Remi Robutel

Member, IEEE Christian Martin

Member, IEEE Cyril Buttay

Member, IEEE Paolo Mattavelli

Fellow, IEEE Dushan Boroyevich

Regis Meuret

Design and Implementation of Integrated Common Mode Capacitors for SiC JFET Inverters

Keywords: Electromagnetic compatibility, Inverters, Multichip modules

This paper deals with the issue of electromagnetic interference (EMI) in SiC-JFET inverter power modules, and proposes a solution to limit conducted emissions at high frequencies. SiC-JFET inverters can achieve very fast switching, thereby reducing commutation losses, at the cost of a high level of EMI. In order to limit conducted EMI emissions, it is proposed to integrate small-value common mode (CM) capacitors, directly into the power module. High frequency noise, which is usually difficult to filter, is then contained within the module, thus keeping it far from the external network. This approach is in line with the current trend towards the integration of various functions (such as protection, sensors or drivers) around power devices in modern power modules. To demonstrate this concept, the resulting CM noise was investigated, and compared with a standard configuration. Simulations were used to design the integrated capacitors, and measurements were carried out on an experimental SiC-JFET half-bridge structure. A significant reduction was achieved in the experimentally observed CM conducted emissions, with a very minor influence on the switching waveforms, losses and overall size of the system. The benefits and limitations of this design are discussed, for the case of mid-power range inverters for aircraft applications.

I. INTRODUCTION

W IDE-bandgap semiconductors are now reaching a sat- isfactory level of maturity for power devices. Their performance and capabilities are very useful and promising for very high efficiency, high temperature or high voltage applications [START_REF] Baliga | Power semiconductor device figure of merit for highfrequency applications[END_REF]. Among wide-bandgap materials, the most technologically advanced is Silicon Carbide (SiC). Beyond their primary power-switching applications, such power devices require an appropriate converter design. Drivers, filters, packaging and various other elements need to be updated, in order to meet the specifications arising from new fields of application. The aerospace industry is now studying these technologies, with the aim of improving avionics and energy management, which would ultimately allow "more electrical" aircraft to be built. By combining the advantages of SiC power devices with improved electronic integration, the concept of the "Smart Electro-Mechanical Actuator (EMA)" has been developed, as illustrated in Fig. 1. With this type of configuration, the converter is mounted directly on the actuator. When compared with conventional aircraft applications in M. XXX is with the Department of Electrical and Computer Engineering, XXXXX Institute of Technology, Xxxxx City, XX, XXXXX COUNTRY email: xxxx.xxxx@xxx.edu.

X. Xxxxxx and X. Xxxxxx are with the Xxxxx University. Manuscript received Xxxxxx XX, 20XX; revised Xxxxxx XX, 20XX. which adjustable speed drives are used, there is no output EMI filter or harness on the AC side of the circuit. Despite its reduced filtering potential, the inverter must be capable of fast switching, whilst at the same time producing a low level of EMI. The solution proposed in the present paper was designed for this type of configuration, which is encountered mainly in embedded power systems.

A. SiC Power Converters 1) SiC Power Devices: SiC devices have recently become commercially available items. Because they can operate at junction temperatures as high as 450 • C [START_REF] Funaki | Power conversion with SiC devices at extremely high ambient temperatures[END_REF], it has now become possible to improve their power density by decreasing the size of the heatsink. Alternatively, a given power device operating range can be achieved at higher temperatures. On the other hand, lower switching losses allow higher switching frequencies to be used, thus leading to a decrease in the volume of the passive energy-storage components. Several highdensity converter designs using SiC-JFET devices have been demonstrated [START_REF] Lai | Development of a 10 kw high power density three-phase ac-dc-ac converter using SiC devices[END_REF], [START_REF] Aggeler | A compact, high voltage 25kW, 50kHz DC-DC converter based on SiC JFETs[END_REF], and the building blocks for high ambient temperature, SiC-JFET inverters have been described by [START_REF] Funaki | Power conversion with SiC devices at extremely high ambient temperatures[END_REF], [START_REF] Dubois | High temperature inverter for aerospace application[END_REF]. The topologies of these devices correspond mainly to inverters and synchronous rectifiers, for aircraft or photovoltaic applications [START_REF] Wood | Evaluation of a 1200 V, 800 A All SiC Dual Module[END_REF]. Moreover, it is noteworthy that the JFET appears to be the most mature and robust SiC power switching structure at the present time [START_REF] Boughrara | Robustness of SiC JFET in short-circuit modes[END_REF].

2) Electro-Magnetic Compatibility (EMC) Issues in Adjustable Speed Drives: The differential mode (DM) current and voltage are confined to the power loop, and depend strongly on the switching frequency. The DM current is filtered mainly by a line inductor and the DC-link capacitors in the voltage switching inverters (VSI). The CM current flows from the power lines through parasitic impedances to the ground, and is produced by high rates of change in voltage (dv/dt) which occur during switching. The CM noise is the main Fig. 2. Electrical diagram of the phase-leg inverter and LISN, including the CM impedances. Note that although the stray inductances and resistances of the power module are not represented, their influence was taken into account in the simulation, as described in section III-A3.

Lconnect/2

Lcm/2 Llisn/2

2.Clisn

ohms

Fig. 3. Common Mode equivalent circuit of a "Smart Electro-Mechanical Actuator (EMA)" with a LISN. V cm is the CM-equivalent noise source, and Cpara is the CM-equivalent parasitic capacitance. This circuit was derived from that shown in Fig. 2, in which only the CM path, from point M to the ground, is considered. contributor to high frequency conducted emission [START_REF] Jettanasen | Common-mode emissions measurements and simulation in variable-speed drive systems[END_REF]. The main issues to be dealt with are the electromagnetic field radiated by long cables, bearing currents, winding insulation stress close to the actuator, and cross-talk with the control signals. Due to the considerable mass and volume of the converter, optimization of the EMI filter is a critical issue for avionics.

The switching speed of SiC-JFET devices is 2 to 10 times higher than that found with IGBT-based inverters [START_REF] Burgos | Design considerations of a fast 0-Ohm gate-drive circuit for 1.2 kV SiC JFET devices in phase-leg configuration[END_REF]. This leads to higher dv/dt and di/dt transients, which in turn produce stronger, wide-bandwidth CM noise. It is also important to bear in mind that in a "Smart EMA" topology, no output filter is used on the AC side of the device. It is thus essential to make use of a suitably designed EMI input filter [START_REF] Heldwein | Impact of EMC filters on the power density of modern three-phase PWM converters[END_REF], [START_REF] Jiang | Study of conducted EMI reduction for three-phase Vienna-type rectifier[END_REF], [START_REF] Akagi | Attenuation of conducted EMI emissions from an inverter-driven motor[END_REF], to protect the electrical network from CM noise generated by the converter, and transmitted through the cable and the electrical machine.

B. Presentation of the Proposed Power Module Concept

1) Towards an Integrated Power Module: Today, considerable research activity is devoted to the integration of power converters, with the aim of improving power density, efficiency or standardization. Although a significant effort has been made in the field of input filter development [START_REF] Baisden | Impedance interaction and EMI attenuation in converters with an integrated transmission-line filter[END_REF], [START_REF] Chen | Improving the characteristics of integrated EMI filters by embedded conductive layers[END_REF], [START_REF] Biela | Passive and active hybrid integrated EMI filters[END_REF] in the mid-power range, i.e. between approximately 1 and 50 kW, a discrete filter is still required for adjustable speed drives. The design presented in this paper extends the filtering capabilities of a discrete input EMI filter, allowing it to cope with the greater noise levels produced by the fast commutations of SiC power devices.

As has already been proposed by [START_REF] Schanen | Built-in EMC for integrated power electronics systems[END_REF], we present a design in which CM filtering capacitors are integrated directly into the power module. The aim of this approach is to allow fast commutation, whilst at the same time ensuring high-frequency EMI containment. Since the high-frequency noise is generated by power switches, and the capacitors are located close to these components, only small capacitances are needed. Since most of the CM current should be "recycled" and contained within a closed loop, lower EMI noise levels can be expected from a SiC-JFET inverter, without taking up an excessive amount of space in the power module.

As shown in the circuit of Fig. 2, and in the CM-equivalent circuit of Fig. 3, an additional filter stage is created for high frequency filtering. This stage is composed of the integrated capacitors, Ccm1, and the stray inductances, Lconnect. In association with the discrete filter, the integrated CM capacitors take advantage of the stray inductance of the conductor between the discrete filter and the power module. Thanks to the additional noise reduction achieved by the use of integrated CM capacitors, a simple design can be found for the discrete input filter. With a standard (i.e. unfiltered) power module, a higher-order input filter would be required to cover the complete frequency range.

