STRUCTURAL PARASITIC CAPACITANCE REDUCTION TECHNIQUES IN PLANAR MAGNETIC INTEGRATED STRUCTURES

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1 STRUCTURAL PARASITIC CAPACITANCE REDUCTION TECHNIQUES IN PLANAR MAGNETIC INTEGRATED STRUCTURES STRUCTURAL PARASITIC CAPACITANCE REDUCTION TECHNIQUES IN PLANAR MAGNETIC INTEGRATED STRUCTURES Lecturer Eng. Adina-Nicoleta RĂCĂŞAN, PhD 1, Prof. Eng. Călin MUNTEANU, PhD 1, Prof. Eng. Vasile ŢOPA, PhD 1, Assis. Eng. Claudia PĂCURAR 1, Eng. Claudia HEBEDEAN 1 1 Technical University of Cluj - Napoca, Electrical Engineering Faculty, Department of Electrotechnics and Measurements, Cluj - Napoca, România. REZUMAT. Obiectivul acestei lucrari este de a prezenta contribuţia autorilor la dezvoltarea unor soluţii de d e reducere a capacităţii parazite structurale, SPC, în dispozitive realizate în tehnologie magnetică planară. Fiind structuri de mare complexitate geometrică, SPC nu poate fi definită prin relaţii directe de calcul şi nici nu poate fi localizată într-un dispozitiv anume, ea fiind distribuită practic în spaţiul dintre înfăşurările bobinelor care alcătuiesc structura. Astfel, in aceasta lucrare sunt propuse cateva tehnici de reducere a capacitatii parazite structurale in structuri integrate magnetic planar. Cuvinte cheie: structura integrata magnetic planara, capacitatea parazita structurala. ABSTRACT. The aim of this paper is to present the authors contribution to developed structural parasitic capacitance reduction solutions from devices realized in planar r magnetic technology. Being structures with an important geometrical complexity, the SPC can not be defined by direct calculation expressions and it can not be localized in one device, it being practically distributed in the space between the coil wirings that form the structure. So, in this paper, are proposed approached techniques to reduce the structural parasitic capacitance in planar magnetic integrated structures. Keywords: the planar magnetic integrated structure, the structural parasitic capacitance (SPC). 1. PRINCIPLE OF PLANAR MAGNETIC INTEGRATION TECHNOLOGIES The fundamental element of any planar integrated magnetic device is the LC integrated structure. Planar integration of LC structures consists in an addition of layers that contain conductors and dielectrics that in some specific cases might be enclosed by a ferrite core. This integrated structure has terminals proprieties similar with those of the structures set-up using discrete components. A detailed representation of such a LC fundamental planar electromagnetic integrated structure is presented in Figure 1 [1]. As it can be noticed from the equivalent circuit representations from Figure 1 this structure has distributed inductance and capacitance and the corresponding circuit characteristics clearly depend by the external connections between the structure s fourterminals. Putting together several basic LC layers one can develop more complex integrated structures, for instance in Figure 2 the integrated structure of a resonant transformer (L-L-C-T) is presented [2]. Fig. 1. Planar integrated L-C structure and possible external connection configurations [1]. a) b) Fig. 2. (a) Resonant transformer electric circuit, (b) Resonant transformator planar electromagnetic integration [2]. Buletinul AGIR nr. 3/2012 iunie-august 1 683

2 WORLD WORLD ENERGY ENERGY SYSTEM SYSTEM CONFERENCE CONFERENCE WESC - WESC 2012 The main advantages of the planar magnetic integrated technology with respect to the classical technologies that uses discrete components for setting up similar devices functionalities are [3]: o low profile structures. Some studies have indicated that the low profile magnetics can have better volumetric efficiency and higher power density for certain applications; o lower leakage inductance. Interleaving can be easily implemented in planar structures, which allows the minimization and control of leakage inductance within the windings o reduced high frequency winding losses. Skin effect is minimized by reducing the thickness of the conductor windings to one or two times the skin depth. o better thermal management. Planar magnetic components have the greater surface area to volume ratio, thus more area contact to the heat sink. The main disadvantage of the planar magnetic technology is that it always leads to a high value of the structural parasitic capacitance, SPC. This fact is motivated by the structural design that puts in closed proximity the wiring layers. Therefore, the very small distances between the conductors play an important role in the global parasitic capacitance value. In this light, the main goal of the paper is to develop and analyses the effectiveness of several SPC-reducing techniques; the present study propose a new method of parasite capacitance reduction that is the staggered winding technique. The effectiveness of these techniques was evaluated by Finite Elements (FEM) simulations. As shown in the well-known parallel plate capacitance calculation equation C=ε 0 ε r A/d, capacitance can be reduced by varying the three parameters: reducing plate area A; increasing distance between plates d and reducing relative permittivity ε r of the dielectric material [4], [5]. From the practical point of view, the reduction of the conductive pathways surfaces is not feasible because it could have an important effect on the practical current capability of filter, thus will be considered only the versions which suppose the distance increasing and the reduction of dielectric permittivity between the conductive pathways. Additionally, a new technique is proposed by the authors will be analyzed in details in this study. It is the appliance of a geometrically staggered configuration between windings. These principles considered and applied in the case of structural parasite capacitance reduction of the planar magnetic integrated structures. To evaluate the effectiveness of the SPC-reducing techniques, the structural parasitic capacitances of the four single winding structures shown in Figures 3-7 are computed using a commercial FEM software package. 2. ORIGINAL STRUCTURE The structure shown in Figure 3 is the original structure, which has two winding layers and six turns per layer. a) compact view b) detailed view c) cross section view Fig. 3. Physical structure of the studied integrated planar structure. The first winding layer is an integrated LC winding, consisting of a thin copper winding, a ceramic layer and a thick copper winding. The second winding layer is a normal copper-foil winding. All the conductors have 2 684

