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Novel High Frequency TransformerConfigurations - Amorphous

Metal vs. FerritesChristian P. Dick, Dirk Hirschmann, Thomas Plum, Dirk Knobloch, Rik W. De Doncker

Institute for Power Electronics and Electrical DrivesRWTH Aachen University

Jaegerstrasse 17-19, 52066 Aachen, GermanyPhone: +49 241 8096936, Fax: +49 241 8092203

Email: di@isea.rwth-aachen.de

Abstract - In this paper a novel configuration of anamor-phous metal tape transformer, with the flux being closedwithin the magnetically anisotropic tape plane, is introduced.After characterization of the metal tape in both tape planedirections, the configuration and basic mechanical and chem-ical processing steps of the transformer construction arepresented.

The new configuration shows high mutual inductance andoffers the possibilities to design leakage inductance andparasitic winding capacitance with tools from literature.

The performance of the newly developed device is charac-terized experimentally and compared to ferrite transformers,showing advantages of the amorphous transformer up tofrequencies of about 100 kHz.

I. INTRODUCTION

Galvanically isolated dc-dc converters, such as seriesresonant [1] [2] [3], series-parallel resonant and single-active or dual-active bridges [4], use an interlinkingtransformer of which the leakage inductance is of majorimportance. It determines power flow in the single activeand dual-active bridge. In the resonant converters thecapacitors have to be designed to match the leakageinductance to achieve a desired switching frequency. Inboth cases the precondition for soft switching of an activehalf bridge, i.e. zero-voltage switching, is an inductiveload, which can be provided by the leakage inductanceof the transformer.

The origin and the description of leakage inductance iswell known. For example, the leakage inductance can becalculated via the energy stored in the magnetic stray field[5]. Yet another major parasitic element is the winding ca-pacitance. If desired, it might be utilized for soft switchingof an output diode rectifier. Design rules for the windingcapacitance can be found in literature [6].

In converters of several hundreds of watts and an opera-tion frequency of 50 ..200kHz mostly ferrite transformersare used to establish defined leakage inductances and par-asitic capacitances. Amorphous metal transformers, withflux flowing in only one direction of the metal tape plane,

are nowadays constructed in two ways, with the followingdisadvantages:

• A set of two C-cores suffers for the cut of the stackedmetal tape, resulting in a reduced mutual inductance.

• Ring cores, when being supposed to achieve a highcopper fill factor, are hard to design for a specificleakage inductance and winding capacitance.

This paper presents a novel amorphous metal configura-tion not showing cuts but showing the capability of con-ventional design of defined leakage and parasitic elements.

The paper is structured as follows: First, a short com-parison between ferrite and amorphous metal based ondata sheets is presented. Two ways of material character-ization, applied in this paper, are qualified. Subsequently,the idea of the new amorphous metal tape transformeris introduced. Test transformers were constructed for acomparison of the effective core behavior. Finally, thesecharacterization steps are presented in comparison withferrite cores.

II. FUNDAMENTALS

A. Loss Behavior and Measurement of Magnetic Materi-als

Loss information of materials in data sheets are mostlygiven in double logarithmic diagrams, with the local lossesindicated as a function of induction with frequency as aparameter. In Table I some results are depicted.

In terms of losses ferrite materials are superior toamorphous metal at high frequencies and low inductionlevels. Amorphous metal is superior to ferrite at lowerfrequencies and high induction levels. However, low-losshigh-frequency amorphous metals are capable to operate atabout twice the induction (550mT) with respect to ferritematerial, reducing also overall volume and thus losses.When being fixed to a frequency of about 100kHz, ahigher power density could be achieved using amorphousmetal.

These characterizations for data sheets are carried outusing standardized ring core shapes and a well known

978-1-4244-1668-4/08/$25.00 ©2008 IEEE

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TABLE IEXEMPLARY DATA OF FERRITE AND AMORPHOUS METAL MATERIAL

Frequency(kHz)

Induction(mT)

Losses Ferrite 3C96 [7]@ T = 100◦C ,

(kWm3

) Losses VITROVAC 6030F [8]amorphous ring core, 7.75 g

cm3

large thermal stability [9],(

kWm3

) Ratio

25 100 6.5 6.2 1:1.025 250 100 50.4 1:0.5

100 100 40 77.5 1:1.9100 250 700 503 1:0.7200 100 170 341 1:2.0200 250 2000 2170 1:1.1

measurement technique, i.e. applying a sinusoidal currentinto the primary winding and measuring this current andthe voltage of the secondary winding. Core losses canbe calculated directly without any measurement-dependenterror [10], [11]. Core excitation and frequency can be setby the input current. This method was applied in this paperas well, however, minor deviations from the measurementprinciple were made:

• Due to limitations in the measurement equipment,the measurements were carried out with triangular,not sinusoidal, current waveforms. This results inreduced losses with a factor of 1.11 [8]. However,this small factor is small enough to be insignificantfor the statements which follow from the experiments.