2) Methodology: The present study is dedicated to adjustable speed drives operating in the mid-power range. In order to provide a demonstration of this concept, the study focused on the design of a half-bridge inverter, also known as a phase-leg inverter. The results could easily be extended to three-phase VSI or synchronous rectifiers.

The present paper is organized as follows: section II presents a customized power module in a standard configuration, without the integrated capacitors. The layout and packaging technologies are described. In section III, the module is modeled and tested under realistic three-phase VSI operating conditions. The model is validated, and simulations are used to determine the most appropriate value for the integrated CM capacitors, in terms of their benefits and impact on the inverter. Finally, in section IV, the integrated capacitors are implemented inside the power module. Two configurations were tested experimentally: the standard configuration and the CM configuration, involving the use of integrated CM capacitors. The results are then analyzed and discussed. The aim of this approach is to experimentally determine the impact of the CM capacitors on the power signals, control signals and EMI.

II. DESCRIPTION OF THE POWER MODULE A power module, intended for applications in a phase-leg inverter topology, was specifically designed for this study. A significant effort was devoted to this aspect of the design, in terms of packaging and manufacturing technologies. High temperature resistance and low parasitic effects are required, as a result of the high junction temperature and high switching speed capabilities of the SiC-JFET. Thermal and electrical considerations are the most important aspects of the power module design: thermal management is dominated by the packaging technologies, whereas the electrical characteristics are more strongly related to the layout and characteristics of the power devices.

A. Packaging Technologies

A conventional assembly was selected for the power module, with the ability to withstand high junction temperatures, since the SiC-JFET has a high junction temperature capability (up to 300 • C). The temperature limitations of the design were driven mainly by the secondary passivation of the die and the solder joint. An additional requirement was imposed by the maximum operating temperature of the integrated CM capacitors (200 • C), since they share the same substrate with the power devices. A description of these packaging technologies is given in [START_REF] Ning | SiC wirebond multichip phase-leg module packaging design and testing for harsh environment[END_REF]. The dies were soldered with high-temperature solder, on an Al 2 O 3 Direct-Bonded-Copper (DBC) substrate. The DBC substrate was comprised of a 635 µm-ceramic layer, which was co-fired with 200 µmcopper layers on both sides. Aluminum-wire bonds connected the top of the dies to the tracks of the DBC, for the gate and source contacts. Several 250 µm diameter wire bonds were used between the source of the JFET and the DBC, in order to reduce on-state resistance. Copper leads were soldered for the external connections. A silicone gel was applied as an encapsulant, for insulation purposes. At the end of the process, the module was attached to a low thermal-impedance heat sink, using a silver-loaded epoxy adhesive.

B. Power Module Layout

The power module layout shown in Fig. 4 represents a phase-leg inverter comprising two power switches. As the selected JFETs (further described in section III) have an intrinsic body diode with acceptable properties [START_REF] Ben Salah | Modelling, analysis, and experimental study of SiC JFET body diode[END_REF], [START_REF] Peftitsis | High-power modular multilevel converters with SiC JFETs[END_REF], no external diode is used. The power module layout can thus be simplified by using only two dies. The current density where the power current flows into the tracks is approximately 5 A/mm 2 . If it is assumed that cooling is achieved by conduction through the bottom layer of the DBC, the copper losses would typically be lower than 0.1 W/cm 2 . When this value is compared with the losses in the power device, which are reported to lie between 100 W/cm 2 and 300 W/cm 2 for SiC-JFET power devices [START_REF] Jiang | Temperature dependent characteristics of SiC devices: Performance evaluation and loss calculation[END_REF], it can be seen that the Joule effect inside the tracks is negligible. The large track width of the design, as well as the copper backside of the substrate, have the additional advantage of reducing stray inductances in the circuit.

A further consideration is the distance separating the power loop and control loop source contacts. Only one wire bond is used as a reference potential for the gate-to-source voltage, and four wire bonds are connected from the source contact to the negative DC voltage, inside the power loop. The clearance between the tracks is set to 1 mm, to provide insulation at voltages up to 1.2 kV. A thermocouple is included, to measure the temperature inside the power module.

In order to anticipate the integration of CM capacitors inside the power module, a track connected to the ground is included, as indicated by the green track in Fig. 4. After having modeled the power module and simulated its behavior and performance, the integrated CM capacitors were surface-mounted at the locations shown in Fig. 4. The same layout was used for all of the configurations described in this paper. The capacitor footprints increase the size of the module by 30 %, when compared to a "normal" power module, in this specific case. It should be noted that the phase-leg inverter with only two dies corresponds to the worst-case scenario. The applications for which this power module is intended are three-phase inverters.

III. SIMULATION MODEL AND ANALYSIS

Simulations were used to set an appropriate value for the integrated CM capacitors, referred to as Ccm1 in Fig. 3. The flow-chart of Fig. 5 illustrates the different steps required to simulate the phase-leg inverter. Before introducing EMC considerations into the simulation, a model based on a real setup was built. This model was then validated by comparing the results produced by the simulation with the measurements at a typical operating point, corresponding to 365 V and 4.2 A on the DC side of the module. Then, CM impedances representing a realistic CM path for an adjustable speed drive were added, and different integrated CM capacitor values were investigated and compared to the standard configuration (without the integrated capacitors). Finally, the most appropriate value in terms of EMI was selected for implementation into the power module.

A. Phase-leg Inverter Modelling

All of our simulations were run with Saber R , a circuitsimulation software using the MAST descriptive language. The phase-leg inverter described in Fig. 6 was modeled from real elements, each of which is described in the following: 1) SiC-JFET: The 4x4 mm 2 SiC-JFET from Infineon was selected for this study. Its ratings are typically 1.2 kV and 15 A. Its on-state resistance is close to 80 mΩ at 25 • C, which is one of the lowest for this type of device. Its static and dynamic performances are evaluated in [START_REF] Burgos | Design considerations of a fast 0-Ohm gate-drive circuit for 1.2 kV SiC JFET devices in phase-leg configuration[END_REF] and [START_REF] Jiang | Temperature dependent characteristics of SiC devices: Performance evaluation and loss calculation[END_REF]. A multi-physics model of this device, described in [START_REF] Morel | A multi-physics model of the VJFET with a lateral channel[END_REF], was used in the simulations. Parameter identification was based on its static electrical characteristics and capacitance measurements.

2) Gate Drive Circuit: A gate drive circuit was designed to meet the SiC-JFET requirements: in the off-state, the gate-tosource voltage, V gs , must be lower than the pinch-off voltage, V p , which is close to -18.5 V , and greater than the punchthrough voltage, V pt , which is nearly -27 V . With fast voltage transients at the middle point M, gate-drive interactions lead to voltage spikes on V gs at the blocked JFET. As an example, if the upper JFET is off and the lower JFET is turning on, an interaction is observed on the upper V gs . During turn-on, the spike is positive and the V gs blocking voltage is increased, with the risk of a short-circuit occurring through the leg. In order to maintain V gs in the off-state, between V p and V pt , the blocking voltage V gs is set to -23.5 V and an additional 2.2 nF capacitor, C g , is inserted between the gate and source potentials to prevent excessive interactions [START_REF] Berry | Minimization of drain-to-gate interaction in a SiC JFET inverter using an external gate-source capacitor[END_REF]. A 12 Ω-gate resistor, R g , is chosen to limit gate-current spikes, whilst ensuring that fast switching speeds can still be achieved. The IXYS-IXDD09 Integrated Circuit (IC), designed for IGBT power modules, was used as a push-pull driver stage. Each gate drive circuit was supplied by an insulated DC/DC supply (the red blocks in Fig. 7). These DC/DC converters (Traco Power THB 3-1215) were selected for their low insulation capacitance (13 pF max.), which can be neglected when compared to the parasitic capacitances associated with the power circuit layout. The switching signals were transmitted to the gate drivers through Texas Instruments ISO721M high speed insulators, which also have a very low input-to-output capacitance (lower than 1 pF). In the model, the gate drive circuit was considered to be an ideal PWM-voltage source with a gate resistor, R g and capacitor, C g .

