Jean Sylvio Ngoua

Teu Magambo

Reda Bakri
email: reda.bakri@ec-lille.fr

Member, IEEE Xavier Margueron
email: xavier.margueron@centralelille.fr

Member, IEEE Philippe Le Moigne
email: philippe.lemoigne@centralelille.fr

Arnaud Mahe
email: arnaud.mahe@fr.thalesgroup.com

Stephane Guguen
email: stephane.guguen@fr.thalesgroup.com

Taoufik Bensalah
email: taoufik.bensalah@fr.thalesgroup.com

Planar Magnetic Components in More Electric Aircraft: Review of Technology and Key Parameters for DC-DC Power Electronic Converter

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Abstract-The More Electric Aircraft (MEA) has motivated aircraft manufacturers since few decades. Indeed, their investigations lead to the increase of electric power in airplanes. The challenge is to decrease the weight of embedded systems and therefore the fuel consumption. This is possible thanks to new efficient power electronic converters made of new components. As magnetic components represent a great proportion of their weight, planar components are an interesting solution to increase the power density of some switching mode power supplies. This paper presents the benefits and drawbacks of high frequency planar transformers in DC/DC converter, different models developed for their design and different issues in MEA context related to planar's specific geometry and technology. Index Terms-MEA; power density; power converters; planar transformer; high frequency losses; parasitic elements.

I. INTRODUCTION

Since the beginning of aviation, engineers have investigated solutions to generate and control electrical power aboard airplanes [START_REF] Hockmeyer | The generation and regulation of electric power in aircraft: a survey of design features of generators and their control[END_REF]. Currently, with More Electric Aircraft (MEA), this topic mobilizes avionic equipment manufacturers with the aim to increase the available power in new aircraft generations [START_REF] Sarlioglu | More Electric Aircraft: Review, Challenges, and Opportunities for Commercial Transport Aircraft[END_REF]. For the last ten years, the power supplied by electrical systems has been multiplied by a factor of 10 (Fig. 1).

The MEA has ambitious objectives in the coming years such as reducing fuel consumption and the emissions of noise and gas (CO2, NOx). In Europe, avionics manufacturers mutualize their skills around different projects (Clean Sky 2 [START_REF]About Clean Sky 2 | Clean Sky[END_REF], Genome [START_REF]CORAC Aéro Recherche -Aéronautique Civile -Le CORAC est le Conseil pour la Recherche Aéronautique Civile[END_REF]) to meet these objectives. Some topics developed in MEA projects deal with the replacement of hydraulic and pneumatic systems by electrical ones [START_REF] Roboam | New trends and challenges of electrical networks embedded in "more electrical aircraft[END_REF], with a trend toward dc solutions [START_REF] Tenconi | Introduction to the Special Section on The More Electric Aircraft: Power Electronics, Machines, and Drives[END_REF] [START_REF] Gao | Modeling and Impedance Analysis of a Single DC Bus-Based Multiple-Source Multiple-Load Electrical Power System[END_REF] or greener taxiing period [START_REF] Heinrich | Regenerative Braking Capability Analysis of an Electric Taxiing System for a Single Aisle Midsize Aircraft[END_REF] [START_REF] Roboam | More Electricity in the Air: Toward Optimized Electrical Networks Embedded in More-Electrical Aircraft[END_REF] for example. Electrical systems have significant advantages such as gain of mass, energy rationalization, availability and maintenance costs [START_REF] Roboam | More Electricity in the Air: Toward Optimized Electrical Networks Embedded in More-Electrical Aircraft[END_REF].

More generally, electrical systems in aircraft relate to various electrical engineering domains from generation to power distribution, energy storage for emergency power supply or electrical loads. All these devices are connected through power converters: Inverters, rectifiers and choppers. Power electronic systems are needed with specific features due to aeronautic specifications: Harmonics, power factor, electromagnetic compatibility (EMC), efficiency, size, weight, harsh environment and isolation [START_REF] Naayagi | A review of more electric aircraft technology[END_REF]. The latter characteristic is necessary for safety and reduction of EMC current [START_REF] Taha | Power electronics for aircraft application[END_REF]. This isolation function is usually made with high frequency transformer, included in power electronic converter.

Magnetic components, transformers and inductors, present an important issue that limits the integration of power converters. As a comparison, power train of static inverters achieve some 6kW/kg [START_REF] Bensalah | Integrated Modular Power Electronics: Achievements and Challenges[END_REF], while DC/DC converters with a high frequency transformer are rather below 2kW/kg [START_REF] Asfaux | Development of a 12kW isolated and bidirectional DC-DC Converter dedicated to the More Electrical Aircraft: The Buck Boost Converter Unit (BBCU)[END_REF]. However, when comparing a DC-DC converter with an isolated inverter, the results are not the same. In [START_REF] Homeyer | Advanced power converters for More Electric Aircraft applications[END_REF], the authors have shown that when using a planar transformer for both a DC-DC converter and a DC-AC inverter, the best power integration and the best efficiency were obtained for the DC-DC converter.

Although those magnetics limit the integration, they are still a key technology to improve power converters performances, in particular in MEA [START_REF] Bourdon | Review of power electronics opportunities to integrate in the more electrical aircraft[END_REF]. Indeed, historically, research on lighter transformers began with the first investigation of electrical power in aviation [START_REF] Stephens | Lightweight aircraft transformers[END_REF].

With the development of planar magnetic components (PMC), high frequency planar self and transformers became an interesting solution in AC/DC or DC/DC power converters to increase power densities [START_REF] Ouyang | Overview of Planar Magnetic Technology; Fundamental Properties[END_REF]. Actually, increasing the frequency allows reduction of the principal inductance value and hence the core size, but true till a certain frequency limit only. Moreover, the low profile of PMC allows them having higher surface/volume ratio as compared to conventional magnetics, and therefore, have a better thermal behavior [START_REF] Ouyang | Overview of Planar Magnetic Technology; Fundamental Properties[END_REF].

Planar magnetics are largely used in automotive for electric vehicles (EV) and more generally in embedded systems. Aeronautics and aerospace industries also investigate benefits provided by the use of new technology of PMC, especially with new constraints in onboard electrical systems.

This paper presents a review of planar technology and benefits that could be obtained with the use of such component in a MEA application. The aim is to discuss transformer key parameters which can help improving performances of a DC-DC converter and its integration. Based on a MEA power converter application, planar magnetic models available in literature are reviewed and analyzed in compliance with aeronautical specifications. The idea is to give readers a complete view of problematics even if models and analytical methods are not deeply developed in the paper. Finally, a planar transformer is designed in order to highlight different parameters and constraints previously reviewed.

The paper is organized as follows: In Section II, a DC/DC converter example serves as a reference to illustrate the interest of using high frequency planar transformers. Section III presents the issue of planar magnetics in MEA. Then the section IV focuses on the key parameters for efficient planar transformer in an aeronautical environment. In the final section, a planar transformer prototype is designed to illustrate steps and benefits of planar magnetics in MEA. Voltage and power flow in avionic systems are expanding with the development of new power electronic converters. Fig. 1 shows the increase of active power linked to the rise of voltage level. Till date, the trend is the use of two DC link of 135V or 270V respectively generated from a 115Vac or 230Vac, 400Hz-800Hz through a rectifier unit. The 270V DC voltages enable to obtain the new standard 540Vdc which is known as the high voltage direct current (HVDC) link of new DC aircraft electric network. Fig. 2 shows an example of a power transfer from an AC generator to a 28Vdc storage battery. It could be noted that most of electric loads in avionics are designed for 28Vdc nominal voltage.

In the following, the aim is to present the study of a DC/DC converter. Such a converter can lead to substantial power density enhancement, reducing the size of the transformer and maintaining a good efficiency level.

A. Example of Converters' Specifications: Triple Active Bridge Converter

The converter under development is a three port DC/DC converter of few kW. From the new 540V HVDC voltage, the two 28V low voltage direct currents (LVDC) must be generated. One output is regulated to 28Vdc while the other one is connected to a 28V battery. Actually, the converter has different operating modes and must be able to operate in buck, boost and buck-boost depending on external constraints. In case of an interruption of the HVDC bus, the LVDC battery side must be able to restart the input voltage or to generate the other 28V LVDC output. The regeneration of the DC bus is an important aspect and an interesting application when dealing with a DC-DC converter in MEA [START_REF] Deng | Regenerative electric power for More Electric Aircraft[END_REF][19] [START_REF] Eid | Modeling and characterization of an aircraft electric power system with a fuel cell-equipped APU connected at HVDC bus[END_REF].

One structure which fits correctly these specifications is the triple active bridge (TAB) [START_REF] Duarte | Three-Port Bidirectional Converter for Hybrid Fuel Cell Systems[END_REF] as presented in Fig. 3. The two LVDC outputs can also be put in parallel to generate the HVDC voltage, becoming a Double Active Bridge (DAB) converter. DAB and TAB architectures are well known in literature [START_REF] Krismer | Modeling and Optimization of Bidirectional Dual Active Bridge DC-DC Converter Topologies[END_REF][23] [START_REF] Baek | Power flow analysis for 3-port 3-phase dual active bridge dc/dc converter and design validation using high frequency planar transformer[END_REF][25] [START_REF] Zhao | An Isolated Three-Port Bidirectional DC-DC Converter With Decoupled Power Flow Management[END_REF] but for MEA, such LVDC 28V application is a key challenge including the safety compliances.

The TAB is an isolated and bidirectional DC-DC converter which is composed of three full bridges, one input H-bridge and two output H-bridges (Fig. 3). Between input and output bridges, a high frequency transformer not only ensures the different operating mode (buck, boost…) but also the galvanic insulation of the converter. Note that in Fig. 3, the 3-winding high frequency transformer is represented with a leakage transformer [START_REF] Margueron | Design of Equivalent Circuits and Characterization Strategy for n-Input Coupled Inductors[END_REF]. This leakage transformer is useful when representing magnetic coupling between transformer's outputs and useless when this coupling is low.