• All measurements were carried out using the trans-former samples, not using standardized ring coreshapes. This leads to deviations in the results, as canbe seen in the example of the used ferrite core. Thedata sheet information from the material [7] differsfrom data sheet information from the specific set ofE-cores [12]. Fig. 1 shows the good correspondencebetween the two provided data points and the mea-surement. In this paper, the specific losses are alwaysthe ratio of total losses and total volume/weight. Thisis important for the metal tape transformers showinga non constant core excitation for the whole core.

• The magnetic properties of metals show large thermalstability [9]. In contrast, ferrites are sensitive totemperature. The presented data was measured atT = 100◦C with the temperature being monitoredusing a thermocouple on the ferrite surface.

B. Loss Factor tan� of Magnetic Materials

Another characterization of magnetics can be carriedout with the loss factor tan� . For example, an induc-tor without copper losses shows a material dependentimpedance. It can be described together with the definitionfor the loss factor by eq. (1). It has to be noted that �r =� ′

r − j� ′′r is still a function of the operation frequency f

and the induction level B - the field strength H respectively- in the material. The test bench shows the disadvantage

Fig. 1. Ferrite loss measurement: 3C96, core set of E22/6/16, T = 100◦C

of being influenced by the copper resistance and of notbeing capable of providing large induction levels. Thus,only the initial permeability could be measured, which isstill an important tool to identify cut-off frequencies.

Z = j�L = j�L0(� ′r − j� ′′

r )= j�� ′

rL0︸ ︷︷ ︸=X

+�� ′′r L0︸ ︷︷ ︸

=R

with L0 = N2 �0Al

(1)

tan� =RX

=� ′′

r

� ′r

plocallosses = ��0�′′r H2

rms

III. AMORPHOUS METAL TRANSFORMER

A. Thermal Treatment of Amorphous Metal

After rapid quenching process an amorphous metal taperesults, which does not show a significant anisotropicmagnetic behavior in the two metal plane directions. Thetape (VITROVAC 6025X) has a low cut-off frequency

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and cannot be operated at frequencies above 100kHz.After thermal treatment one preferential direction of thetape plane is introduced which is also indicated in datasheets (VITROVAC 6025F||) 1. The effect is similar tothe grain orientation of large low-frequency power trans-formers. Therefore, nowadays either two C-cores or onering core are mostly used. Operating the material in theperpendicular direction results in the material propertiesVITROVAC 6025F⊥. This data is not available in datasheets.

B. Low Excitation Characterization

The different behavior of isotropic material VITROVAC6025X as well as the anisotropic material VITROVAC6025F||, VITROVAC 6025F⊥ respectively, is character-ized. The setup of this measurement is depicted forVITROVAC 6025X in Fig. 2. The metal tape is woundand isolated by thin plastic foil. Using a few turns, atest ”inductor” is built. The two probes of the thermallytreated materials VITROVAC 6025 || an ⊥ were smaller,i.e. 3 ·3cm2 wrapped in both directions. The impedance,compare eq. (1), is measured at frequencies up to 1MHz.

Fig. 2. Wrapped VITROVAC 6025X tape for characterization

The measurement was carried out at low constant cur-rent over the whole frequency range, i.e. at a constant fieldexcitation level H = const. For the different measurementsetups the excitation is low (0.1 mA

cm ..0.2 mAcm ). Hence, in

Fig. 3, the initial complex permeability �r = � ′r − j� ′′

r isrecorded, always weightened with a constant k, being theequivalent inductor L0 from eq.(1). The loss factor tan�is derived from the measurements.

Fig. 3 illustrates the differences between VITROVAC6025X, VITROVAC 6025F|| and VITROVAC 6025F⊥.Note that, after the temperature treatment for optimizingone preferential direction, also the performance of VIT-ROVAC 6025F|| is significantly improved with regard toVITROVAC 6025X. The eddy-current cut-off frequencyis increased from about 50 to 200kHz. The frequency ofthe loss maximum (� ′′

r is maximum), is moved to higherfrequencies as well. The loss factor tan� is reduced byhalf an order of magnitude.