3) Power Module Layout: To take the influence of the coupling effects and stray elements associated with the conductors into account, a 3D model was built using the Par-tial Element Equivalent Circuit (PEEC) technique [START_REF] Martin | Analysis of electromagnetic coupling and current distribution inside a power module[END_REF] and the INCA3D R software. A perspective view of the layout is shown in Fig. 4. Potentials composed of phase inputs, transistor potentials, d, s and g, and the middle point M, are defined in the mesh. A matrix impedance, calculated at 1 MHz by the software, is provided by the model. The resistive and inductive components of the conductors (self and mutual inductances) were also modeled. The 3D model includes the wire bonds. The complete workflow, from the 3D description to the generation of a circuit model compatible with the Saber R software is described in detail in [START_REF] Schanen | Impedance criterion for power modules comparison[END_REF]. As this circuit model contains 32 ports (16 impedances and the coupling between any two of these), it is too large to be shown here. As an example, the stray inductance between the source terminal of JFET #2 and the DC(-) terminal (see fig. 4) was found to be 4.5 nH. None of the inductances exceeded a value of 9 nH.

4) Shunt: A 25 mΩ-coaxial shunt (T&M Research) was inserted in order to accurately measure the current across the lower SiC-JFET. Because of its low resistance compared to the on-state resistance of the SiC-JFET, and its low series inductance, measured at approximately 10 nH, the shunt has a very low influence on the dynamic response of the phase-leg. It can be modeled by an R-L equivalent circuit. All of the signals were referenced to the shunt during the time-domain acquisitions.

5) R-L Load: Adjustable speed drives are comprised essentially of a VSI and an actuator, which behaves like an inductive load. An 8 Ω power resistor and a 2.3 mH air inductor were used to test the phase-leg inverter. The air inductor is an excellent current source, which has linear characteristics since no magnetic material is used and it has a low parasitic capacitance. The latter were measured with an Agilent-4294A impedance analyzer, and the equivalent parasitic capacitances were estimated at respectively 9 pF and 17 pF, for the power resistor and the air inductor. Note that this constitutes a very simplified model of a machine, in which many phenomena have not been considered (non-linearity of the inductance, distributed parasitic capacitance. . . ). This model was chosen for the purposes of comparison, since it can be more easily introduced into the circuit simulator.

6) Test Board: A test board was built in order to connect the various elements together, as shown in Fig. 7. The stray inductances resulting from the presence of the power leads and connections are modeled by the inductance, Lconnect, which was estimated to be 100 nH. This relatively large value is due to the position of the power module, which (for practical reasons) was installed approximately 20 mm below the test, resulting in a large switching loop. Decoupling capacitors, C dc , were placed as close as possible to the power module, to limit the parasitic inductance of the commutation loop. Three 7 µF film capacitors were inserted in parallel. Using an impedance analyzer, the equivalent series resistance and series inductance were measured at respectively 10 mΩ and 10 nH. In addition, 450 V-100 µF electrolytic capacitors were inserted at the DC-link input, in order to filter the low frequency voltage ripple (these low-bandwidth capacitors were not taken into account in the dynamic modeling). Finally, a line inductor, Ldc, was inserted between the phase-leg and the power supply, to limit the current ripple. Ldc was rated at 500 µH when carrying a current of 5 A.

B. Model Validation

The operating point was chosen to be representative of an inverter functioning at a power level of a few kilowatts: the set points were thus 365 V and 4.2 A on the DC side and a switching frequency of 15 kHz, with a duty cycle of 36.6 %, was used at room temperature.

The instruments used during the tests are listed in Table I. A high-bandwidth oscilloscope and probes were chosen in order to measure the switching waveforms of the following signals: V ds , V gs and i g . Only passive current and voltage probes were used for the acquisitions. The differential voltages computed from two voltage measurements using the P6139A, high-frequency probes were preferred. Because of its low CM immunity, the differential probe was used only to trigger the oscilloscope, or to measure differential voltages which were not subject to very fast and large voltage transients.

The results of the simulation and the experimental measurements, corresponding to the switching waveforms of the phase-leg inverter, are compared in Fig. 8 and can be seen to be in good agreement. The differences can be attributed mainly to errors introduced by the measurement devices themselves, and to parasitic effects such as parasitic capacitances, which are not included in the PEEC model. Some limitations also arise from the accuracy of the measurements carried out during the preliminary identification of the components. Nevertheless, the dynamic behavior of the converter can be seen to be well modeled. The commutation energy E, defined as the integral of V ds x i s during switching, is a good criterion for validation of the model. The energies E on and E of f , measured at respectively 202 µJ and 164 µJ, and predicted as 206 µJ and 182 µJ by the simulations, are thus consistent. At this stage, the phase-leg inverter model is thus considered to be validated.

C. Addition of Common Mode Impedances

Following validation of the phase-leg inverter model, the simulations were extended to study the influence of the integrated CM capacitors. Realistic CM impedances were introduced into the model, and the influence of a range of CM capacitance values was studied. The full model, including the CM impedances, is shown in Fig. 2 and described in the following. The values of these elements are listed in Table II.

An important point to be considered is the modification of the (ground) reference. In a standard EMC setup, the reference is imposed by the Line Impedance Stabilization Network (LISN) and the copper ground plane. There is no particular difficulty with this in the simulation, since all of the signals can be accessed. However, in the experimental setup, the shunt is no longer the reference and it is not possible to accurately measure the source current. For example, at the lower JFET, i s cannot be measured. The simulations are thus very useful when it comes to estimating the influence of the integrated capacitors, CM, on the losses.

1) Line Impedance Stabilization Network: EMI levels are defined by standards. Reproducible measurements are ensured by a LISN and a standard set-up for conducted emissions. The LISN, specified by the DO-160-F standard [START_REF] Eurocae-Rtca | Environmental conditions and test procedures for airborne equipment -RTCA DO-160F/EUROCAE 14F[END_REF], was chosen. It was modeled with a realistic equivalent circuit, since at frequencies below 1 MHz its phase-to-ground impedance is less than 50 Ω. The frequency range to be considered was from 150 kHz to 30 MHz.

2) Discrete EMI Filter: The discrete EMI filter is a singlestage L-C filter. This filter was modeled by Lcm and Ccm2. Typical values for CM chokes, Lcm, are in the range of several mH. A 2.7 mH choke and two 50 nF ceramic capacitors were selected. The resulting cut-off frequency of the discrete filter was close to 10 kHz, i.e. below the switching frequency (15 kHz).

Because the CM emissions flow between the phases and ground, the overall capacitance is equal to the positive phaseto-ground capacitance plus the negative phase-to-ground capacitance. CM capacitance values (Ccm1 or Ccm2) are thus twice as great as the physically implemented values. The integrated CM capacitance of 6 nF was physically composed of a 3 nF capacitance between the positive phase and ground, in addition to a 3 nF capacitance between the negative phase and ground.

3) Parasitic Common Mode Impedance: The parasitic CM impedance is mainly capacitive. In adjustable speed drives, this is created mainly by a long-shielded cable and/or the actuator, between the windings and the chassis. A typical value of 1 nF was chosen for the simulations.

4) Integrated Common Mode Capacitors: The integrated CM capacitance under investigation ranges between 1 nF and 10 nF. It was initially assumed that lower values would have no impact on the EMI. On the other hand, values greater than 10 nF would be difficult to integrate into the power module. The upper limit of the CM capacitance is also imposed by the required frequency range. In accordance with the aim of this study, the integrated capacitors must filter the high frequency noise. The bandwidth of larger capacitors, with larger values and larger parasitic capacitances, could also be a limitation. The parasitic capacitances chosen for the model were derived from real discrete and surface-mounted components. In addition, a damping resistor was inserted at the ground connection, in order to prevent excessive oscillations. Dissipation due to damping was considered to be negligible.

For the purposes of the simulations, the operating point and load and gate circuits were kept constant. Only Ccm1 was allowed to vary. In order to compare the CM emissions, the CM voltage at the LISN, defined in (1) by V cm, was computed. V lisn1 and V lisn2 are the voltages across the 50 Ω termination impedances in Fig. 2. The EMI spectra of V cm are plotted in Fig. 9 for Ccm1 equal to 2 nF, 6 nF and 10 nF. The frequency bandwidth under consideration ranges between 100 kHz and 30 MHz, since the range covered by the standards is 150 kHz-30 MHz [START_REF] Eurocae-Rtca | Environmental conditions and test procedures for airborne equipment -RTCA DO-160F/EUROCAE 14F[END_REF]. The commutation energies are also compared in Table III. The simulation shows that there is no significant difference in the losses determined with the integrated CM capacitors.