With such topology and voltage levels, semiconductors are made of SiC MOSFET for the HVDC side and Si MOSFET for the LVDC side. The converter must operate with a switching frequency around few tens of kHz. Table I summarizes the converter's specifications. To reduce the switching and conduction losses, an optimization on the control strategy is made to minimize the rms current through the components [START_REF] Krismer | Modeling and Optimization of Bidirectional Dual Active Bridge DC-DC Converter Topologies[END_REF]. The basic control strategy commonly used is the phase-shift modulation (PSM) which consists in controlling all MOSFET with same duty cycles (D=0.5) delayed with a phase (i=2, 3 corresponding to the output bridges) between input and output bridges. These two setting parameters ( ) allow to tune the power flow (1). This strategy can lead to good efficiency while operating around nominal conditions. For voltages higher than the nominal, others strategies such as alternative modulations can be privileged [START_REF] Everts | Optimal ZVS Modulation of Single-Phase Single-Stage Bidirectional DAB AC-DC Converters[END_REF].

The control strategy which consists in minimizing the rms current is used not only to reduce switching losses but also allows reducing copper losses in the transformer because these losses are linked to the current. The control must also avoid DC current which could quickly saturate the magnetic core.

In order to minimize switching losses at the turn on, all full bridge legs are controlled with zero voltage switching (ZVS), helped by each MOSFET capacitance.

To simplify the study of the TAB, one output H-bridge can be considered as opened. Then, in the equivalent two port, with a PSM strategy on both full bridges, the power flow between the primary and one secondary winding (which is not opened) for example, is expressed as follows [START_REF] Zhao | An Isolated Three-Port Bidirectional DC-DC Converter With Decoupled Power Flow Management[END_REF]:

i k sw i i i i L F V V P 1 2 1 1 1 1 2 ) ( π ϕ π ϕ - = (1)
With the phase between primary winding voltage and secondary winding i voltage , is the fundamental switching frequency and the leakage inductance between the two windings from primary side.

B. Discussion on Power Density

Regarding power considerations, this converter must be able to operate in a high temperature environment with a nominal efficiency of 94%. The aim is to reach at least 1.5kW/kg (or 1kW/L) of power density.

The heatsink represents a non-negligible element because of its weight and size, and because MEA power converters are generally in a harsh environment [START_REF] Mastromauro | SiC MOSFET Dual Active Bridge converter for harsh environment applications in a more-electric-aircraft[END_REF]. Thermal performances impact the component's functioning (semiconductors, insulation, core…) and so the reliability of the converter. For example, Fig. 4 shows a 3D view of a prototype of the DC/DC converter, including planar transformer and inductor, with a heatsink placed on the two faces (top and bottom) of the PCB. Note that it is not a final optimized structure. Some adjustments have to be done regarding dimensions and the components' distribution.

C. Toward the integration of magnetics for TAB converter

Research has shown its interest for transformer's technology since the first generation of voltage in airplanes. Initially, transformers were designed for each embedded electric load, operating at the AC frequency of voltage generators and power network. Power density was not yet a topic of interest. Actually, with the development of power transistors and power converters, transformers and more generally magnetics have become key components for the achievement of high power density.

For the TAB converter, as it was introduced previously, the transformer leakage inductance has to be set to a specific value to obtain the nominal power value [START_REF] Hockmeyer | The generation and regulation of electric power in aircraft: a survey of design features of generators and their control[END_REF]. Leakage inductances of planar transformers are known to be low [START_REF] Carsten | The low leakage inductance of planar transformers; fact or myth?[END_REF], especially when a good efficiency of the transformer is required. Therefore, the value of the leakage inductance is lower than expected ( in Table I) at fundamental frequency. The difference could be completed either by adding a resonant self or a bus bar on winding connections. Both solutions increase size and weight of the converter.

Another specification for the TAB converter deals with the fact that both secondary windings can be set in parallel to transfer power to the HVDC side. As a consequence, leakage inductance values, between primary winding and secondary windings ( , must be as close as possible to parallel both windings.

With such converter topology and specifications, the use of an efficient and lightweight high frequency transformer is mandatory. It can be noted that in power electronics, high frequency for PMC means from few tens of kHz to several hundred of kHz. The high frequency range is rarely beyond 1MHz because of skin and proximity effects regarding magnetic core behavior, conductor's size and the conductors' arrangement chosen.

Planar technology presents a lot of benefits for this kind of application but also some drawbacks. The expansion of PMC in MEA is constraint with the perfect control of some component's parameters.

III. PLANAR MAGNETICS: ISSUES IN MEA

A. Generalities on Planar Components

PMC are interesting alternatives to classic high frequency transformers [START_REF] Ouyang | Overview of Planar Magnetic Technology; Fundamental Properties[END_REF]. Based on the combination of a low profile high frequency magnetic core and printed circuit board (PCB) or copper foil windings technology, PMC enable to obtain significant gain in terms of weight and size but also regarding electrical and thermal performances.

Planar magnetics presents some benefits as compared to traditional high frequency wound magnetic components:

• Low profile component.

• Lightweight.

• High power density.

• Good reproducibility based on winding's precision. • Easy manufacturing for industrial and cost effective.

• Good thermal behavior [START_REF] Bakri | Planar transformer equivalent thermal resistance variation with ambient temperature and power losses[END_REF] due to their surface/volume ratio.

• Low leakage inductance.

• Low AC copper losses due to the rectangular shape of the conductors.

On the opposite, some drawbacks can be listed:

• Restricted to high frequency use.

• Limited core size and shape.

• Limited power range, few kW.

• Too weak leakage inductance value for some resonant converter applications. • Penalizing parasitic capacitances.

• Longer mean turn length (MTL) and consequently higher DC resistances due to core shape.

• Complexity of wires connections in case of multilayers windings. Some of these benefits and drawbacks don't have the same impact when dealing with MEA power converters. Let's now detail some topics to investigate how to use efficiently those advantages and how to circumvent those limits in MEA context.

B. Planar Magnetic Cores

Due to the industrial context, it is better to begin investigation using existing core on the market, otherwise it is necessary to control the manufacturing techniques and so, the prototyping cost can quickly increase.

The core size of a transformer is often chosen according to the magnetizing inductance desired and the desired active power through the area product of the core [START_REF] Mclyman | Transformer and Inductor Design Handbook[END_REF]. The typical planar core made of ferrite are given in Fig. 5: EE, ER, EQ and PQ [START_REF]Ferroxcube[END_REF][33] [START_REF]Ferrites and Accessories | Products | TDK Product Center[END_REF]. The EE shape requires straight windings while ER, EQ and PQ match with circular ones. The advantages of ER, EQ and PQ cores are their lower copper losses and parasitic elements thanks to the copper shape which reduce the mean turn length. In [START_REF] Shin | Adapter design using planar transformer for aircraft[END_REF] the author has shown that a planar transformer with a PQ core leads to the best efficiency for an aircraft application.

Table II presents planar core size and weight for smallest and biggest core in each shape. The core volume represents the external dimensions and not the volume of material. Data are extracted from a manufacturer datasheet [START_REF]Ferroxcube[END_REF] but similar shapes and dimensions can be found with other core manufacturers [START_REF]Ceramic Magnetics Homepage[END_REF] [START_REF]Ferrites and Accessories | Products | TDK Product Center[END_REF]. As it can be seen in Table II, EE shape seems to have the best compromise between size/weight and area product for bigger planar cores. The PQ core is a little bit lighter but is penalized by its volume. This is due to its non-low profile core. Others shapes also present interesting characteristics but are limited to smaller core for EQ while the ER one cover the biggest range, from the smallest one (ER9.5) in terms of volume and weight to the biggest one (ER64). One can note that combination of two E is not the only one solution for planar E and EQ core. For these two kind of cores, they can be combined with PLT to reduce volume and weight of the component.

It can be obvious that PMC are more interesting to achieve higher power integration as compared to conventional magnetic shapes. Nevertheless, an example of comparison is given in Table III andTable IV. Those tables compare smallest and biggest EE planar cores supplied by [START_REF]Ferroxcube[END_REF] and their equivalent in conventional cores with approximately the same area product. On one hand, it can be noted that planar cores are lighter and have less volume. On the other hand, they have higher surface/volume ratio. These characteristics make them better candidate not only for cooling but also for increasing the power density for a same given power value. If PMC frequency and power level are restricted to specific ranges, temperature and magnetic core can also act as limiting parameters. Indeed, one of the main drawbacks of planar transformers is their limited maximum area product for fixed core shapes supplied by manufacturers.

Magnetic material has an important influence. This parameter is linked to the magnetic performances of the component but also to the thermal characteristic. For a given switching frequency, magnetic material is selected to obtain specific and maximal value of magnetizing inductance. Generally, manufacturers give a tolerance of 20 to 25 % on magnetic characteristics of ferrite cores. An air gap can then be added to control the magnetizing value and to avoid core saturation in case of converter control imbalance. Choosing to add an air-gap leads to problems as fringing flux that can create supplementary losses. Conductors have to be placed far away from air gap [START_REF] Zhang | Modeling and design optimization of planar power transformer for aerospace application[END_REF] to reduce those additional eddy currents generated by this fringing flux.

C. Windings' Technology

As previously said, planar windings are either made of PCB or copper foil. Sometimes Litz wire can also be used as in classical high frequency wounded magnetics. Fig. 6 presents two examples of planar transformer. The first one (Fig. 6a) is a simple 2-layer PCB planar transformer. The second one (Fig. 6b) is an 8-layer copper foil planar transformer.

