resistance $R_{\text{AC}}$ [m $\Omega$ ]	8–187	8–222

${}^{*}$ C2025 results for measured $R_{\text{s}}$ ( $R_{\text{AC}}$ ) in the frequency range (50 kHz–4.5 MHz), CN20 (50 kHz–980 kHz).

Since the AC resistance value has a large variation within the analyzed frequency range, we also provide measured resistance as a function of frequency for analyzed inductive structures and for cases without core and for the different Ni-Zn ferrites as cores, as can be seen in Fig. 5. It can be noticed that resistance increase with increasing of frequency due to the losses in ferrite materials. Material C2025 has lower high frequency resistance comparing with CN20.

Figure 5.

Resistance as a function of frequency without core and with two ferrite materials for cores.

For an inductor made by ferrite core saturation parameters are very important, especially saturation current. A test circuit was realized [26] and from the measured graph current as a function of time, the maximum current for the proposed inductive structure could be noticed around 13.5 A. In addition to this, B-H curves were measured for two presented ferrite materials and obtained values were: Bs $=$ 0.4 T for CN20 and Bs $=$ 0.39 T for C2025. Bearing in mind that focus of this paper is on structural and mechanical properties of ferrite materials which are used, the further work in this field will be oriented towards measuring the dependence of inductance as a function of DC current, which can provide additional information about performances of the proposed structure.

3.3 Mechanical characterization

Two ferrite materials: C2025 and CN20, have been characterized through nanoindentation testing. These tests were conducted by means of an Agilent (Keysight) Nanoindenter G200, which provides repeatable and reliable measurements. Multiple indentation (15 indentations per sample were made) tests provide measurement repeatability for the mechanical properties of analyzed samples. The system has resolution of load and displacement of less than 50 nN and 0.1 nm, respectively. Nanoindentation tests were performed with a Berkovich three-sided pyramidal diamond tip, with the face angle of 65.27 ${}^{\circ}$ . Figure 6a and b show the representative load-displacement curves measured on the C2025 and CN20 samples respectively, whereas Fig. 6c compares average load-displacement curves for two studied ferrite samples.

Figure 6.

(a) Load versus displacement curves for C2025, (b) Load-displacement curves for CN20 (c) Average Load-displacement curves for C2025 and CN20 samples.

All tests were performed at max load of 350 mN with one indentation per place, 1 s peak hold time and time to load of 15 s. It can be concluded from Fig. 6 that sample C2025 has the lower hardness in the comparison with the sample CN20, bearing in mind deeper penetration of the tip into this sample, with the same applied force. Average displacement into surface for C2025 was 3442 nm and for CN20 was 3369 nm. Table 3 displays the results for measurement of hardness (average values of 15 tests) of C2025 sample and CN20 sample. Average elastic modulus of two analyzed ferrite cores is also presented in Table 3. The obtained results are in accordance with the data from the slope of the load-displacement curves. Standard deviation of results is also calculated and presented at Table 3, as well.

Table 3

Results of hardness tests on C2025 and CN20 samples

	Modulus at max load (GPa)		Hardness at max load (GPa)
	Mean	Std. Dev.	Mean	Std. Dev.
C2025	27.8726	11.903	2.0963	0.9784
CN20	31.2090	9.803	2.0714	0.6847

It can be concluded that ferrite core from CN20 material has higher value of hardness module/ coefficient. This can be explained by internal structure of this sample with bigger grain size comparing to C2025 Ni-Zn ferrite material. Consequently, the sample CN20 can be exposed to higher level of mechanical stress from the application point of view, for example in DC/DC convertors. The application of proposed six-phased compact inductive structure in DC/DC convertors has been already described in our earlier paper [26].

4. Conclusions

A novel design of six-phase coupled-inductor with customized Ni-Zn ferrite core has been presented in this work, intended to be used in high frequency range (up to 1 MHz) in DC/DC converters. Two types of Ni-Zn materials were selected for cores manufacturing, one with low permeability (C2025) and the other with high permeability (CN20) and their structural characterization was performed by SEM imaging. Inductance and quality factor of inductor samples with nickel-zinc ferrite cores have been studied as a function of frequency as well as values of coupling coefficients have been given. Nanoindentation testing were completed on two ferrite core samples and the results showed better surface properties for sample CN20 and excellent repeatability of the results for both samples. In accordance with obtained results, proposed component is promising candidate for application in DC/DC converters which can be exposed to higher level of mechanical stress or vibrations.

Footnotes

Acknowledgments

This work was partly supported through the projects TR32016 and III45021, financed by the Ministry for Education, Science and Technological Development of the Republic of Serbia.

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