V cm(t) = V lisn1(t) + V lisn2(t) 2 (1)
The EMI spectrum is typical for an inverter. The CM noise is well filtered up to several megahertz, after which the attenuation of the discrete filter drops off, due to the presence of parasitic elements, whereas the noise is still high. Consequently, a spike appears with a significant amplitude. As initially assumed, the lowest value of Ccm1 (2 nF) does not lead to any decrease in the EMI, when compared to the case without integrated capacitors. When Ccm1 is equal to 6 nF, a 6 dBµV decrease is achieved for V cm, at the highest spike, and a slight attenuation is observed over the whole frequency range. Up to a value of 10 nF, although V cm is decreased by 1 to 2 dBµV, this is insufficient to compensate for the increase in capacitance, which is indirectly related to the footprint area. These results show that a capacitance of 6 nF provides a good trade-off between attenuation and the size of the integrated CM capacitors.

In Fig. 10, time-domain waveforms are compared for two different configurations: without the CM integrated capacitors, and with a 6 nF capacitance. The parasitic CM impedance produces source current oscillations at turn-on. These are reduced by the presence of the integrated CM capacitors, as a consequence of decoupling in DM mode. However, no significant difference is observed in the instantaneous power and control signals during commutation.

D. Conclusions on the simulations

The model was initially validated by measuring switching waveforms and losses, following which CM impedances were added. The simulations show that a value of 6 nF for the integrated CM capacitors provides a good trade-off between attenuation and the size and weight of the components. At high frequencies, close to 7 MHz, a 6 dBµV decrease in the CM voltage, V cm, is achieved. This level of high frequency attenuation is useful, because the discrete filter is less effective power module described in section II above. The resulting prototype layout is shown in Fig. 11. As described above (see section III-C-4), the value of 6 nF was selected for the integrated CM capacitors. These had to be placed as close as possible to the power devices, and connected to the ground. The ground potential was therefore introduced into the power module. High-temperature, NP0 Multilayer Ceramic Capacitors (MLCC), supplied by Presidio Components and rated at 500 V-1.5 nF, were selected. They were surface mounted and connected in parallel in order to obtain the required capacitance. The critical parameters of these capacitors, such as insulation resistance, capacitance and dissipation factor, are very stable over a wide range of temperatures, from -55 • C to 200 • C. It is interesting to note that capacitors with even higher temperature ratings (up to 260 • C) are available, and that technologies (film capacitors), which can be used at temperatures as high as 300 • C, are currently being developed [START_REF] Robutel | Design of a high temperature emi input filter for a 2 kw hvdc-fed inverter[END_REF].

B. Experimental Results

The experiments were carried out in a standard EMC setup (compliant with the DO-160-F standard) which consisted mainly in the LISN, provided by Solar Electronics (9403-5-PB-10-BNC), and the conductive ground plane. A 5-meter shielded cable, grounded at both ends, was used to introduce a parasitic CM impedance between the phase-leg and the load. For the purposes of comparison, the same operating point, set to 365 V-1.7 A on the DC side, was used for all measurements and the converter was run continuously. As shown in Fig. 4, two different cases were investigated: the standard configuration, without the integrated CM capacitors, and the CM configuration, with a 6 nF CM capacitance. The time domain waveforms were measured, to determine the influence of the integrated CM capacitors during switching. The drain-to-source voltages at the lower JFET, V ds , were measured and compared during turn-on and turn-off, as shown in Fig. 12, in which the two measurements have been time shifted, to improve their readability.

As shown in Fig. 12, at turn-on the switching speed is increased by the presence of the integrated capacitors. The switching time was approximately 50 ns with the integrated capacitors, as opposed to 55 ns without them. It can also be noticed that in the absence of the capacitors, the V ds waveform is disturbed by the parasitic inductance inside the commutation cell. This waveform is improved by the presence of the integrated capacitors, as a result of improved decoupling in the DM mode. This small change was predicted by the simulations, as shown in Fig 10-(a). At turn-off, no large over-voltage was measured on either of the V ds waveforms. However, the switching speed was approximately 5 ns faster when these capacitors were not present. This result is consistent with the losses predicted by the simulations in Table III: the integrated CM capacitors reduce the losses at turn-on, but increase the losses at turn-off. This leads to equal losses for both configurations. The temperature measurements given in Table IV confirm this observation. The temperature increase inside the power module was measured with a thermocouple sensor, in order to compare the losses. Steady-state thermal conditions were achieved after three minutes.

Although the absolute increase in temperature was low, since the thermocouple did not measure the junction temperature directly, it provided a good means of comparing the losses with both configurations. In addition, the phase-leg inverter losses were very low, and the overall system-level efficiency was found to be nearly 98 %. Fig. 13 compares V gs and I g , measured at the lower JFET during turn-on. Although the control signals were not strongly affected, it can be seen that I g was slightly disturbed by the oscillations, and that V gs followed almost identical curves in both cases.

The CM EMI emissions, defined in (1) by the term V cm, were measured in the time domain at the LISN, and computed using a Fast Fourier Transform (FFT). When connected to the power module by means of coaxial cables, the oscilloscope served the purpose of a 50 Ω load. The time-domain variations of V cm are plotted in Fig. 14, using two different time scales, and the frequency-domain spectra produced by the FFT computations are shown in Fig. 15. For the FFT calculations, a total of 50,000 points were acquired by the oscilloscope, at 2 ns sampling intervals. The FFT was computed using a rectangular window. This approach led to an accurate determination of the harmonics present in the selected frequency range, between 100 kHz and 30 MHz.

In the time domain, a significant reduction in V cm was achieved with the integrated capacitors. Fig. 14 shows that the spike amplitude was reduced by a factor of five, and the oscillation frequency was also reduced, as can be seen in Fig. 14-(b).

The same trends are reproduced in the frequency domain, as shown in Fig. 15. The strongest spike observed with the integrated capacitors occurs at a lower frequency and a lower level, i.e. approximately 10 dBµV lower than the strongest spike observed without the capacitors. The additional highfrequency L-C stage, comprising the integrated capacitors and the stray inductance of the connections, also contributes towards improved attenuation at lower frequencies: a reduction of approximately 6 dBµV was measured between 100 kHz and 1 MHz. The measured CM attenuation was slightly greater than the value predicted by the simulations.

V. CONCLUSION

This study has demonstrated that a significant reduction in CM noise can be achieved at the LISN, by inserting CM capacitors directly inside the power module, as close as possible to the noise source. The design procedure, based on the use of simulation software, has been validated by experimental results. With appropriate modeling, such simulations can be used as an effective design tool.

The negative effects of the CM integrated capacitors on the system are very low, in terms of power switching waveforms, driver circuit, and power module losses. No prohibitive voltage spikes were measured during switching of the V ds waveforms, the control signals were not disturbed, and the losses remained unchanged. Consequently, no additional constraints were added to the converter.

As initially expected, the concept of integrated CM capacitors makes it possible to achieve fast switching with low losses, and contained EMI at high frequencies. The use of additional integrated CM capacitors leads to an approximately 30 % increase in the size of the power module layout, when compared to a conventional design, and to the need for an additional copper grounding pad. However, in the case of a three-phase VSI, the overall impact of these modifications would be reduced, as a consequence of the greater surface area occupied by the dies. When compared to the alternative of adding a second L-C stage, the overall volume would nevertheless be reduced for a mid-power adjustable speed drive. This outcome is of considerable interest for aerospace applications.

Finally, the assembly technologies used to demonstrate this concept allow excellent performance to be achieved with SiC-JFET power devices. The authors are convinced that, in the near future, new generation converter packaging concepts, based on the use of wide-bandgap semi-conductors, will allow even greater benefits to be derived from the the concepts presented .

Fig. 1 .

 1 Fig. 1. Electrical diagram of an adjustable speed drive including a LISN, generally referred to as a "Smart Electro-Mechanical Actuator (EMA)

Fig. 4 .

 4 Fig. 4. Perspective view of the SiC-JFET power module used for PEEC modeling, and corresponding module circuit diagrams, with and without the integrated CM capacitors, as described in section IV. The position of the capacitors and JFETs can be seen in the perspective view.

Fig. 5 .Fig. 6 .

 56 Fig. 5. Simulation procedure.

Fig. 7 .

 7 Fig. 7. SiC-JFET power module test board. The power module terminals were soldered directly onto the bottom layer of this board.

Fig. 8 .

 8 Fig. 8. Comparison between simulated and experimental switching waveforms at the lower JFET of the phase-leg inverter. (a) Drain-to-source voltage, V ds , and source current Is. (b) Gate-to-source voltage, Vgs, and gate current, Ig. (c) Instantaneous power, P .

Fig. 9 .