The choice of conductor's size is linked to several parameters: the desired DC current density, the skin depth (2) calculated at the operating switching frequency including harmonics and the eddy currents in the conductors as known as proximity effect. Usually, copper thickness is quite thin due to technology but also in order to limit those eddy currents in layers. As a consequence, planar windings are generally made of multiple layers connected in parallel to support high current. The copper winding does not fill the core window and generally leads to low filling factor.

µ f ⋅ ⋅ = π ρ δ (2)
With f is the operating frequency, µ the permeability, and ρ the copper resistivity. Multilayer PCB fits well with manufacturing process and automated mass production. The copper thicknesses as well as the insulation between conductors are fixed by standard manufacturing techniques. Moreover these thicknesses are the same for primary and secondary windings. Then, these characteristics limit the flexibility to optimize the design of multilayer PCB planar transformer. Transformer can be integrated directly in converter's PCB but this specific case is limited to small power applications. For transformer designed for some hundreds of watts and more, it is preferred to design a specific multilayer PCB for the winding transformer. Multilayer PCB needs vias to connect different layers. This creates additional eddy currents and hotspot inside the windings [START_REF] Zhang | Modeling and design optimization of planar power transformer for aerospace application[END_REF] [START_REF] Ropoteanu | Thermal investigation of a planar core power transformer[END_REF]. Finally, multilayer PCB does not allow easy insertion of a magnetic layer to increase potentially the leakage energy and so the leakage inductance [START_REF] Ouyang | Optimal Design and Tradeoff Analysis of Planar Transformer in High-Power DC-DC Converters[END_REF].

For copper foil planar components, windings' conductors are cut inside foil whose thickness can be chosen differently for primary and secondary. Insulation between layers is made with kapton foil, for example, also cut to be sized for planar magnetic core. Such winding technology usually allows more current for power transformer because of the greater thickness that leads to bigger conductors' across section. As opposed to multilayer PCB, a leakage layer can be added easily between layers. However, a problem still remains, dealing with connections of layers that have to be made one by one.

In both technologies, the maximum number of layers will be limited by the window's core height. Moreover, industrial standards require a minimum space between the conductors and the core and a minimum insulation between conductors of the same layer and conductors of different layers to warranty a level of dielectric rigidity. For every planar component, an attention must also be payed to connections between layers but also between the component and its environment. When they are not well chosen, connections can strongly modify windings and transformer performances.

D. Modeling PMC: Analytical vs Numerical

In order to design efficient and optimized planar magnetic components, accurate models are needed. The increase of switching frequency, thanks to new active SiC and GaN semiconductors, can be problematic when dealing with modeling.

In literature, analytical models [START_REF] Prieto | Magnetic Component Model for Planar Structures Based on Transmission Lines[END_REF][40] [START_REF] Tria | Implementation of a Nonlinear Planar Magnetics Model[END_REF] are generally based on the 1-D magnetic field hypothesis, known as the Dowell hypothesis [START_REF] Dowell | Effects of eddy currents in transformer windings[END_REF]. Other derivative models close to Dowell's one have been developed to be more accurate when the conductors do not fill the core window width (porosity factor (3) 7 . 0 ≤ η ) whereas Dowell's model accuracy is proved when

1 7 . 0 ≤ ≤ η [43]
. This porosity factor is usually higher for traditional high frequency transformer than for planar ones. Note also that porosity factor is different from the filling factor which is the ratio of the total area of conductors on the core window surface. This filling factor is generally lower for PMC (< 0.4) as compared to traditional transformer or self (> 0.75). Such approach is also limited with the frequency evolution because of the field hypothesis that becomes inacurate. With n the number of turns in a layer, a the conductor width and b the core window width Some other semi-analytical computational models based on the resolution of Poisson's equation such as PEEC formulation [START_REF] Bui-Ngoc | New µPEEC formulation for modeling 2D core transformer. Principles, academic and industrial applications[END_REF] or Roth's method [START_REF] Hammond | Roth's method for the solution of boundary-value problems in electrical engineering[END_REF] can be used for 2-D transformer's winding cross section. These methods are efficient only when material in rectangular region is homogeneous. Moreover, they require more computational time than analytical ones.

Regarding numerical modeling, 2-D finite element analysis (FEA) are often performed [START_REF] Chen | Model and design of PCB parallel winding for planar transformer[END_REF][47] [START_REF]Finite Element Method Magnetics: HomePage[END_REF]. 3-D FEA are more complex to achieve and require important computational ressources and time computing. Simplifications are sometimes done, based on the use of 2-D methodology [START_REF] Aime | Numerical Methods for Eddy Currents Modeling of Planar Transformers[END_REF] or modelorder reduction (MOR) [START_REF] Taylor | Model-order reduction of magneto-harmonic problems based on POD: application to planar magnetic components[END_REF] to shorten resolution time.

Main advantages of 3-D FEA are the possible coupling between electromagnetic and thermal solvers (Maxwell 3D, CFD…) [START_REF]ANSYS -Simulation Driven Product Development[END_REF]. With such coupling, impact of losses in temperature's distribution is facilitated and can lead to the identification of potential hotspots inside windings, which can be destructive for transformer. This thermal aspect is also an indicator of the PMC's lifetime, focusing on the insulation's aging.

E. Planar self for EMC

Apart from the application of filtering inductors in power converter, planar self can also be used to design EMI filters [52][53]. This is possible only when the stress current through the filter does not saturate the core. As a consequence, planar self are more dedicated to common mode (CM) suppression filters than differential one, except adding air gaps through the magnetic core. One can note that CM filters which are coupled, allow compensating the main flux generated by the power current. Some configurations of planar transformers are also interesting while they allow reduction of the EMI conducted noise without adding EMI filters but electrostatic screen between conductive layers [START_REF] Pahlevaninezhad | An Improved Layout Strategy for Common-Mode EMI Suppression Applicable to High-Frequency Planar Transformers in High-Power DC/DC Converters Used for Electric Vehicles[END_REF]. For MEA, these Electromagnetic compatibility (EMC) aspects must be taken into account by designers to meet aeronautical standards such as DO-160 and it begins with qualification tests [START_REF] He | Investigation of test methods for DO-160 qualification tests[END_REF].

IV. PLANAR MAGNETICS: KEY PARAMETERS

The design of planar magnetic transformer as well as planar self, must consider some key parameters that need to be controlled or tuned to reach good performances for the component and the power converter.

A. Multiphysics Component

Modeling and analysis phenomena inside PMC are based on multiple domains of physics, as for traditional high frequency transformer. The main fields involved in the component are: Electrical, electromagnetic, electrostatic and thermal. Fig. 7 presents complete high frequency equivalent circuit for a 2winding transformer, including elements (resistances for losses and inductances) that vary with frequency as well as capacitors for the electrostatic behavior [START_REF] Margueron | Identifying the Magnetic Part of the Equivalent Circuit of -Winding Transformers[END_REF] [START_REF] Schellmanns | Representing electrical behaviour of transformers by lumped element circuits: a global physical approach[END_REF]. Generally, engineers design transformers to have low losses and low leakage inductance, but capacitances are rarely considered. However, the reduction of the leakage inductance will be made at the expense of parasitic capacitance which will increase. Both aspects are coupled through the thickness of the dielectric material. Different methods can be used to reduce winding capacitances such as using thicker dielectric or inserting a grounded electrostatic screen, but, presently, no method allow reducing both leakage inductance and winding capacitances [START_REF] Ouyang | Overview of Planar Magnetic Technology; Fundamental Properties[END_REF]. In some designs, these parasitic aspects are desired and even enhanced, especially for LLC power converters.

These electromagnetic and electrostatic aspects are effective to represent the equivalent electric circuit of planar transformer, but this one does not include thermal behavior which is linked to transformer's geometry and boundary conditions. Same equivalent circuit (Fig. 7) can be obtained for 3-winding transformer and more generally for n-winding transformers. Such circuit is a really interesting tool that can be included in circuit software. The focus is now on some design rules and modeling aspects which must be considered when dealing with planar transformers.

B. Losses and Thermal performances

Copper losses are a major topic in high frequency power electronic transformer. As mentioned before, copper losses can be evaluated based on analytical formulae or using 2-D and 3-D FEA. Skin and proximity effects increase with frequency and thus create more losses and heat to dissipate.

In high frequency, current densities are concentrated in the corner of the rectangular conductors (i.e PCB or copper foil). Fig. 8 presents current density repartition along a 7x0.5 mm² conductor for different frequencies and for a sinusoidal excitation. Moreover, planar rectangular conductors are sized thin to limit eddy current effects. In order to withstand enough current, conductors have to be connected in parallel. It has been shown in [START_REF] Margueron | Current Sharing Between Parallel Turns of a Planar Transformer: Prediction and Improvement Using a Circuit Simulation Software[END_REF] that even with such connections, high frequency currents are not distributed uniformly between layers. One solution to reduce these high frequency effects, and thus the AC equivalent resistance of transformer, is to interleave copper layers. Regarding copper tracks, it has been shown in [START_REF] Magambo | Impact of PCB track shape on HF copper losses of planar magnetic components[END_REF] that the shape of the windings also have an impact on copper losses in conductors located outside the magnetic core. Mixing different copper shapes (square/round) for primary and secondary windings can improve the reduction of high frequency copper losses.

The other losses to evaluate are the core losses and the dielectric losses, but, the latter are generally considered null regarding the two other types of losses. Core losses can be calculated using data of magnetic material [START_REF]Ferroxcube[END_REF], in particular complex permeability. For more precision, magnetic core losses can be calculated with Steinmetz's equation ( 4) [START_REF] Steinmetz | On the Law of Hysteresis[END_REF] but only when the excitation is sinusoidal. For non-sinusoidal power electronic waveforms, Improved General Steinmetz Equation (iGSE) [START_REF] Li | Improved calculation of core loss with nonsinusoidal waveforms[END_REF][62] is preferred.

c peak sw c core V B F K P * * * β α = (4)
Where Kc, α and β are parameters given by manufacturer's datasheet and Vc is the core volume.