The new information in the measurement of VIT-ROVAC 6025F⊥ can be explained as follows. The eddy-current cut-off frequency drops below the minimum mea-

1The operation of VITROVAC 6025F in the preferential direction isindicated with ||, the operation perpendicular to the preferential directionis indicated with ⊥

Fig. 3. Initial permeability of amorphous metals and loss factor tan�

sured frequency of 3kHz. Thus, |�r| decreases over theentire measured frequency range. The same applies to theloss maximum which is below 3kHz. There is a secondcut-off frequency close to the sole cut-off frequency ofVITROVAC 6025F||. It is reasoned by measuring thesame tape in both directions such that the designed cut-off frequency of VITROVAC 6025F|| is also visible inVITROVAC 6025F⊥. As expected, the loss factor oftan� of VITROVAC 6025F⊥ is at its maximum for lowfrequencies and approximates to tan� of VITROVAC6025F|| beyond the high cut-off frequency, where isotropiceddy currents dominate the magnetic behavior of the tape.

These measurements are feasible, since the absolutevalue of tan�3kHz = 0.02 of VITROVAC 6025F|| matchesthe data sheet information of similar materials. Thesematerials show deviations in tape thickness and in cut-offfrequencies resulting in a loss factor varying from 0.0005to 0.17 [8].

The measurement equipment did not allow to operate athigher induction levels. With the material under operationmuch higher core excitations are given. At a field strengthof H =60 mA

cm , i.e. B=150mT, the value of �r

in thepreferential direction increases more than a factor of 3[8], resulting in a much lower loss factor coded in Table Icompared to the measurement.

It is assumed, that the permeability of VITROVAC6025F⊥ shows a similar behavior at higher inductionlevels, e.g. also at higher induction levels the loss factorof VITROVAC 6025F⊥ dominates the loss factor VIT-ROVAC 6025F|| similar to Fig. 3.

C. Novel Amorphous Metal Transformer Configuration

The main idea is to use the amorphous metal tapes alsoperpendicular to the preferential direction. It is proposedto built a lamination stack, with the main flux beingclosed within a single tape plane. Thus, as indicated inFig. 4, for a short distance the material is operated inthe non-preferential direction, being a new approach tobuilt amorphous metal transformers. However, lamination

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stacks of large low frequency power transformers are alsooperated with a deviation from the direction of grainorientation at the corners [13].

a

preferentialdirection

mainflux

b

c

d

2a average distance for flux in preferential direction2b average distance for flux perpendicular to preferential directione effective thickness of lamination stack2ce core cross section for flux in preferential direction2de core cross section for flux perpendicular to preferential direction

Fig. 4. Proposed transformer cross section, i.e. shape of lamination

A qualitative optimization can already be carried outnow. Due to the fact that VITROVAC 6025F|| (in thepreferential direction) shows an optimized performancein contrast to VITROVAC 6025F⊥, the relation a

b shouldbe maximized. Thus, a low profile transformer will bethe result. Furthermore, the cross section perpendicularto the preferential direction could easily be increased toreduce flux density and thus, minimize core losses (choosed ·e > c ·e). With the winding configuration, which isdepicted exemplary and could also be interleaved, theleakage inductance and parasitic capacitances could bedesigned with available tools in literature [5], [6].

D. Construction of Amorphous Metal Tape TransformerCore

Two amorphous metal tape transformers were con-structed corresponding to Table II and Fig. 4 using thematerial VITROVAC 6025F. The rectangular copper win-dow in Fig. 4 was kept rounded due to constructionalconstraints.

TABLE IITRANSFORMER CONSTRUCTIONAL DETAILS

Trafo A Trafo B

a 34 mm 42 mmb 11 mm 11 mmc 4 mm 4 mmd 4 mm 12 mm

Lamination height (= 2b+2c) 30 mm 30 mmLamination width (= a+d) 38 mm 54 mm

Number of laminations 100 24e (lamination tickness = 22�m) 2.2 mm 0.53 mm

In a first step, the more than 12m long VITROVAC6025F tape was electrically isolated using plastic spray.

The tape was cut to the laminations widths indicatedin Table II and stacked. Subsequently the stack wasmechanically fixed by aluminum plates and the copperwindows were drilled and milled carefully. The resultingelectrical shorts coming from the mechanical processingwere etched away by dipping the laminations to a 4minutes bath of acid. It consisted of one part H2O, onepart H2O2 in 30% concentration for a fast oxidization ofthe VITROVAC iron metal and one part of HCl in 37%concentration for etching the oxide. The etching processwas first tested with electrically isolated and untreatedsamples, showing the stability of the electrical isolationwith regard to the acid. Transformer A is depicted in Fig. 5and the success of the etching process is illustrated inFig. 6.