 9 Fig. 9. Simulated EMI spectra for different values of the integrated CM capacitors.

Fig. 10 .

 10 Fig. 10. Comparison between the simulated switching waveforms, during turn-on at the lower JFET of the phase-leg inverter, with no integrated CM capacitors, and using a 6 nF capacitance (Ccm1=6 nF). (a) Drain-to-source voltage, V ds , and source current Is. (b) Gate-to-source voltage, Vgs, and gate current, Ig. (c) Instantaneous power, P .

Fig. 11 .Fig. 12 .

 1112 Fig. 11. SiC-JFET power module prototype with integrated CM capacitors.

Fig. 13 .Fig. 14 .

 1314 Fig. 13. Vgs and Ig at the lower JFET during turn-on, for the two studied configurations.

Fig. 15 .

 15 Fig. 15. FFT of V cm(t), for the two studied configurations.

TABLE II PARAMETERS

 II USED FOR THE CM-EQUIVALENT IMPEDANCES

	P arameters	L	esr	epr	epc
	Lcm	2.7 mH	50 mΩ	9 kΩ	30 pF
	Llisn	5 µH	0.2 Ω	none	none
	P arameters	C	esr	esl	Damping
	Cpara	1 nF	1 Ω	none	none
	Ccm1	1 to 10 nF	20 mΩ	10 nH	3 Ω
	Ccm2	100 nF	50 mΩ	10 nH	none
	Clisn -HF	280 nF	10 mΩ	60 nH	none
	Clisn -BF	7 µF	100 mΩ	none	none

TABLE III COMMUTATION

 III ENERGIES DETERMINED BY SIMULATION, FOR DIFFERENT INTEGRATED CM CAPACITOR VALUES

	Ccm1	Eon	E of f	E total
	none	253 µJ 158 µJ	411 µJ
	2 nF	246 µJ 162 µJ	408 µJ
	6 nF	245 µJ 164 µJ	409 µJ
	10 nF	242 µJ 162 µJ	404 µJ

ACKNOWLEDGMENTS

The authors would like to thank the Direction Generale de l'Armement (DGA) and the Hispano-Suiza company, a subsidiary of the Safran Group, for their contributions. They also extend their thanks to the staff and students of the Center for Power Electronics Systems (CPES), and would like to acknowledge the assistance provided by Dr. Puqi Ning, Dr. Fang Luo and Mr. Abderrahime Zaoui.

Rémi Robutel received a technology degree in electrical engineering from the Universitè Paris-Sud, in 2005, the M.S. degree in electrical engineering from the Grenoble Institute of Technology, in 2008, and the Ph.D. degree in electrical engineering from the Université de Lyon, France, in 2011. He is now working as a R&D engineer in the Oil and Gas industry.

His major field of interest includes hightemperature and high-density electronics, high reliability packaging, and MEMS sensor technologies. He is the author or co-author of about 15 technical papers, and a co-author of an U.S.patent. He received the Innovation Safran Power Electronics Center Award for his contribution to high-temperature power electronics projects, in 2011. Régis Meuret is 55 years old, French national. He graduated as an engineer from the "Institut des Sciences de l'Ingenieur de Montpellier" in 1981. His professional background includes 12 years with a world leader of variable speed drive as an Electronics Design Engineer, 5 years with a manufacturer of submarine propulsion as Electronics Manager for motor drive. For more than ten years he has been working aircraft applications, including 2 years as Senior Design Engineer for Thales-AES in Power Electronics, with a special focus on electrical network and converter simulation. He is currently a Power Electronic and Motor control Expert with Hispano-Suiza, and is in charge of the research at SAFRAN POWER ( Hispano-Suiza Division ). He leads the SAFRAN Power Electronic Center (SPEC) to work on the breakthrough technologies and their maturation for a more electrical aircraft .

Christian


Claudio Price
email: claudio.price@alumnos.usm.cl

Yuliya Tarabalka
email: yuliya.tarabalka@inria.fr

Ludovic Brucker
email: ludovic.brucker@nasa.gov

GRAPH-BASED METHOD FOR MULTITEMPORAL SEGMENTATION OF SEA ICE FLOES FROM SATELLITE DATA

Keywords: Multitemporal Segmentation, Region Growing, MODIS, Sea Ice Floes

Automated segmentation of the sea ice evolution would allow scientists studying climate change to build accurate models of the sea ice meltdown process, which is a sensitive climate indicator. In this paper, we propose a novel approach which uses shape analysis and graph-based optimization for segmentation of a multiyear ice floe from time series of satellite images. Differently of the state-of-theart sea ice segmentation techniques, the new method does not rely on the coherence of the intensity values between successive time moments, but only on the coherence of the shape. We successfully validated the performance of the proposed approach on a set of AMSR-E and MODIS images and estimated the area of a sea ice floe as a function of time.

INTRODUCTION

The sea ice cover is both a sensitive climate indicator and an active participant of the Earth's climate system [START_REF] Le Treut | Historical Overview of Climate Change[END_REF]. Therefore, it is very important to monitor the sea ice meltdown process. Satellite-based remote sensors acquire data for the continuous monitoring of ice floes at global scales. There is thus a need to develop methods for automated segmentation of the sea ice evolution.

Most of the state-of-the-art techniques for automatic ice floe segmentation from radar or optical remote sensing data sets use adaptive thresholding for distinguishing ice from water areas (Zhang, 1992, Haverkamp and[START_REF] Haverkamp | Using temporal information in an automated classification of summer, marginal ice zone imagery[END_REF]. Recently, iterative region merging methods were proposed to track ice floe [START_REF] Yu | Segmentation of RADARSAT-2 dual-polarization sea ice imagery[END_REF][START_REF] Tarabalka | Shapeconstrained segmentation approach for arctic multiyear sea ice floe analysis[END_REF]. Even though the partition obtained by applying hierarchical step-wise region merging, such as in [START_REF] Tarabalka | Shapeconstrained segmentation approach for arctic multiyear sea ice floe analysis[END_REF], is optimal regarding the minimization of the internal heterogeneity of regions, this method does not cope well with segmentation of noisy and low-contrast images, where internal heterogeneity of the ice floe intensity can be higher than the intensity difference between floes and a background.

In this paper, we propose a new method for multitemporal segmentation of multiyear ice floes from temporal sequences of satellite images. The method performs hierarchical step-wise optimization to construct an initial graph of connected adjacent image regions, and then seeks for a connected subgraph corresponding to an ice floe region, by minimizing a foreground energy. The advantage of the new method when compared to the previously proposed techniques is that it does not rely on the coherence of the intensity values between successive time moments, but only on the coherence of the shape. It thus succeeds in segmenting images where both foreground and background intensity distributions vary significantly over time.

The rest of the paper is organized as follows. In the next section, the data set used for experiments is described. Section 3 presents the proposed segmentation approach. Experimental results are discussed in Section 4, and conclusions are given in Section 5.

DATA SET

We analyzed data from two instruments onboard the NASA Aqua satellite: Advanced Microwave Scanning Radiometer -Earth Observing System (AMSR-E) and Moderate-Resolution Imaging Spectroradiometer (MODIS). This satellite has a polar orbit enabling several measurements per day over the Earth's polar regions. The objective was to segment a multiyear ice floe, which left the Arctic sea ice pack in December 2007, continuously moved with the Arctic ocean currents and melted in June 2009.

The position of the ice floe of interest was detected by using the AMSR-E data at 89 GHz mapped to a polar stereographic grid at 6.25 km spatial resolution. These data have been extracted from the "AMSR-E/Aqua Daily L3 6.25 km 89 GHz Brightness Temperature Polar Grids" product distributed by the National Snow and Ice Data Center. A multiyear ice has a low microwave emissivity and thus appears dark in AMSR-E images, and in this way distinguishable from clouds and younger ice which has a higher emissivity. However, the low spatial resolution of the AMSR-E data does not allow to accurately segment the ice floes from the background.

In accordance with these tracking measurements, a time series of the AMSR-E and MODIS images was built with the ice floe of interest. We used band 1 (0.620 -0.670 µm, spatial resolution of 250 m) of the MODIS data, because it has the highest MODIS spatial resolution and provides the best contrast between the sea ice and the ocean water. Level 1B calibrated and geolocated MODIS swath data reprojected onto a polar stereographic grid was the main data source. The geolocation uncertainty is ⇠50 m [START_REF] Wolfe | Trends in modis geolocation error analysis[END_REF].