As total losses have been determined, the thermal behavior of the planar transformer can be estimated considering these losses as input. The increase of transformer's temperature due to total transformer losses has to be minimized with the use of cooling technology. The thermal behavior can be obtained by performing FEA with numerical tools, but under some assumptions, such as the transfer mode considered, boundary conditions and the exchange coefficient values.

For PMC design, analytical models and tools were recently developed to help designer with the thermal modeling of transformer. In [START_REF] Bakri | Planar transformer equivalent thermal resistance variation with ambient temperature and power losses[END_REF], the equivalent thermal resistance variation with ambient temperature and power losses was established analytically. The simplest method to estimate the global temperature rise is based on the following expression: th R . One can note that radiative effect with thermal emissivity is not taken into account, but if so, it can contribute to reduce relatively the temperature rise.

In order to be more precise and to locate potential hotspots inside the component, some computational methods [START_REF] Buccella | A computational method of temperature distribution in high frequency planar transformers[END_REF] as well as 3D FEA can be performed. In [START_REF] Lewaiter | A thermal model for planar transformers[END_REF][65], temperature inside planar transformer is calculated based on the segmentation of transformer in thermal resistances. The ( )

f r p ( ) f R m ( ) f L m ps η ( ) f L lk ( ) f r s 3 C 1 C 2 C
authors in [START_REF] Buccella | A Coupled Electrothermal Model for Planar Transformer Temperature Distribution Computation[END_REF] proposed an analytical coupled electro-thermal model (1-D) to estimate the transformer's temperature with a good accuracy, but only in transient.

C. Leakage inductance

Leakage inductance has a major role in power electronics. Most of applications require low values of leakage inductances because they can induce supplementary losses, stress on active components as well as electromagnetic interference (EMI) due to high frequency voltage oscillations [START_REF] Ouyang | Optimal Design and Tradeoff Analysis of Planar Transformer in High-Power DC-DC Converters[END_REF] [START_REF] Pahlevaninezhad | An Improved Layout Strategy for Common-Mode EMI Suppression Applicable to High-Frequency Planar Transformers in High-Power DC/DC Converters Used for Electric Vehicles[END_REF]. For some applications [START_REF] Ferrell | The role of parasitic inductance in high-power planar transformer design and converter integration[END_REF], an important leakage inductance value is useful as in TAB converter in ZVS conditions.

Analytically, the leakage inductance is often calculated using leakage energy [START_REF] Heinrich | Regenerative Braking Capability Analysis of an Electric Taxiing System for a Single Aisle Midsize Aircraft[END_REF], based on the 1-D Dowell's assumptions [START_REF] Dowell | Effects of eddy currents in transformer windings[END_REF] and with windings excited as in ( 9) so that the flux generated is only linked to the leakage energy.

Where and are respectively primary and secondary rms currents, and η is the transformer ratio.

Other calculations are performed assuming that the current is uniform across the conductor's section [START_REF] Margueron | Complete Analytical Calculation of Static Leakage Parameters: A Step Toward HF Transformer Optimization[END_REF]. This hypothesis leads to low frequency inductance values. For higher frequencies, leakage inductance can decrease up to 50% [START_REF] Bahmani | Accurate Evaluation of Leakage Inductance in High-Frequency Transformers Using an Improved Frequency-Dependent Expression[END_REF][70] depending on the frequency range chosen. To take into account 3-D effects, FEA can also be performed and are generally more accurate to compute leakage energy.

Interleaving windings (Fig. 9) is the best solution for reducing high frequency copper losses but it also leads to small leakage inductance values. Some recent studies [71][72] have involved in finding the way to increase the magnetic energy in a core window adding a magnetic layer such as C350 [START_REF] Zhang | Design of the planar transformer in llc resonant converters for micro-grid applications[END_REF] ferrite polymer composite (FPC) for example. Models were developed and results seems pomising since they could improve significantly the leakage inductance but they must also maintain the efficiency. However, this last solution is only valid for planar transformers made of copper foil or several 2-layer PCB assembled together.

D. Parasitic capacitances

Parasitic capacitance is also a topic of huge interest in high frequency power electronic converters. Transformers' capacitances exist between different winding's layers. Different electrostatic representations exist but each one is linked to the electrostatic energy stored in the component [START_REF] Schellmanns | Representing electrical behaviour of transformers by lumped element circuits: a global physical approach[END_REF] depending on the number of independent voltage across the component: 3 for a 2-winding transformer (Fig. 7) and 6 for a 3-winding transformer. These capacitances cause resonance, vibration and additional losses in the transformer. That effect could be desired in some resonant converters such as LLC converters [START_REF] Zhang | Design of the planar transformer in llc resonant converters for micro-grid applications[END_REF] but it could also create additional losses while coupling to a consequent leakage inductance [START_REF] Li | Modeling, simulation and optimization design of PCB planar transformer[END_REF]. Finally, it could be dangerous for the reliability of the converter. Parasitic capacitances are also paths for common mode currents inducing conducted EMI noises. This phenomenon is more important in circuit where the voltage dynamics are important as in the application presented in Fig. 3 where HVDC SiC MOSFETs can switch up to 650V (Table I) within tens of nanoseconds.

Some capacitances between two layers can be determined with the expression of a plan capacitor assuming there is no voltage drop in a same layer of conductors. With a complete component, including multiple layers and magnetic core, the analytical calculations of parasitic capacitances becomes quickly complex. Even if some models have been developed based on Electric Field Decomposition Method [START_REF] Tan | An analytical method for modeling parasitic capacitances of planar components for EMI filters[END_REF] or Green's functions [START_REF] Tan | An Improved Parasitic Capacitance Cancellation Method for Planar Differential Mode Inductor in EMI Filters[END_REF], such computation is cumbersome and FEA will be preferred to extract capacitance matrix. Moreover, in transformers with distributed turns between windings, conductors of the same layer should be spaced as far as possible to enhance the fringing effect of the electric field, which reduces the stray capacitance [START_REF] Chan | Stray capacitance calculation of coreless planar transformers including fringing effects[END_REF] by reducing the electrostatic energy between two parallel plate conductors.

V. APPLICATION DESIGN: FROM AREA PRODUCT TO THERMAL BEHAVIOR

In order to illustrate previous topics, an example of planar transformer design is developed. In the following, a design for the TAB 3-winding transformer (Fig. 3) is thus detailed. This transformer has a maximum power of kW P 5 . 4 max = and a switching frequency of kHz F sw [START_REF] Ouyang | Calculation of Leakage Inductance for High-Frequency Transformers[END_REF] =

. Using expression of area product [START_REF] Gu | A study of volume and weight vs. frequency for high-frequency transformers[END_REF], the size of the core can be determined with the expression [START_REF] Naayagi | A review of more electric aircraft technology[END_REF]. Table II, this area product matches with the biggest sizes of planar cores. To achieve the best power density, the core chosen is the EE64. With this core size, conductors' dimensions can be set up taking into account a minimal distance between conductors and core to ensure dielectric insulation. In order to have a good transformer's efficiency, full interleaved windings (Fig. 9) are fixed to reduce the resistive factor r F [START_REF] Taha | Power electronics for aircraft application[END_REF]:

w sw peak u f c w p J F B K K P A A A max = × = (10
( ) DC r AC R f F f R ⋅ = ) ( (11)
One of the possible transformer's configurations (Fig. 9) is made of 18-layer PCB with a copper thickness of 0.21mm and a width of 4mm for primary winding to achieve w J fixed in [START_REF] Naayagi | A review of more electric aircraft technology[END_REF]. Primary and secondary windings have the same thickness because of PCB manufacturing constraints. Each winding is made of six layers; the secondary windings are large enough to limit their DC current density under 20A/mm², which is the limit we assumed for a better thermal dissipation. With the conductors' arrangement chosen, the utilization factor is 137 . 0 = u K , which is closed to the assumed value. All layers are separated by FR-4 epoxy with a thickness of 0.25mm. Primary conductors on the same layer have an insulation of 0.5mm. 12) is a simplification of the resistive factor, in literature [START_REF] Dowell | Effects of eddy currents in transformer windings[END_REF], for plate conductors. Skin effect (first term in ( 12)) and proximity effect (second term in ( 12)) can be expressed as following:

        + + - - - + - + + - = - - - - ) 2 cos( 2 ) sin( 2 ) 1 ( 3 2 ) 2 cos( 2 ) 2 sin( 2 2 2 2 2 2 2 2 X e e X e e m X e e X e e X F X X X X X X X X r (12)
With X defined in [START_REF] Asfaux | Development of a 12kW isolated and bidirectional DC-DC Converter dedicated to the More Electrical Aircraft: The Buck Boost Converter Unit (BBCU)[END_REF] and m the number of layers in a winding portion.

δ h X = (13)
With h the thickness of the conductor and δ the skin depth expressed in (2).

A similar expression is used to model the decrease of the leakage inductance from low frequency to high frequency [START_REF] Dowell | Effects of eddy currents in transformer windings[END_REF].

The DC resistances can easily be expressed knowing the windings' dimensions [START_REF] Homeyer | Advanced power converters for More Electric Aircraft applications[END_REF]. Finally, the windings' AC resistances can be calculated based on [START_REF] Taha | Power electronics for aircraft application[END_REF], ( 12) and ( 14). Fig. 10 presents AC resistances for the primary and one secondary winding.

N MTL S R DC * * ρ = ( 14
)
Where N is the turn's number, MTL is the mean turn length and S the current cross-section. to DC R ) for primary winding in series is between 2 and 3 for a fully interleaved transformer with frequency between [50kHz; 150kHz] but could be higher than 10 for non-interleaved windings. It can be noted that for secondary winding, made of one large conductor per layer, resistive factor is generally higher than 4, especially when secondary layers are put in parallel. Then, depending on the cases, the AC copper losses could be well distributed between layers.