3 STRUCTURAL PARASITIC CAPACITANCE REDUCTION TECHNIQUES IN PLANAR MAGNETIC INTEGRATED STRUCTURES the same dimensions, which are 1.2 x 0.3 mm. The thickness of the insulation kapton between winding layers is 0.1 mm. The relative permittivity of the materials used in the simulation is given in Table 1. Table 1 Material properties used in the simulation Materials Ferrite Air Cooper Kapton Ceramic ε r AIR SPACER REDUCING RELATIVE PERMITTIVITY OF INSULATION MATERIAL Reducing ε r can be achieved by inserting an air spacer between winding layers instead of using the normal insulation material since the relative permittivity of air is approximately 1, while the relative permittivity of other widely-used insulation materials is in the range from 4 to 10. However, there is no solid air spacer at room temperature; winding with air spacer is not a mechanically stable structure and it is not feasible. Therefore the staggered winding structure shown in Figure 7 is proposed by the authors. The conductors and the thin insulation materials on their surfaces provide the mechanical support to form the virtual air spacer. The structure shown in Figure 6 replaces kapton in Figure 5 with air. Fig. 4. Detailed cross section view and the structural parasitic capacitance calculated for the original structure (i). 3. INCREASED INSULATION THICKNESS Increasing the distance between plates can be achieved by increasing insulation layer thickness. The structure shown in Figure 5 is similar to that of Figure 4, except the insulation kapton thickness is increased to 0.5 mm. Fig. 6. Detailed cross section view and the effect of the air spacer on the structural parasitic capacitance values (iii). 5. STAGGERED WINDING Fig. 5. Detailed cross section view and the effect of increasing the insulation thickness on the parasite capacitance values (ii). The cross section view of the proposed staggered winding (iv) propused by the authors is presented in Figure 7. To avoid the overlapped windings, the number of winding layers is increased to four and the number of windings on each layer is reduced to three. Two cases were tested for 3D analysis of the staggered winding structure method based on how the staggered winding was realized, see Figure 8. In the first case the two copper layers that form the staggered winding start both from the same side, and in the second case the second winding 22 starts from the point where the winding 21 ends. The staggered winding is obtained connecting both windings. Buletinul AGIR nr. 3/2012 iunie-august 3 685

4 WORLD ENERGY SYSTEM CONFERENCE WESC 2012 WORLD ENERGY SYSTEM CONFERENCE - WESC The two correspondent models of the proposed methods for the staggered winding method are presented in Figure 9 for Case 1 and Figure 10 for Case 2. As it was mentioned above the only difference between these cases is how the staging is realized, the other boards are the same. Fig. 7. The staggered winding structure. Fig.9. 3D view of staggered winding usage Case 1. a) Case 1 Fig D view of staggered winding usage Case 2. b) Case 2 After both cases simulations, the obtained capacitance matrices are presented in Figure 11. Fig. 8. The staggered winding achievement