All swath images did not provide opportunities to quantify the ice floe areas either due to limited solar illumination, extensive cloud cover, or weakness of contrast between the multiyear ice and the neighboring young ice. We chose for our analysis T = 50 images with spatial dimensions of H ⇥ W =800⇥ 800 pixels, acquired on 21 different days during the two-months period from mid-August to mid-October 2008 (from the 227th day to the 286th day of 2008). Figure 1 shows examples of AMSR-E and corresponding MODIS images from the considered set.

PROPOSED METHOD

The proposed method belongs to the category of causal, or feedforward, temporal segmentation techniques [START_REF] Paris | Edge-preserving smoothing and mean-shift segmentation of video streams[END_REF]

Floe detection and alignment

First, each AMSR-E image is smoothed by Gaussian lowpass filter (σ =8 , size = 300 ⇥ 300) and normalized between 0 and 1. The obtained smoothed image is thresholded to detect a multiyear ice floe, yielding a black and white image with a blob-like element corresponding to the floe of interest (see Figure 2(b-c)). Because a multiyear ice has a very low microwave emissivity, we recommend to choose a small positive value of the binary threshold Θ (we used Θ=0 .1 in our experiments). Then, the position (approximate coordinates of the floe center and the rotation angle) of the ice floe is detected, by applying an ellipse fit algorithm on the extracted blob. This position information is used to align each MODIS image with the MODIS image in the preceding time moment from the considered time series. Example of the MODIS image alignment is shown in Figure 2.

At the next stage, we exhaustively partition each MODIS image into three non-overlapping regions:

1. Floe core, which is a reliable region in the center of the floe.

2. Background mask, which contains reliable background pixels.

3. Working area, which contains both floe and background pixels, including a border of the floe, and which is located between the floe core and the background mask. We detect the floe core (1) by applying an erosion on the AMSR-E-derived blob-like element with the disk structuring element of radius r =3 0pixels [START_REF] Soille | Morphological Image Analysis[END_REF]. The background mask (2) is similarly computed by dilating the blob-like element with the disk structuring element of radius r =60pixels. In the following steps, we seek for a segmentation contour only within the working area.

Hierarchical step-wise optimization

At this step, best merge region growing is performed [START_REF] Beaulieu | Hierarchy in picture segmentation: a stepwise optimization approach[END_REF] within the working area of each MODIS image, yielding its exhaustive partitioning into M homogeneous regions, which we call macro-pixels. The algorithm is initialized by assigning a new region label for each pixel of the working area. Then, at each iteration the two most similar spatially adjacent regions are merged, until working area is split into M regions (see Figure 4).

The measure of dissimilarity is computed by using the square root of mean squared error criterion (RMSE), combined with the standard deviation spatial feature (SDSF), as described in [START_REF] Tilton | RHSEG users manual: Including the Core RHSEG open source release[END_REF]. The RMSE between two image regions Ri and Rj, with region intensity means µi and µj, and region sizes (number of pixels) ni and nj, respectively, is defined as:

RM SEij =  ninj (ninj) (µi -µj) 2 1 2
.

(1) The SDSF we used is equal to the mean normalized standard deviation. For regions consisting of two or more pixels, the SDSF of a region Ri with the intensity mean µi and region size ni is computed as:

sdsfi = σi = q 1 n i -1 P xp2R i (xp -µi) 2 µi . (2)
The dissimilarity criterion between each two adjacent regions Ri and Rj is then computed as:

DCij = RM SEij  1.0+ |sdsfi -sdsfj| (sdsfi + sdsfj) W , (3)
where the parameter W sets the importance of taking into account the SDSF. We set W =4in our experiments.

Figure 3(c) shows an example of the image partitioning into a set of macro-pixels, obtained by applying the hierarchical step-wise optimization step. Once the partitioning is performed, an image graph is built with a set of nodes {floe core, macropixel 1, ..., macropixel M} and edges connecting spatially adjacent nodes. The objective of this step is to reduce the number of nodes in the image graph, resulting in a lower computational time of the overall method. The value of M is set empirically and it should be high enough to avoid undersegmentation. We used M =8 0 in our experiments.

Energy minimization on the resulting graph

Finally, we seek for a connected subgraph S = {floe core, {subset of macropixels}} of the constructed image graph, which corre-sponds to the ice floe region. For this purpose, a foreground energy is defined as a sum of shape and data terms, which depend on the shape of the considered region and on the gradient at the border of the region, respectively:

E = E shape + αE data . (4)
The weighting parameter α balances the contribution of these two energy terms. In our experiments we used α =0 .25. The shape term for the considered subgraph Gi is computed as the dissimilarity between a shape of the current region (i.e., region corresponding to Gi) and an average shape of the segmentations at k previous time moments, which we denote by Gpr (the choice of k depends on the speed of the shape change between consecutive frames; we set k =3):

E shape i = 360 • X θ=0 • dif f i,θ . (5)
We use a Manhattan distance between normalized shape profiles as a dissimilarity criterion:

dif fi = |di(θ) -dpr(θ)| , (6)
where di(θ) is a distance from the centroid point of the region Gi to the region border at the angle θ. Before applying (6), regions Gi and Gpr must be aligned according to their centroid points and major axes. Since θ can take an unlimited amount of values between 0 • and 360 • , we must set beforehand a dθ in order to have a sufficient accuracy and a reasonable computational speed for each comparison. In our experiments we used dθ =3 • .

The data term is computed as a normalized gradient sum along the border of the current region. Because we seek to minimize the total energy E, we use the following edge indicator function for each point x of the region border C [START_REF] Schoenemann | Globally optimal image segmentation with an elastic shape prior[END_REF]:

g(x)= 1 1+rC(x) (7)
This function is averaged along the curve C, so not to discourage segmentations with longer perimeters:

E data i = P l(C i ) p=0 g(Ci(p)) l(Ci) . ( 8
)
The total energy is thus computed as

Ei = 360 X θ=0 dif f i,θ + α P l(C i ) p=0 g(Ci(p)) l(Ci) . (9)
The ice floe region is segmented by finding a connected subgraph S that minimizes the foreground energy.

RESULTS AND DISCUSSION

We applied the proposed method to the time series of AMSR-E and MODIS images described in Section 2, and compared results with those obtained by performing hierarchical step-wise optimization [START_REF] Beaulieu | Hierarchy in picture segmentation: a stepwise optimization approach[END_REF] to segment an image into two regions: a floe and a background. It can be seen in Figure 5 that the proposed approach yields consistently more accurate results than the hierarchical step-wise optimization. For quantitative comparison between the methods we used the Dice score [START_REF] Dice | Measure of the amount of ecological association between species[END_REF]:

D = 2| Ŝ \ S| | Ŝ| + |S| , (10)
where Ŝ and S are manually and automatically segmented ice floe regions, respectively. It can be seen in Figure 6(a) that the hierarchical step-wise optimization is able to deliver good segmentations in several cases, but its effectiveness is inconsistent in a time series. The proposed method gives accurate results for all the images (D>0.92 8 images), and yields consistently decreasing (with non-significant variations) melting ice profile, as shown in Figure 6(b).

CONCLUSIONS

We have proposed a new method for segmentation of multiyear sea ice floes from time series of Aqua satellite images. The new graph-based approach performs best merge region growing, followed by the energy minimization on the image graph, where the energy consists of two terms describing the floe shape (shape term) and the gradient between the floe and the background (data term), respectively. We validated the performance of the proposed method for segmentation of a shrinking ice floe from a sequence of AMSR-E and MODIS images acquired in August-October 2008. The obtained results did show both the effectiveness of the proposed approach and its robustness to low-contrast data.

  Figure 1: (a)-(c) MODIS and (d)-(f) AMSR-E images acquired on the 5th, 10th and 19th day, respectively.

  , which consider only past data for iteratively segmenting each next image. It allows thus to segment ice floes in real-time, given the low acquisition rate. In the following, each step of the proposed iterative technique (each iteration corresponds to one time moment) is described. At the input, T 1-band AMSR-E and MODIS images including the ice floe of interest are given. We denote each of the AMSR-E and MODIS images by At and It, t =1 ,...,T, respectively. The goal is to obtain for each time moment t a segmentation map St = {st(x, y) 2 [0, 1],x =[ 1 ..H],y =[ 1 ..W ]}, where each pixel has label 1 if it belongs to the considered floe, and 0 otherwise.

Figure 2 :

 2 Figure 2: Floe detection and alignmentt An example of these three regions is shown on Figure 3(b). We detect the floe core (1) by applying an erosion on the AMSR-E-derived blob-like element with the disk structuring element of radius r =3 0pixels[START_REF] Soille | Morphological Image Analysis[END_REF]. The background mask (2) is similarly computed by dilating the blob-like element with the disk structuring element of radius r =60pixels. In the following steps, we seek for a segmentation contour only within the working area.