To estimate the temperature rise of the transformer, total losses are considered for a sinusoidal excitation. Therefore, in the following example, losses are under-estimated because TAB's waveforms contain harmonics. In the design example, losses due to the sinusoidal voltage are equal to 31W and 9W for the copper losses and the ferrite core losses, respectively. Fig. 11 presents the resulting temperature's distribution in the planar transformer designed as well as the one obtained for and equivalent conventional transformer. In both cases, the area product is approximately the same (Table IV). For this simulation, a natural convection by air was considered with a coefficient h=14W/m².K [START_REF] Hurley | TRANSFORMERS AND INDUCTORS FOR POWER ELECTRONICS: THEORY[END_REF], in steady-state and with an ambient temperature of 25°C. As boundary condition, the bottoms of transformers are considered to be put on a cooling heatsink at 70°C. Same total losses (i.e. 40W) were considered in both cases. To perform 3D-FEA, an equivalent thermal conductivity was calculated according to the transverse direction [START_REF] Bourdon | Review of power electronics opportunities to integrate in the more electrical aircraft[END_REF] and according to the normal direction [START_REF] Stephens | Lightweight aircraft transformers[END_REF]. The calculated coefficients are reported in Table V. This table shows that most of thermal exchange made by conduction are in the X-Y plane. As it can be seen in Fig. 11, the maximum temperature in the planar transformer is lower than the conventional one. It is principally due to the surface/volume ratio which is higher for the planar technology (Table IV) and hence, its equivalent thermal resistance is lower. This is an important result because it allows weight savings on heatsink and so a better power integration. Regarding leakage inductances, their analytical values are plotted in Fig. 12 for both secondary windings. Both leakage inductances are close. This can easily be explained by the symmetrical configuration of the arrangement (Fig. 9). Their equivalent inductance in primary winding is about 0.5µH in low frequency, which is very low, and lower again in high frequency (especially 70kHz which is the switching frequency). This value is not enough according to the desired value 30µH (Table I). Then, this value should be enhanced without largely decreasing transformer's performances. As mentionned in section II.C, an external self (for example from [START_REF]Coilcraft -RF chip inductors, power inductors, and other magnetics[END_REF]) can be added in series with the windings. Another solution is to add bus bar on secondary windings' connections considering about 10nH/cm loop inductance. This short example has helped to highlight the link between some of planar transformer's parameters: Design, conductors' layout, losses and leakage inductances. Of course, this design is not complete and could be extended and deepened in order to optimize a planar transformer prototype.

VI. CONCLUSION

In this paper, challenges and issues on planar magnetic components for MEA have been presented, based on the example of a specific DC/DC converter. Potential benefits of PMC in aircrafts have been highlighted, as well as drawbacks that have to be limited. PMC's key parameters to be optimized have been presented to improve the power converter performances.

Some models were reviewed and discussed with the objective of helping designers to tune or, at least, to control parameters of planar transformers. An example of a high frequency planar transformer was designed and compared to a high frequency conventional transformer, highlighting the planar technology's interest.

Finally, because they can reach higher power densities with better thermal behavior, these planar magnetic components are going to be increasingly used for the MEA in aeronautics as well as for the EV in automotive.

Fig. 1 .

 1 Fig. 1. MEA, throughout the ages II. A MEA APPLICATION: A DC-DC POWER CONVERTER

Fig. 2 .

 2 Fig. 2. Example of an Aircraft Electric Network with industrial power converters

Fig. 3 .

 3 Fig. 3. TAB simplified electric diagram

  Fig. 4. 3D view of a prototype (a) HVDC side (b) LVDC side

  Fig. 5. Available planar core shapes: (a) EE (b) ER (c) EQ and (d) PQ

Fig. 6 .

 6 Planar technology: (a) PCB (b) copper foil

Fig. 7 .

 7 Fig. 7. High frequency equivalent circuit of a planar transformer

Fig. 8 .

 8 Fig. 8. Example of Eddy current effects on planar rectangular conductors

R

  the thermal resistance depending on the transformer's geometry and the thermal coefficient as following for example: h the thermal coefficient by convection, λ K the thermal conductivity, l the characteristic length, ext S and c S the orthogonal surface of the losses flux. The equivalent thermal resistance th R is calculated considering parallel and series associations of different ′ th R and ″

  inductance which depends on core and conductors dimensions and arrangement and I the current through the considered winding.

) With w A

 w the winding window area, c A the core window area, 4 = f K for a non-sinusoidal excitation, u K is the utilization factor of the window by conductors, m B and w J are the maximum magnetic induction and the primary current density, respectively. The factor u K has a maximum value of 0.2 for planar conductors [79]. In our design, u K is set to 0.14. The peak induction is assumed to mT B peak 130 = in order to avoid the core saturation and the DC current density is fixed

Fig. 9 .

 9 Fig. 9. 2D representation of the planar transformer To evaluate high frequency copper losses with the arrangement in Fig. 9, 1-D analytical modeling is used on a wide frequency range [10kHz; 1MHz]. Equation (12) is a simplification of the resistive factor, in literature[START_REF] Dowell | Effects of eddy currents in transformer windings[END_REF], for plate conductors. Skin effect (first term in (12)) and proximity effect (second term in (12)) can be expressed as following:

Fig. 10 .

 10 Fig. 10. Analytical winding AC resistances

  and cu h the number of copper layers and their thickness, ins N and ins h are the number of insulating layers and their thickness, respectively.

Fig. 11 .

 11 Fig. 11. Temperature distribution in planar vs conventional transformer

Fig. 12 .

 12 Fig. 12. Analytical leakage inductances from secondary windings

TABLE I :

 I

	Converter's electric specifications
	VHVDC	[450V; 650V]
	VLVDC	[26V; 32V]
	Vbat	[16V; 32V]
	Transformer ratio	17:1:1
	Leakage inductance (from primary side)	30µH
	Fsw	[70kHz; 150kHz]

TABLE II :

 II Core size and weight of standard Ferroxcube planar cores -Data

				from [32]
				Dimensions (mm 3 )	Core size Volume Area product (mm 3 ) (mm 4 )	Weight (g)
		Min	EE14	14x5x7	490	200	1.2
	EE	Max	EE64	64x50.8x20 .4	66324	115000	200
	ER	Min Max	ER9. 5 ER64	9.5x5x4.9 64x50.8x25 .4	232 82580	23.7 99616	0.7 304
	EQ	Min Max	EQ13 EQ38	12.8x8.7x5. 7 38.1x25.4x 16	634 15483	69.6 4334	1.6 43
	PQ	Min Max	PQ20 /16 PQ50 /50	21.3x14x16 .2 51x32x50	4830 81600	1450 98400	13 195

TABLE III :

 III Planar vs conventional core size of same area product -Smallest planar core available in[START_REF]Ferroxcube[END_REF]

		Planar core	Conventional core
		E14	EFD15
	Area product (mm 4 )	200	222
	Effective area (mm²)	14.3	15
	Set weight (g)	1.2	2.8
	Material volume (mm 3 )	300	510
	External volume Vext (mm 3 )	490	1046
	External surface for cooling Sext (mm²)	70	69.75
	Ratio (Sext/Vext) (mm -1 )	0.14	0.066

TABLE IV

 IV

		Planar core	Conventional core
		E64	E55
	Area product (mm 4 )	115000	117000
	Effective area (mm²)	519	420
	Set weight (g)	200	260
	Material volume (mm 3 )	40700	52000
	External volume Vext (mm 3 )	66324	77275
	External surface for cooling Sext (mm²)	3200	1375
	Ratio (Sext/Vext) (mm -1 )	0.048	0.017

: Planar vs conventional core size of same area product -Biggest planar core available in

[START_REF]Ferroxcube[END_REF]

TABLE V :

 V Equivalent thermal conductivity Thermal conductivity (W.m -1 .K -1 )

	Copper	380
	FR-4	0.25
	Core	5
	Transverse XY K	174
	Normal Z K	0.46

ACKNOWLEDGMENT

This work is part of the French project GENOME involved in MEA with the support of Thales Avionics Electrical Systems S.A.S.


Behrooz Rasuli
email: rasouli@students.irandoc.ac.ir

Joachim Schöpfel
email: joachim.schopfel@univ-lille.fr

Hélène Prost
email: helene.prost007@gmail.com

Electronic Theses and Dissertations Programs after Two Decades Exploring Impact

Keywords: Electronic Theses and Dissertations (ETDs), ETD programs, Academic Impact, Citation Analysis, Open Access

Objective: Electronic Theses and Dissertations (ETDs) programs have been recognized as a relatively new and effective channel through which information resources can be made available to academics and beyond. This study aims to provide empirical evidence of the impact of ETD programs in the scientific communities. Method: Records containing citations to theses and dissertations (TDs) in Scopus database from 1970 to 2017 are retrieved through a structured query. Trends of the citations of TDs in academic literature in different subject areas are examined. In additions, online search of a random sample of citation records on Google Scholar and BASE provided more insight of the impact of ETD programs, with a focus on open access. Findings: According to the Scopus database, there is an obvious increasing trend in citing TDs in academic literature. In all four broad subject areas, there is an increasing trend in citing TDs, in particular from 1996 onward. The analysis of the random sample and the survey provide complementary elements for a better understanding of this trend, of the role of ETD programs and of the impact of accessibility via institutional repositories.

INTRODUCTION

Electronic Theses and Dissertations (ETDs) programs are new channel to distribute students' research. ETDs programs were launched in the early 1990s and there is an increasing interest in implementing such programs around the world. ETD programs' advantages including cost and space savings, increasing access to research, increasing visibility, increasing stability and authority, making connections, increasing authorial control, and monitoring usage statistics.