5 STRUCTURAL PARASITIC CAPACITANCE REDUCTION TECHNIQUES IN PLANAR MAGNETIC INTEGRATED STRUCTURES The conclusion of complete analysis in order to reduce the SPC is that the air spacer method is the most effective. Applying this method, the structural parasitic capacitance is much reduced, that means an efficient method but, unfortunately from the mechanical point of view it s an unstable structure and it s practically difficult to be realized. Therefore, the staggered winding method developed by authors is proposed for structural parasitic capacitance reduction. 6. CONCLUSIONS a) Case 1 The paper outlines several techniques to minimize the SPC for devices realized in planar magnetic technology, such as: increased insulation thickness, air spacer and staggered winding. Following these techniques, as it was outlined in the paper, the structural parasitic capacitance can be reduced several times with respect to the original traditional arrangement of the winding structure. Thus, the high frequency performances of a devices realized in planar magnetic technology can be significantly improved by applying the proposed SPC reduction techniques. BIBLIOGRAPHY b) Case 2 Fig. 11. Parasite capacitance results using the staggered winding structure propused by authors. It is visible that for C 33 small differences are obtained comparing these matrices. The C 33 values for all four simulated structures are collected in Table 2. Table 2 C 33 for all four structures Structure (i) (ii) (iii) (iv_1) (iv_2) C 33 [pf] Following the numerical modelling results it s visible that using the method of increasing the insulation thickness causes a four times reduction of the structural parasitic capacitance. The kapton 2 layer replacements with an air spacer reduce the structural parasitic capacitance with about 11 times related to the standard, original structure. [1] Strydom, J.,T., Van Wyk, J.,D., Volumetric Limits of Planar Integrated Resonant Tranformes: A 1 mhz Case Study. IEEE Transactions on Power Electronics, Vol. 18, [2] Wang, S., Lee, F.,C., Odendaal, G.,W., Van Wyk, J.,D.: Improvement of EMI Filter Performance With Parasitic Coupling Cancellation. IEEE Transactions on Power Electronics, Vol. 20, [3] Wang, S: Modeling and Design of Planar Integrated Magnetic Components, MsC Thesis, Blacksburg, Virginia, [4] Wang, S., Chen, R., Van Wyk, J.,D., Lee, F.,C., Odendaal, G.,W., Developing Parasitic Cancellation Tehnologies To Improve EMI Filter Performane\ce for Switching Mode Power Supplies. IEEE Transactions onelectromagnetic Compatibility, Vol. 47, [5] Wang, S., Lee, F.,C., Odendal, W.,G., Characterization and Parasitic Extraction of EMI Filters Using Scattering Parameters. IEEE Transactions on Power Electronics, Vol. 20, [6] Racasan, A.,N., Munteanu, C., Ţopa, V., Racasan, C., Techniques to Reduce the Equivalent Parallel Capacitance for EMI Filters Integration. Mathematics in Industry, Vol. 11, Springer, [7] Racasan, C., Racasan, A.,N., Ţopa, V., Munteanu, C., Modelarea numerică a câmpului electromagnetic. Casa Cărţii de Ştiinţă, Buletinul AGIR nr. 3/2012 iunie-august 5 687

6 WORLD WORLD ENERGY ENERGY SYSTEM SYSTEM CONFERENCE CONFERENCE WESC - WESC 2012 About the authors Lecturer. Eng. Adina-Nicoleta RĂCĂŞAN, PhD Adina.Racasan@et.utcluj.ro Received the M.Sc. degree in electrical engineering from Technical University of Cluj-Napoca, in 2004 and the PhD. degree in electrical engineering in She joined the Electrotechnics Department from Technical University of Cluj- Napoca in as PhD student. Since 2012 she is Lecturer. Her scientific work is related to electromagnetic fields, numerical computation, optimal design techniques and EMC. Prof. Eng. Călin MUNTEANU, PhD Calin.Munteanu@et.utcluj.ro Received the MSc. degree in electrical engineering from Technical University of Cluj-Napoca, in 1989, and the PhD. degree in electrical engineering in He joined the Electrotechnics Department from Technical University of Cluj- Napoca in Since 2003 he is Professor and the Head of the EMC Laboratory. His scientific work is related to EMC, electromagnetic fields, numerical computation, optimal design techniques. Prof. Eng. Vasile ŢOPA, PhD Vasile.Ţopa@et.utcluj.ro Received the M.Sc. degree in electrical engineering from Technical University of Cluj-Napoca, in 1982, and the Ph.D. degree in electrical engineering in He joined the Electrotechnics Department from Technical University of Cluj- Napoca in Since 2001 he is Professor of Electric Circuits and Electromagnetic Field Theory and head of the CAD in electrical engineering Laboratory. His scientific work is related to optimal design of electromagnetic devices and numerical computation methods in electromagnetism. Assis. Eng. Claudia PĂCURAR Claudia.Pacurar@et.utcluj.ro Received the M.Sc. degree in electrical engineering from Technical University of Cluj-Napoca, in 2004, and she is a Ph.D. student in electrical engineering. She joined the Electrotechnics Department from Technical University of Cluj-Napoca in Her scientific work is related to electromagnetic fields, numerical computation and optimal design techniques. Eng. Claudia HEBEDEAN Claudia.Hebedean@et.utcluj.ro Received the M.Sc. degree in electrical engineering from Technical University of Cluj-Napoca, in She joined the Electrotechnics Department from Technical University of Cluj-Napoca in 2009 as PhD student. Her scientific work is related to electromagnetic fields, numerical computation, optimal design techniques and EMC