  Figure 3: MODIS image partitioning

Figure 5 :

 5 Figure 5: Comparison of results for images acquired on the 4th, 16th, 17th and 19th day, respectively. Manual segmentation ice floe contour is shown in green, hierarchical step-wise optimization result in red, and new graph-based approach in blue.

  Figure 6: (a) Comparison of the performance of the proposed method versus hierarchical step-wise optimization (HSWO) technique in terms of the Dice score. (b) Area [km 2 ] of the segmented ice floe as a function of time.

ACKNOWLEDGEMENTS

This research has been supported in part by the Science and Technology Research Council of Chile (CONICYT) in the framework of INRIA (Sophia-Antipolis) and CONICYT cooperation agreement. The authors would like to thank A. Ivanoff from the NASA GSFC, USA, for usefull discussions about multiyear ice floe analysis.


Stefano Moretti
email: stefano.moretti@dauphine.fr

Fioravante Patrone
email: patrone@diptem.unige.it

Special Issue on ICT-based strategies for environmental conflicts

The special issue on ICT-based strategies for environmental conflicts -to be published in the journal Strategic Behavior and the Environment -contains four peer-reviewed articles devoted to the analysis of multi-agents decision problems arising from the exploitation of environmental resources, and where Information and Communications Technologies are crucial for the design and the implementation of environmental policies. In this short presentation of the special issue, the Guest Editors provide a brief summary of the contributed papers, together with some insights gained from their original motivations and main results.

Introduction

Information and Communication Technologies (ICTs) can be critical for new environmental policies that may affect human communities, both at local and at international levels. For instance, technological advances require systems that can optimize the use of telecommunications infrastructures and standardized software to address environmental management needs. Or, they may demand the implementation of algorithms reducing the energy consumption of computing and of managing communication networks. However, most of these ICT applications involve multiple actors interacting via complex protocols that make it difficult to implement coordinated strategies aimed to improve resource management, networking, and information exchange.

The goal of the Special Issue on ICT-based strategies for environmental conflicts is to present contributions from different areas of strategic behaviour analysis dealing with multi-agents decision problems arising from the exploitation of environmental resources, and where ICTs are crucial for the selection or the implementation of environmental policies. With this objective, particular emphasis has been given to the discussion of ICT applications addressing one or more of the following environmental issues.

The first issue deals with the analysis of policies providing a valuable contribution to sustainable environmental management by improving the monitoring and the coordination of different stakeholders. As discussed in Dovers [START_REF] Dovers | Clarifying the imperative of integration research for sustainable environmental management[END_REF], this kind of analysis requires the "integration" of environmental, social, economic aspects and other multi-disciplinary issues as well as stakeholder interests, and where the meaning of the term "integration" is intended to be established in a very broad manner, including the response across social systems, the collection of information and knowledge sharing. In this direction, Bottero et al. [START_REF] Bottero | Enabling public participation in strategic environmental assessment: an application of multicriteria analysis[END_REF] in this collection, introduce an application of a Multiple Criteria Decision Aiding method to support the preparation of a land-use plan for a small town in Italy, taking into account the main steps of the process aimed to involve the population in planning decisions.

Another issue treated in this book is the formulation of protocols enabling a more efficient allocation of natural resources or providing incentives for reducing the consumption of energy, water and other natural resources through more efficient procedures. For example, the carbon footprint of ICT technologies themselves represents an important and ever-increasing contribution of the global CO2 emissions. In order to reduce their carbon footprint, emerging ICT technologies are embedded with energy-aware protocols and energy saving techniques (Bolla et al. [START_REF] Bolla | Energy efficiency in the future internet: a survey of existing approaches and trends in energyaware fixed network infrastructures[END_REF]). The role of interactions among agents is taken into account in Bianzino et al. [START_REF] Bianzino | The green-game: Accounting for device criticality in resource consolidation for backbone ip networks[END_REF] in this collection, re-introducing and discussing some energy-aware routing approaches in backbone networks, which take into account the criticality of devices in the considered network scenario, the relevance for network connectivity of the devices and the amount of traffic that they are routing.

The third issue deals with the analysis of socio-ecological communities and the amount of data that is generated during our everyday activities on Internet and other modern communication technologies, which opens new questions in organizational and societal behaviour. For example, how can we explain the organizational behaviour of communities by gathering network data through the interaction among people? Can we explain how human communication dynamics in natural resource governance is linked to the organizational formation of social networks (Bodin and Crona [START_REF] Bodin | The role of social networks in natural resource governance: What relational patterns make a difference?[END_REF])? Moreover, designing ideal platforms for communication and debate about environmental problems is central to overcome physical and social barriers and allowing for the formation of specialinterest social networks and virtual communities. Moretti [START_REF] Moretti | On the role of coalitional network games in modelling the social dimension in ecosystem management[END_REF] re-introduces some applications from the literature of coalitional games, aimed at modelling social dimensions like Resilience and Social Capital, and shows some direction to empirically test on data such applications within the MAS (Multiple Agent System) framework applied to the evolution of social ecosystems.

The fourth and final issue is related to the definition of incentives and mechanisms to support the right of access to information related to the quality and the utilization of environmental resources. A way to achieve it is to support the concept of open source for wireless, mobile phone and web-based applications. Open source systems are very vulnerable to malicious attacks, and policies aimed to guarantee the correct users' access need to be implemented. Tembine [START_REF] Tembine | Energy-constrained mean field games in wireless networks[END_REF] in this book, analyzes the problem of ensuring security for cognitive radio in wireless networks using mean field stochastic games. The first function of cognitive radio networks is to serve the demand of spectrum of licensed users (primary users), allowing for the accommodation of the demand of spectrum of external secondary radio devices (secondary ussers). In this context, Tembine [START_REF] Tembine | Energy-constrained mean field games in wireless networks[END_REF] proposes an optimal strategy against possible malicious attakers and focuses on exploiting mobility and energy savings.

In the following sections we introduce the individual papers presented in this special issue, the original motivations, the methodology employed and the main results.

Enabling public participation in Strategic Environmental Assessment: an application of Multicriteria Analysis

Marta Bottero, Valentina Ferretti, Giulio Mondini, Silvia Pomarico Background: In the last decade, the design and the implementation of public policies in the domain of sustainable development has determined a rapidly growing interest for techniques of environmental impact assessment aimed at better linking the decision-making process and the opinions of the different stakeholders involved. Methodology: This paper proposes a Multiple Criteria Decision Aiding method to support the preparation of a land-use plan for a small town in Italy, aimed at helping Decision Makers (DMs) to integrate the different opinions of the stakeholders (public authorities, citizens, associations, etc.). In this context, the Analytic Network Process (ANP) represents a theory of relative measurement on absolute scales of both tangible and intangible criteria based on both the judgement of experts and on existing data. The application of ANP has focused on the evaluation of the importance of the different areas with respect to the development of the Municipal Plan based on the information coming from the participatory process that had been conducted to develop the plan. Results: Starting from the most relevant planning goals of citizens (and the actions that citizens would like to see implemented to achieve those goals), the municipal territory has been divided into five strategic areas according to their importance for the future development of the town. It has been shown that, via the ANP method, it is possible to achieve a transparent and traceable decisionmaking process facilitating deliberation, supporting communication with the DMs and granting a mutual understanding among stakeholders.

The Green-Game: accounting for device criticality in resource consolidation for backbone IP networks Aruna Prem Bianzino, Jean-Louis Rougier, Claude Chaudet, Dario Rossi Background: Telecommunication infrastructures are responsible of a large part of the carbon offprint of ICT technologies. As a consequence, the research community is showing an ever increasing interest in studying techniques and algorithms to reduce the energy consumption of ICT systems. Methodology: In this paper, the problem of reducing power consumption in backbone networks is studied according to an energy-aware routing approach and keeping into account the contribution of devices in the network to provide a good quality of service (QoS) level. Different models have discussed, with the objective to summarize the contribution of devices using power indices (and in particular the Shapley value) of particular coalitional games defined over the set of the elements of a backbone network. Such coalitional games incorporate the information about the network structure (e.g., the connectivity of sub-networks), the amount of traffic that the devices are routing and the network robustness (i.e., possible failure scenarios). Results: The ranking provided by the Shapley value of such games has been used to drive a resource consolidation process, i.e., the selection of those devices that should be switched off first in order to reduce the energy consumption keeping into account the expected global performance of the network. Such a ranking has been compared with different ranking methods on a realistic network scenario. The results yielded by this comparison show that the Shapley value ranking may provide high energy savings with a smaller impact on the expected QoS levels on the network.