Another key promises of ETD programs is increasing impact of students' works [START_REF] Bangani | The impact of electronic theses and dissertations: a study of the institutional repository of a university in South Africa[END_REF][START_REF] Ratanya | Institutional repository: Access and use by academic staff at Egerton University, Kenya[END_REF]Buehler 2013: 65). Through increasing the visibility of research, ETDs programs can help researchers, practitioners, business owners, and other stakeholders to search and retrieve new findings in different fields. Therefore, it can be expected that ETDs programs increase the impact of TDs. However, research impact includes academic, health, economic, societal, cultural, legal, environmental, etc. (Bornmann 2012). Among them, academic impact of research is much easier to measure, because counting citations is one of the methods to measure such impact. Then, the citations can indicate academic impact of pervious research. A new method entitled 'citation analysis' has been developed to study of citations in academic works and citation indexes, such as Web of Science and Scopus, have implemented such methods in their designs. However, citation analysis is complicated sometimes since research shows that citations in academic publications are not always correct. For example, O'Connor (2002) andLukic et. al. (2004) believe that citation errors are common in medical journals. Lukic et. al. (2004) study revealed that 27% of the 199 randomly selected references from articles of three major gross anatomy journals include errors. [START_REF] Al-Benna | Accuracy of references in burns journals[END_REF] found that 3.3% of cited references in two major burns surgery journals are incorrect. According to [START_REF] Adhikari | Accuracy of references in different speciality medical journals of Nepal[END_REF], more than 40% references of medical journals' articles published in Nepal are incorrect. But the errors in cited references are not limited to medical journals. In a study on library and information science's research articles published in academic journals, [START_REF] Davies | Reference accuracy in library and information science journals[END_REF] believes that error rates in the journals are considerable. Her findings showed the overall error rates were: author (56 percent), page number (22 percent), article title (15 percent), volume (3 percent), publication year (2 percent) and journal title (2 percent). After the study of citations in five leading environmental science journals, [START_REF] Lopresti | Citation accuracy in environmental science journals[END_REF] found that more than 24% of citations contain errors. The accuracy of cited references in unpublished materials, including theses and dissertations, is lower than journals. A study on five psychology theses submitted to the University of Mysore showed that seventy seven percent of the references had citation errors (Harinarayana, Chikkamanju, and Raju 2011). [START_REF] Rahmani | A Survey on citation accuracy of thesis in Faculty of Psychology and Educational sciences in Shahid Beheshti University[END_REF] reported that about 78% of cited references in Shahid Beheshti University's theses and dissertations contain errors. Since one of the most important goals of ETD programs is to increase visibility, usability, and impact of TDs [START_REF] Ferreras-Fernández | Providing open access to PhD theses: visibility and citation benefits[END_REF][START_REF] Ezema | Electronic Theses and Dissertations and the Promotion of Scholarship in African Universities: Prospects and Challenges[END_REF][START_REF] Ahmed | Theses and dissertations in institutional repositories: an Asian perspective[END_REF], then measuring success of these programs is important. This study aims to measure the impact of ETD programs through analyzing citations to theses and dissertations (TDs). Increasing citations to TDs distributed through ETD programs can indicate the impact of such programs and make programs' manager sure that their resources are not wasted.

METHODOLOGY

Our approach combines two different related methods. (1) The first step was a citation analysis with the Scopus database. The Scopus scientometric database (one of Elsevier's product) has been used to identify TDs cited in academic literature (articles, reviews, book chapters, conference papers, etc.). Scopus citation database supports cited reference searches for different document types and indexes more literature than similar databases. To extract citations to TDs, the most popular citation styles in academic works has been surveyed. 1 (2) Based on a representative random sample of TD records, in the second step try to estimate the contribution of ETD programs, via combined searches with the Bielefeld Academic Search Engine (BASE) and Google Scholar (GS) in order to determine the part of ETD in institutional or aggregating repositories, portals or other platforms (eg NDLTD Global Search, DART Europe, EThOS, Thèses.fr, Shodhganga, and Theses Canada). Particular attention will be paid to accessibility (open access, embargo etc.) and retrodigitization programs. Studying the trend of citations and 1 search query: (((REFSRCTITLE("(Dissertation)" OR "[dissertation]" OR "[master's thesis]" OR "[master's thesis]" OR "[thesis]" OR "dissertation" OR "Doctoral dissertation" OR "doctoral thesis" OR "honors thesis" OR "M Eng. Thesis" OR "M.S. thesis" OR "M.Sc. Thesis" OR "MA thesis" OR "Master" OR "Master's thesis" OR "Master's thesis." OR "Masters level dissertation" OR "master's theses" OR "masters thesis" OR "Master's thesis" OR "MS thesis" OR "MSc Thesis" OR "MSc. Thesis" OR "Ph.D. diss." OR "Ph.D. dissertation" OR "Ph.D. thesis" OR "PhD [dissertation]" OR "PhD diss." OR "PhD dissertation" OR "PhD thesis" OR "PhD. Dissertation" OR "Senior thesis" OR "thesis" OR "M.A. Thesis" OR "M. A. Thesis" OR "dissertation on the Internet" OR "dissertation on microfiche)") AND PUBYEAR < 2018))); search date: July 2018 random samples that whether they contain citations to TDs which are available in an ETD database or not, the real impact of ETD programs will be clarified.

FINDINGS

A preliminary search on the Scopus database retrieved 869969 documents from 1923 to 2017 (all documents = 66839703) which already provide a general trend of the impact of TDs on academic publications. Worded differently, about 1.5% of all indexed documents on Scopus database contain (at least) one citation to TDs. Figure 1 presents the percent of documents which have cited TDs from 1970 to 2017. According to figure 1, there is an obvious increasing trend in the percent of documents containing (at least) a citation to TDs (black line). As well, this increasing trend occurred in different broad subjects areas (red, green, blue, and grey lines).2 As it is showed in the figure, Art and Humanities academic literature are more willing to cite TDs in their works. In 2015, about 4% of documents in this subject area included (at least) one citations to TDs, while in the same year only 0.6% of medical sciences' documents contained citations to TDs. According to the findings, more than 10% of indexed books in the Scopus contained (at least) on citations to TDs, followed by book chapters (2.59%), retracted (1.51%), articles (1.41%), reviews (1.08%), and conference papers (0.61%). Although, among countries with more than 100000 publications in Scopus, Nigeria is more willing to cite TDs in academic literature; more than 3.1% of all documents published by Nigerian researchers contain (at least) on citations to TDs, followed by Russian Federation (3.07), South Africa (2.94), Netherlands (2.46), New The second analysis is based on a random 384 sample 3 of all publications citing TDs from 16696 to 2017. For this smaller corpus, we assess the format and the accessibility of the cited TDs. In other words, we tried to determine for each cited TD if it has digital format, and if it is a digital format (ETD), if the TD is freely available or not, for instance via an institutional repository. However, 384 sample documents contain 15340 references, which 705 references of them include TDs. After deep analysis, we found 65 documents with no citation to TDs (16.93% of 384 documents). These documents contain some terms of search query (for example, "synthesis","synthesis","prostheses","hypothesis","master plan","Master Abstr Int","master brewers","masters of the big house","master plan","master lecture","master course"). However, after removing these records from the sample, 319 correct documents were analyzed, which contain 476 references. More analysis showed that 76 references (~16% of 476 references) do not contain a title; many of them have just author name, year, and the level of TDs (for example, "Schulze, J., ( 2009) PhD Thesis"). As well, these references withdrew from the sample records. Analysis of 400 references shows that the most of them cited a doctoral dissertation (~70%).

Figure 2 shows the frequency of citations to TDs of different educational level. national repositories (such as EThOS, Trove, HAL, Theses.fr, CiNii, INFOUNIVERSIDADES, BDTD, etc.), researcher's CVs pages, and regional repositories (such as, DART, DiVa portal, OATAD, Ohiolink, etc.). However, to investigating the impact of ETD programs, the trend of accessible TDs in past years can provides a better insights. In figure 4, the percent of accessible TDs in different years from 1996 to 2017 is presented.

CONCLUSIONS

This study is a first attempt to assess ETD programs' impact after more than two decades from establishment of these programs. The originality of our study is the application of scientometric measures to assess the academic impact of ETD programs. Trends of citations to TDs in academic literature can be categorized in three time periods: 1) From 1970 to 1992: In this period we observe increasing trends in citations to TDs. Probably, the main reason of increasing trends is the increasing universities and academic institutions with more and more students, and accordingly increasing theses and dissertations. 2) From 1993 to 2002: In this period we observe decreasing trends in citations to TDs (ignoring the sudden decrease of citations in 1996). Probably, the main reason is empowering access to journals through electronic, as well as online channels. But TDs in this period were on the libraries' shelves. Researchers and authors of research articles had to go to the libraries to read and cite TDs. Therefore, because of restricting access to TDs the number of readers and citing authors have been decreased. 3) From 2003 onward: In this period we observe increasing trends in citations to TDs again. It can be argued that the main reason of the increasing trends is ETD programs, which were developed in the beginnings of 90s and increased gradually from 2000s onwards.

To sum up, it can be argued that ETDs programs at all levels (i.e. institutional, national, regional, and global) are reviving TDs' impact (at least in academic literature). The trend of the percent of open access references from 1996 to 2017 showed that there is a significant increase in the number of open access ETDs through ETD programs. However, surveying authors citing TDs in their works is needed for more clarification of their access to TDs.

The results of our study will provide a better understanding of the real impact of ETDs in terms of citations. This may be helpful for institutions and operators of ETD programs and repositories, in order to assess the academic "return on investment" of their programs and projects and to improve, if necessary, their way to produce, disseminate and exploit ETDs in order to strengthen their contribution to open science.