Fig. 5. Transformer core A

Fig. 6. Losses before and after etching of trafo A

Transformer B was designed with a number of lamina-tions of 100. However, amorphous metal is very hard andbrittle and during the mechanical processing many lamina-tions were destructed. Probably the mechanical fixing wasnot rigid enough. In the following etching process of trafoB the acid started cooking (probably initiated by a wrongtweezers) such that the electrical isolation separated fromthe metal at some laminations. Subsequently, the metal of

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the affected laminations was not protected against the acid.The defect laminations were removed from the laminationstack.

For the first test, chemical and mechanical processingshowed good results, but the processes should be devel-oped further.

IV. DISCUSSION

A. Amorphous Metal Tape Transformers

The two constructed metal tape transformer cores werecharacterized as indicated in Fig. 7.

Fig. 7. Cross section modification

Transformer core B is superior to transformer core Aby about a factor of 3 to 5 on this scale with regard toweight. The material increase per lamination of trafo B isbelow a factor of 2. Thus, also overall losses of trafo Bare reduced with regard to trafo A. Since core excitationin the preferential direction is kept constant, total lossesin the region - with the flux being perpendicular to thepreferential direction - are reduced in trafo B.

The cross section where the magnetic flux is perpendic-ular to the preferential direction is increased by a factorof 3:

(dBdA

= 3)

. Two effects come into play and lead tothe same statement:

• Increasing the core cross section where material isoperated perpendicular to the preferential direction(increasing d) results in a reduced flux density andthus, reduced losses.

• Increasing the core cross section where material isoperated perpendicular to the preferential direction

(increasing d) to large values results in higher reluc-tances for the long flux lines (permeability is large,but not infinity). Thus, from a specific length ofd onwards there is no significant flux in the tape,but weight and volume increase. Thus, with a largeincrease of d the volume and weight related lossesof the whole transformer decrease.

Consequently, the longer the distance d, the betterthe efficiency as shown in Fig. 7. However, the secondargument is of mathematical nature and not of practicalrelevance. In order to determine an appropriate size ofd, the flux distribution for d −→ ∞ can be consideredfor example by using FEM or calculating a reluctancenetwork similar to [14]. With the assumption of constantpermeability in both metal tape plane directions (Fig. 3),the depth of impression of flux can be estimated as a func-tion of b. Thus, an appropriate size of d can be estimated.Before concise theoretical investigations can be presented,improvements have to be achieved in the mechanical andchemical processing. There are still unknown variables,e.g. in the etching process, showing a direct influence onthe core cross section and thus, on the results.

B. Comparison to Ferrite Transformer

In Fig. 8 trafo B is compared to a ferrite transformercore.

Fig. 8. Comparison of trafo B vs. ferrite transformer core [12]

As expected the novel amorphous metal transformerconfiguration is superior at lower frequencies and highcore excitations. Above frequencies of about 100 kHz,ferrites show a superior loss behavior.

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As a consequence, this novel metal tape transformerconfiguration offers similar design possibilities as a ferritetransformer, i.e. design of leakage inductance and parasiticcapacitance at high copper fill factor. At the same time,the metal transformer shows a better loss behavior forfrequencies below 100 kHz.

V. CONCLUSIONS AND OUTLOOK

In this paper, a novel configuration of an amorphousmetal tape transformer, with the conservative flux beingclosed within the magnetically anisotropic tape plane, isintroduced. Up to frequencies of 100 kHz, measurementsshowed that performance is superior compared to a ferritetransformer, with the boundary condition of a highersaturation flux density. The novel configuration allows - incontrast to common ring cores with high copper fill factor- to design a defined leakage inductance and a definedwinding capacitance with the methods found in literature[5], [6]. Furthermore, in contrast to amorphous cut-coreconfigurations, this solution leads to higher mutual induc-tances.

After the anisotropic tape is characterized in both tapeplane directions, the construction of the novel transformersis presented. The characterization of the novel transformeris carried out, however several unknown variables stillexist. For example, the etching process shows a directinfluence on the core cross section and thus, on the results.

However, varying the metal cross section with the fluxperpendicular to the preferential direction shows signifi-cant results. It was qualified that with increased materialeffort (factor <2) the weight related losses are reducedby a factor 3 to 5. Thus, overall losses are reduced. Incomparison to ferrites the constructed prototype (trafo B)is shown being superior for frequencies up to 100 kHz andin particular for high induction levels.

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