On the role of coalitional network games in modelling the social dimension in ecosystem management

Stefano Moretti

Background: Most of the computational approaches in the literature concerning the management of natural and social systems have not been rigorously validated using empirical data. For example, models based on Multi-Agent Systems (MASs) have been widely used to compare different self-governance policies aimed at avoiding the collapse of environmental resources, but they present huge difficulties to be validated on complex scenarios dealing with large social interaction datasets. Methodology: This paper discusses two important families of indicators for social-ecological systems: Resilience, aimed to analyse the ability of social ecosystems to absorb changes on both human and ecological variables, and Social Capital, intended as an assessment of the set of all those human relations which are important for the sustainability issue. In particular, the paper presents a review of the main applications based on coalitional games that model these two fundamental indicators for social ecosystems on a micro scale, and they have proved their feasibility and efficiency even on macro situations, where the number of nodes of networks scales up dramatically. Results: The paper discusses several models from the literature of coalitional games aimed at measuring multi-dimensional social attributes of Resilience and Social Capital. Such models could be empirically tested within the MASs framework in order to verify their ability to predict the evolution of social ecosystems.

Several numerical examples from different fields of network analysis are provided.

Energy-constrained mean field games in wireless networks

Hamidou Tembine

Background: Cognitive radios are wireless devices that can autonomously and dynamically adapt the characteristics of their transmissions (waveform, energy, channel, spectrum, networking protocols etc.) in order to better serve the primary users (PUs), who are those licensed to use the spectrum. However, cognitive radio aims at accommodating the spectrum access of extra radio devices referred to as secondary users (SUs) under the constraint that PUs have to be as little affected as possible by the presence of SUs. Moreover, due to intrinsic characteristics of dynamic spectrum access, cognitive radio networks are very vulnerable to malicious attacks that may concern both PUs and SUs. Methodology: This paper proposes a feedback strategy against the attackers and focuses on exploiting mobility and energy consumption by considering wireless networks where users are distributed in an Euclidean space and have a limited energy, and where SUs decide their transmission power depending on their remaining battery state and the signal strength of PUs. The modelling approach is based on mean field games, that are used to define and evaluate macroscopic properties of dynamic wireless networks, representing wireless networks as a collection of interacting nodes characterized by dynamic random processes in an Euclidean space. Results: The proposed mean field framework allows for the description of the evolution of the density distribution and the associated performance metrics using coupled dynamical equations. Explicit formulas and algorithms are given for the management of the power of PUs and SUs keeping into account attackers' strategies. A complete characterization of the optimal distribution of the energy and of the payoff function (based on the Shannon information rate) is provided.

Future Directions

At the end of the nineties, the Guest Editors of this special issue were involved in a project concerning the analysis of cost allocation methods for waste management (Moretti [START_REF] Moretti | A model for cooperative inter-municipal waste collection: cost evaluation toward fair cost allocation[END_REF], Moretti and Patrone [START_REF] Moretti | Cost allocation games with information costs[END_REF]). The project was aimed at studying the actual protocols used by consortia of Italian municipalities to share the costs of waste collection and disposal, and at comparing those protocols with the allocation rules proposed by cooperative game theory. It turned out that the rules used for cost allocation were very simple, and relied on the proportionality criterion (Moretti and Patrone [START_REF] Moretti | Cost allocation games with information costs[END_REF]) applied, respectively, to population size, quantity of waste and surface of the municipalities (or, in some cases, to a linear combinations of these values). Moreover, the project pointed out to the difficulties of finding real data on waste management, and to the role played by costs related to the collection of the relevant data for building a game, due to the fact that those data were somehow estimated from very partial and incomplete informations.

Nowadays, things seem quite different, at least with respect to the availability of data on waste management. In fact, due to an increasing demand of transparency for public policies, associated with a substantial improvements of distributed technologies for the continuous data monitoring and sharing on several waste operations, an impressive number of large datasets about all stages of the waste life-cycle can be used to retrieve a very detailed account of the production, the collection and the disposal of waste over broad regions. By means of sensors inside trash containers, for instance, researchers my monitor on real time several variables related to the dynamic of waste production (Vicentini et al. [START_REF] Vicentini | Sensorized waste collection container for content estimation and collection optimization[END_REF]); the integration of several technologies of geographical positioning with geographic information systems allows users to continuously monitor the location of collection trucks via web-based applications (Arebey et al. [START_REF] Arebey | Integrated technologies for solid waste bin monitoring system[END_REF]).

However, even if it seems more and more clear that game theoretic models (and in particular the cooperative ones) can be potentially used to analyse the interaction among several stakeholders and the stability of agreements on fair allocations of costs arising from waste management systems (Karmperis et al. [START_REF] Karmperis | Decision support models for solid waste management: Review and gametheoretic approaches[END_REF]), still we have no evidence of cost allocation protocols based on game theoretic considerations that have been implemented in the standard practice of waste management. Fifteen years after the beginning of the project, it is then reasonable to ask whether the absence (with a few exceptions) of game theory in the standard practice of cost sharing in waste management is due to different reasons, other than the cost of collecting the pertinent information. Are there other cost items affecting a concrete application of game theoretic models? Are those cost items specifically referred to waste management, or they are connected to more general problems dealing with the analysis and the elaboration of large environmental datasets?

The idea of editing a special issue of Strategic Behaviour and the Environment devoted to ICT-based interaction situations, was partially inspired by the purpose of addressing the above questions. Now, the picture that emerges from the papers collected in this volume point out that the problem of computational complexity related to games can be to some extent overcome by the use of approximation techniques aimed to make good estimation both of the structure of the game and of the solution concepts associated with it (see, for example, Bianzino et al. [START_REF] Bianzino | The green-game: Accounting for device criticality in resource consolidation for backbone ip networks[END_REF] and Moretti [START_REF] Moretti | On the role of coalitional network games in modelling the social dimension in ecosystem management[END_REF]). On the other hand, looking at the different applications of game theoretic models in the literature, it seems that the environmental applications of game theoretic models on real data are still in an embryonic stage, despite the great interest shown for these applications by the scientific community (Bahn et al. [START_REF] Bahn | Stability of international environmental agreements: an illustration with asymmetrical countries[END_REF], Parrachino et al. [START_REF] Parrachino | Cooperative game theory and its application to natural, environmental, and water resource issues: 3. application to water resources[END_REF]).

In addition, nowadays, the possibility of quick access to large databases on various environmental issues, has made more evident the difficulties related to the game theoretic modelling process through the data or, more precisely, associated with the methods used to transform the big amount of information available in modern databases in the proper language of game theory. In other words, it seems that game theory still lacks the analytical substrate that has allowed, for example in biology and medicine through the specialization of informatics and statistics, to translate the large masses of data provided by new experi-mental technologies to intelligible information in the light of modern biomedical theories. For example, as more and more detailed records on the behaviour of individuals in interaction situations are registered, such as in social, ecological or business networks, it is not at all clear how statistical or data-mining techniques can be used, in general, to aggregate this data at the level of larger communities (or, conversely, if only the information on communities exists, how to disaggregate it at the level of interaction between subgroups or individuals) (Kleinberg et al. [START_REF] Kleinberg | A microeconomic view of data mining[END_REF]). Furthermore, due to the large dimension of environmental datasets, the applications of game theoretic models to the practice of environmental management need specific techniques of data processing that should be performed in an automatic or semi-automatic way (Elovici and Braha [8]).

In conclusion, the lesson learned from the collection of papers presented in this volume, tells us that the effective practice of game theory in the management of environmental issues (using models that are able to cope with the ever increasing availability of data concerning the interaction of the different actors involved) can not -and should not -disregard a good definition of general rules aimed at transforming (in a systematic way) the observed data into usable information in the context of a game. As it was previously the case for other disciplines, such as for biomedical applications or experimental physics, the definitions of these general rules should take place in parallel with the definition of statistical and data processing protocols specifically tailored to the language of game theory.

Acknowledgements

The Guest Editors are grateful to the Editors-in-Chief of Strategic Behavior and the Environment for their very professional support and for the opportunity of organizing this special issue. The Guest Editors wish to express their sincere gratitude also to all the anonymous referees for their prompt cooperation and the time spent to review the papers collected in this book, and to Zac Rolnik for his very important editorial assistance. Last but not least, a special thank is dedicated to all the authors of the contributed papers, for their professionalism and the very high quality of their contributions.