Figure 1 .

 1 Figure 1. Percent of documents cited theses and dissertation in Scopus (1970-2017)

Figure

  Figure 2. Citation to TDs of different educational level; Scopus (1996-2017)

Figure 4 .

 4 Figure 4. Percent of accessible and open access reference (TDs) on the web; Scopus (1996-2017)

The sudden decrease of citations between 1995 and 1996 is probably related to Elsevier's database production and not to authors' behaviour. We asked the possible reasons of this atypical curve from Scopus team, we have not received a certain answer yet.


Promoting Electronic Theses and Dissertations

The Networked Digital Library of Theses and Dissertations

Missions

NDLTD is an international organization dedicated to promoting the adoption, creation, use, dissemination, and preservation of electronic theses and dissertations (ETDs). NDLTD supports electronic publishing and open access to scholarship in order to enhance the sharing of knowledge worldwide. Topics include how to find, create, and preserve ETDs; how to set up an ETD program; legal and technical questions; and the latest news and research in the ETD community.

Membership

NDLTD is present in 24 countries, with

• 119 institutional members

Social media

On Twitter @NDLTD Account On LinkedIn NDLTD Group On Facebook NDLTD Page On Google+ NDLTD Group and List

Contact: http://www.ndltd.org

Poster by: Joachim Schöpfel Layout by: Aurélie Halipré

Goals

• To be the leading international organization for promotion of ETDs worldwide.

• To provide useful and innovative resources, standards, and technology for development of ETD programs.

• To encourage institutions of higher education to use NDLTD resources and participate in NDLTD activities.

Global ETD Search at http://search.ndltd.org/

Activities

• Support of institutions of higher education to develop their own ETD programs by adopting the submission, collection, and archiving of electronic theses and dissertations to their own repositories and to international digital libraries and repositories.

• Support of the Open Access movement.

• Awareness raising at institutions of higher education of the benefits of ETDs, including:

• the facilitation of the writing process for students,

• the increased speed of sharing research methods and results,

• the improved distribution of research methods and results through electronic publication and archiving, leading to improved graduate education and scholarship, and

• the reduced costs of printing, processing, and storage.

series and awards 19 international conferences in Memphis, Blacksburg, St Petersburg (FL), Pasadena, Provo, Berlin, Lexington, Sydney, Quebec, Uppsala, Aberdeen, Pittsburgh, Austin, Cape Town, Lima, Hong Kong, Leicester, New Delhi and Lille.

	48 Innovative ETD Awards for PhD students from Yale, Caltech, Melbourne, Pretoria,
	West Virginia, Antioch, Robert Gordon, Waterloo

• 22 consortium members • 31 individual members Conference , British Columbia, PUC Rio de Janeiro, Kansas State, Texas, Rochester RIT, Ohio, Saskatchewan, Witwatersrand and Manitoba. 23 ETD Leadership Awards for Monique Joly, Peter Murray-Rust, Shu-Hsien Tseng, Linda Goodfellow, David T. Palmer, Felix N. Ubogu, Laura Hammons, Monica Hammes, Nancy Stuart, Tim Brace, John H. Hagen, Thomas Dowling, Susan Copeland, Gail McMillan, Peter Schirmbacher, Angela McCutcheon, Sharon Reeves, Jose Mutt, Maggie Dougherty, Marian E. Bate, Ana Pavani, Edward A. Fox and Shalini Urs.

NDLTD archive -gateway to 4.5m ETDs

							Swedish	Spa nish 2%	Others , n/a 2%	Languages	4000000
								4%	3500000
							Germa n	3000000
							9%	Engl ish	2500000
							French	40%	2000000
							9%	1500000
							Portuguese	1000000
							10%	500000
								0
								Chi nese
								24%
							Disciplines	Sources
								Engi neering
							Others , n/a 22%	14%	Ohi oLink 1%	Others 23%	OCLC 27%
							Hea lth	Educa tion 11%	South Afri ca ETD Porta l 2%
							6%	Ca na da ETD
							Ps ychol ogy 6%	Sci ences 10%	Repository 4%
							Studi es 6%	Ma na gement 7%	Computer 8%	10% Soci al	7% IBICT Bra zil DIVA Sweden 5%	10% Ethos UK	21% NDLTD Ta iwan
		SURA SURA SURA SURA
	Ann Arbor Ann Arbor Ann Arbor Ann Arbor	Work Work Work Work		501(c)3 501(c)3 501(c)3 501(c)3	Cape Cape Cape Cape
	Meeting Meeting Meeting Meeting	shop NDLTD shop NDLTD shop NDLTD shop NDLTD	organization organization organization organization	Town Town Town Town	Lille, France Lille, France Lille, France Lille, France
	1991 1991 1991 1991	1996 1998 1996 1998 1996 1998 1996 1998		2005 2005 2005 2005	2013 2013 2013 2013	2017
	1987 1987 1987 1987	1994 1994 1994 1994		2003 2003 2003 2003		2011 2011 2011 2011	2016 2016 2016 2016
			1st 1st 1st 1st
	Virginia	ETDdb ETDdb ETDdb ETDdb	Symposium Symposium Symposium Symposium	Sydney Sydney Sydney Sydney	Global	20th ETD Conference
	Tech	software software software software				ETD	at Washington
						Search


Joachim Schöpfel

Thomas Etd4oa Severiens

ooS

  Scientometric study on publishing models o Evaluation of representativeness, comprehensiveness, best practices, and openness of ETDs in European repositories Survey on usage and uptake of ETDs in open o workshops, conferences, o Survey on usage and uptake of ETDs in open repositories Social networking and visibility, results publications Communication to general public o Communication to general public o Partnership with developing countries o Partnership with developing countries WP5 Project Management o Organize projects meetings C di t d i j t ti o Coordinate and review project reporting o EC liaison Contact o EC liaison o Budget follow-upContactJoachim Schöpfel (F) joachim schopfel@univ-lille3 fr o Budget follow up Joachim Schöpfel (F) joachim.schopfel@univ lille3.fr Thomas Severiens (D) severiens@isn-oldenburg.de g

WP1 Dissemination and Communication

  Support EU infrastructures and OA policies for ETDs o Suppo t U ast uctu es a d O po c es o s Address barriers and access restrictions o Address barriers and access-restrictions o Take actions to promote input, openness and o Take actions to promote input, openness and impact of ETDs in existing repositories and portals impact of ETDs in existing repositories and portals

	El t Electronic Theses and i Th d Electronic Theses and
	Di Dissertations for t ti f Dissertations for
	Open Access Open Access Open Access
	Objectives Objectives Objectives
	C o Creation of European ETD network ti f E ETD t k

o o Community management and promotion o Community management and promotion o Guidelines, recommentations, and best practices o Guidelines, recommentations, and best practices o Evaluation of progress

WP2 Policy Coordination WP2 Policy Coordination

	P j t O t Project Outputs t Project Outputs

o Mapping of open access to European ETDs o Mapping of open access to European ETDs o Benchmarking and case studies o Benchmarking and case studies o Comparison of legal provisions and rules o Ethics and deontology in decision making


Joachim Schöpfel

Hélène Prost

Marjorie Piotrowski

Eberhard R Hilf

Thomas Severiens

Paul Grabbe

Restricted vs. Open Access for Electronic Theses and Dissertations -A Challenge for Public Science

Introduction

Academic electronic theses and dissertations (ETDs) should be openly accessible to serve best the scientific progress, and inform the scientific community as a whole on the results. This is technically supported at universities by their institutional repositories, by global open archives, and by some publishers. Over the years the percentage of ETDs which are open access (OA) is rising worldwide and in all scientific disciplines. Open, stated Peter Murray-Rust in his keynote at the ETD2014 conference at Leicester, UK, should be the default, not the exception. However, the reality is different. Even in institutional repositories (IRs) created to provide access to the scientific output of an academic institution and as a central sector of the so-called green road to open access, more or less important parts of the scientific production are missing. This is because of lack of awareness, embargo, deposit of metadata without full text, confidential content etc. ETDs, in particular, are disseminated with different status types -some are freely available, others are under embargo, confidential, restricted to campus access (encrypted or not) or not available at all. Contrary to the OA philosophy, many IRs are not fully OA (Prost & Schöpfel 2014). But while other papers may be available through alternative channels (journals, monographs etc.), ETDs most often are not. Our paper summarizes empirical evidence from different studies and describes some main results from a small French-German survey. It proposes a conceptual approach designed not only for a better understanding of access restrictions but also for action and policy-making to increase the part of ETDs to be open access. At the end, the paper will present the outline of a European infrastructure project called "Electronic Theses and Dissertations for Open Access" (ETD4OA) to be submitted in the framework of the Horizon 2020 program.

Empirical evidence

A first preliminary study based on a review of recent papers and empirical evidence of a small set of European and American repositories, networks and institutions confirmed that ETDs are disseminated with different status types -some are freely available, others are under embargo, confidential, restricted to on-campus access (encrypted or not) or not available at all (Schöpfel & Prost 2013). The number of hidden theses may amount for some repositories to 40% or more. Figure 1 provides an overview1 .

Figure 1: Open and restricted access of ETDs in different OA servers Figure 1 shows a situation that academic librarians in charge of ETDs know well more or less, the large spread of access-type per repository. Yet, surprisingly only very few studies contain empirical data on access restrictions and allow for comparison and analysis. Also, in some cases, figures are only available for limited periods and relatively small numbers of items. Three points should be emphasized:

1. In the surveyed repositories, the part of openly available ETDs varies largely, from less than 40% to more than 95% (ProQuest OA database). 2. We identified three main types of restricted access: availability limited to authorized users (often, on-campus access or intranet), embargoed ETDs (delayed access) and confidential content or topic. Sometimes, the type of restricted access is not specified. 3. The nature of these types of access restrictions show considerable differences. In some repositories, all non-OA ETDs are under embargo while elsewhere their access is limited to the campus. Also, embargo and oncampus access can take different forms (embargoes for six months to two years or more; access in library, on the campus, via intranet etc.). The part of confidential ETDs appears of be rather low.

Survey data

There is an astounding decrease since 2009 in Germany in the OA theses as archived by the National German Library collected from the official OA institutional repositories of the universities. In spite of the growing success of the OA initiative, a significant part of ETD remains unavailable on the web or circulates with restrictions.

Which are the reasons and determining factors? We started in 2013 a French-German project called EDAR Electronic Theses: Access and Restrictions 2 to collect data on the present status of the workflow of ETD and on the local rules for handling restricted access, via a survey of academic libraries and graduate schools. The survey has been published in detail (Schöpfel et al. 2014). Here are some major results.

France

• The part of OA theses compared to all theses increased from 12% of the PhD theses in 2009 to 24% of the PhD theses in 2012.

•

There is an increase of embargoed PhD theses, from 1% of the ETD in 2009 to 7% in 2012.

•

On-campus access restrictions of ETD increased from 5% in 2009 to 28% in 2012.

•

The part of confidential theses remains small and stable (2% of ETD).

Germany •

The number of OA theses increased slightly from 41% of all theses in 2009 to 47% in 2012.

• Furthermore, in Germany, very few libraries reported access restrictions. There are however some individual cases where the author asked for restrictions because of confidential material etc. These cases are in total less than 1% of the total. Yet, attention should be paid to two limitations (see discussion in [START_REF] Schöpfel | Electronic theses and dissertations: access and restrictions[END_REF]. First, because of the small sample size, the survey did produce only preliminary data, interesting for empirical evidence but not necessarily representative. Second, the results reflect also different ways of handling ETDs -in France, all ETDs are deposited in IRs whenever possible, if necessary and requested with embargo or limited availability; in Germany, only OA ETDs are handed in for deposit so that ETDs with restricted access remain virtually invisible, "out of scope". The EDAR survey confirms former empirical evidence on restricted access to ETDs, especially because of embargo periods and on-campus availability. Obviously, there is no need for new facilities and infrastructures on local, national or European levels. Today, the problem of OA ETDs lies upstream in local contexts that facilitate decisions in favor of OA as the regular case, instead of embargoes or restricted access (on-campus access, intranet). To put it in a simple way, pipes exist, but it lacks the fuel and the pressure for ETDs and OA.

Conceptual approach

In a third approach we tried to conceptualize the context of access restrictions -who are the stakeholders, which are the reasons for decision-taking, how can the result be described in terms of different degrees of openness? Here are some results; for more details, see Schöpfel & Prost (2014). Who are the stakeholders, who takes part in the decision on open or restricted access? At first glance, the situation appears to be rather simple. PhD theses being intellectual work, the student is the only person holding the right to decide about dissemination. Of course, this view is by far much too simplistic. Different actorspeople and institutions -can be distinguished who have a more or less impact on the process of decision-making, with different reasons, motivations and objectives. Figure 2 tries to map these players in a joint sphere of decision-making of dissemination and access to ETDs. Each player sets his own goals, fulfils specific functions, plays his particular role, sometimes consistent with others, sometimes in opposition. For instance, PhD students may deposit their non-reviewed papers in open archives "off-campus", outside of their institution and without any validation or authorization, even when the jury rejects the disclosure. All these people, groups and institutions act in different ways, for different reasons, with different objectives and strategies. The literature review and survey results reveal several components, such as a publishing project, individual attitude toward OA, copyright law or institutional OA policy that may be understood as independent variables of the final decision (figure 3). Each of these aspects acts in a different way. Some may point to decide on dissemination or non-dissemination, while others are limited to embargo or on/offcampus decisions. Moreover, some are case-by-case decisions while others reflect general attitudes and stable behaviors. This model may need empirical confirmation and perhaps, more details. Yet, its central characteristics are the multi-factorial or multivariate approach to the prediction of decisions on dissemination or concealment of ETDs. Even if individual publishing strategies and attitudes towards open access may play a major role, other variables such as personal advice from the PhD director, easiness of decision and references should not be neglected, in particular when discussing ways of improving accessibility and availability of PhD theses and designing the respective policies. With regards to accessibility and availability of PhD theses, our analysis showed so far that openness is not a simple, binary concept but that the documents can be more or less open, depending on different variables. Some of those variables are similar to articles published in journals or books, but others are specific to PhD theses. In October 2012, the Scholarly Publishing and Academic Resources Coalition (SPARC), "an international alliance of academic and research libraries working to create a more open system of scholarly communication", released a guide called "How Open Is It" that outlines the core components of open access (e.g., reader rights, reuse rights, copyrights, author posting rights, etc.) covering the continuum from "open access" to "restricted access". Compared with our multivariate approach, this Open Access Spectrum3 (SPARC 2012) helps to get a realistic view on the problems of openness, disclosure and concealment of theses. Figure 4 shows a possible adaption of the SPARC guide to the specific conditions of the dissemination of PhD theses. This table needs further discussion and comparison with real situations. Also, some openness-scales may be more complicated then those shown in figure 4, as for instance the copyright dimension merging intellectual property, confidentiality and privacy. But it may help to better understand and evaluate the great variety of IRs, also with regards to potential action in favor of OA.

ETD4OA

Following the EDAR project, the project team prepares a proposal for the European Horizon 2020 Framework Programme for Research and Technological Development. H2020 supports the achievement and functioning of the European Research Area in which researchers, as well as scientific knowledge and technology circulate freely, by strengthening cooperation both between the Union and the Member States, and among the Member States, in particular through the application of a coherent set of rules. Also, a set of activities aims at optimizing the use of national facilities by integrating them into networks and opening their doors to all European researchers. The project "Electronic Theses and Dissertations for Open Access" (ETD4OA) will support the coordination of European infrastructures and open access (OA) policies in the field of electronic theses and dissertations. Together with stakeholders and OA initiatives, it will address barriers and access restrictions, and it will take actions (active communication, recommendations, advice) to promote and develop input, openness and impact of ETDs in existing open repositories and portals. The objectives of the ETD4OA project are (see figure 5):

1. to produce and disseminate reliable and consistent empirical data on the processing of electronic theses and dissertations (ETDs) in a wide set of European countries with their different infrastructures and legal environments, in particular to address the problems and variations of access restrictions; Project ETD4OA Some of the research questions the project will address: Which are the (apparently) increasing demands by the authors for keeping the document not open to the public for fear of copyright infringement, e. g. of figures used? Is there increasing pressure of commercial publishers to not allow OA distribution if a book is planned? Are there more recent other ways of posting an ETD in OA which are not seen and thus not counted by the institutional repository network, such as for instance institutional servers, personal websites, gold OA journal publications? Language barriers may lead to publish a thesis as a book to make use of the publisher's editorial office. Figure 6 resumes the five work packages. The project is coordinated by the GERiiCO laboratory at the University of Lille 3 (France) and the ISN Oldenburg (Germany). Actually more than ten member states take part in the project, represented by public or private institutions, experts and networks (see figure 6).

Five work packages

ETD4OA will be submitted within the H2020 project call H2020-INFRASUPP-2014-2, "Support to innovation, human resources, policy and international cooperation", a sub call of the H2020-INFRASUPP-2014-2015 program. The deadline of this call is September 2, 2014.

The project members

2Figure 2 :

 2 Figure 2: People and institutions taking part in decision-making

Figure 3 :

 3 Figure 3: Different aspects of decision-making on OA of ETDs

Figure 4 :

 4 Figure 4: Degrees of openness for ETDs in IRs (adapted from SPARC)

  2. to contribute to the coordination of open access policies regarding ETDs in the European research era and to build a sustainable European community focusing on the open access to research theses; 3. to contribute to the understanding and development of the acceptance, uptake and usage of open access ETD infrastructures, by the research communities, by industry and business and by the Internet community; 4. to develop practical and helpful guidelines and recommendations in order to raise awareness about the challenge, and to support cooperation with developing regions and countries, in order to increase the part of ETDs which are widely and fast disseminated in institutional repositories without any restrictions as the standard solution.

Figure 6 :

 6 Figure 6: The five work packages of ETD4OA

Figure 6 :

 6 Figure 6: ETD4OA project members (draft July 2014)

  Figure 5: Objectives of the ETD4OA projectThe European infrastructure for this already exists, with the interconnection of institutional repositories and other open archives and concurrently registered which can be searched by the DART-Europe portal. The ETD4OA project has not the ambition to develop a new infrastructure or prototype. Instead, today, the problem of open access to ETD lies upstream in local contexts that facilitate decisions in favor of embargoes or restricted access (on-campus access, intranet). To put it in a simple way, pipes exist, but it lacks the fuel and the pressure.The input to the central portal varies depending on the country and the research field and the institution. To increase the OA-percentage is a problem on many levels, technical (workflow) as well as legal, administrative (regulatory) and tradition and ethics. The answer proposed by the project is awareness raising, lobbying at the national and European policy level, a thorough communication net both for candidates as well as for the responsible local staff, as well as the production of training material for them. ETD4OA aims at increasing the use of national facilities, i. e. institutional repositories and OA infrastructures, by integrating them into an emerging network of the people, those responsible and involved for European research theses. Its aims are to exploit synergies between national and Union initiatives by setting up partnerships between relevant policy makers and funding bodies or advisory groups, in order to facilitate the development of global research infrastructures and the cooperation of European infrastructures with their non-European counterparts (in particular US and Russia), ensuring their global interoperability and access.

For the sources, seeSchöpfel & Prost (2013).

See SPARC, PLOS, and OASPA (2012). How Open Is It? Open Access Spectrum at http://www.plos.org/wpcontent/uploads/2012/10/OAS_English_web.pdf