Whatever Happened to Dignity? The Politics of Citizenship in Post-Revolution Tunisia

Nadia Marzouki At this time, near the end of 2021, the prospects for social justice and democracy in Tunisia are quite bleak. In the aftermath of President Kais Saied's coup of July 25, 2021, the state of power relations among political parties and civil society organizations is uncertain.

Most of the well-known political players-such as the Islamist party Ennahda, the Tunisian General Labor Union (UGTT,) former regime politicians and military officials-have settled back into a common modality of political strategizing in Tunisia: the politics of waiting, a political strategy of buying time. [START_REF] Meddeb | l'Attente comme Mode de Gouvernement[END_REF] In the economic realm, the waiting mode of governance has meant that Tunisian governments-before and after the revolution of 2011-prefer ad hoc projects aimed at periodically providing employment to a disgruntled group rather than comprehensive economic restructuring. Politically, the waiting strategy has manifested in a tendency toward conflict avoidance shared by the main political parties, who were aware of the high cost of abrupt political moves under a repressive and authoritarian regime. After the revolution, the strategy of buying time pointed to a shared awareness of the fragmentation of political forces. More fragmented than ever since this year's coup, Tunisian political players are still waiting and hedging their bets.

Since the popular uprising that began in late 2010 and succeeded in ousting former president Zine El Abidine Ben Ali, many analysts of Tunisian politics have framed the complex processes at play in terms of success and failure. By focusing on evaluation and prediction they overemphasize specific junctures and decisions. For example, the academic literature is overflowing with discussions of whether Ennahda acted as an obstacle or a contributor to democratization. Analysts have singled out actors and variables that are presented as key to the democratic transition at the expense of examining more informal processes and less legible initiatives. A whole range of social realities are deemed unworthy of analysis because of their inability to induce immediate changes in political institutions.

With a few notable exceptions, scholars of Tunisia have devoted relatively less attention to groups that played a key role in the December 2010 and January 2011 uprisings. [START_REF]Couscous: Les Graines de la Dignité (Inside Production[END_REF] These actors include the youth that were injured and killed when they took to the streets in the first months of the revolution (known in Tunisia as the martyrs and wounded of the revolution), the unemployed graduates and the impoverished of the poorest regions of the hinterland, far from the more affluent coastal cities. Scholarly neglect echoes the unease with which successive governments have dealt with these marginalized groups. The selfimmolation by 26-year-old Neji Hefiane on September 6, 2021, after the government failed to offer him any compensation for his injuries incurred during the 2011 uprisings, epitomizes the enduring despair of the youth that sacrificed the most for the revolution.

Rather than isolating a single causal relationship as a possible indicator of the success or failure of democracy, I examine the changes that occurred in the last decade through the lens of the demand for dignity (karama) that triggered the 2010 uprisings. Whatever happened to dignity as a political claim and project? Initially, protesters translated dignity claims into concrete attempts at imagining alternative polities through a practice of reappropriating and cleaning spaces. Saied's populist politics later capitalized on the collective demand for dignity as cleanliness, while at the same time re-signifying it toward an exclusionary project of purity (nathafa). But the fluidity of a politics of dignity, and the apparent vulnerability of such a politics (being easily derailed toward a politics of purification) is also paradoxically what makes it an enduring resource for democratic revival.

Reclaiming and Cleaning Spaces

The uprisings that led to the fall of Ben Ali's rule rejected the authoritarian bargain that had governed state-society relationships for decades in Tunisia. Exchanging obedience for a mirage of social services and stability was no longer good enough. Protesters who took to the streets in December 2010 and January 2011 demanded a reconfiguration that would affirm Tunisians' rights to both economic prosperity and political freedoms. The demand for dignity that was consecrated in the 2014 Constitution acknowledges distinct rights (such as employment, free speech and gender equality) as part of a broader attempt at redefining the foundation of Tunisian politics. The preamble of the Constitution defines the uprising as a "revolution for freedom and dignity." Article 4 declares the motto of the Tunisian republic as "freedom, dignity, justice and order." Saied's decision, on September 22, 2021, to suspend the Constitution and to extend his ability to govern by decree put a halt to this remarkable effort to consecrate through law the concept of dignity. The daunting work of the Truth and Dignity Commission (TDC), established in 2014, was one attempt at institutionalizing the concept of dignity in the postrevolution political order. This independent institution, chaired by human rights activist Sihem Bensedrine, was tasked with investigating human and political rights violations perpetrated by state representatives since 1955 with the goal of bringing corrupt officials and perpetrators to justice and organizing compensation to victims. The TDC was subjected to violent smear campaigns and hostility from political forces linked to the former regime, such as the party of late president Beji Caid Essebsi, Nida Tounes. As a 2016 International Crisis Group report explains, the coalition governments at the time "favored selective amnesia over remembrance," notably in the realm of social and economic justice. The Economic Reconciliation Bill that was signed into law in the summer of 2017, for example, effectively granted amnesty to corrupt businessmen in exchange for their commitment to invest in impoverished regions.

Yet, dignity has inspired a whole range of practices that have often remained below the radar of scholarly analysis, which prioritizes the study of institutional reform, elections, constitutional drafting and party politics. In addition to the legal discussions that have taken place around the work of the Constitutional Assembly and the TDC, dignity has been the underlying principle in a new set of practices by citizens aimed at claiming sovereignty over specific spaces. But affirming dignity in these cases is not simply an acknowledgment of an abstract set of rights. Rooted in vivid memories of injury and humiliation, like the ones recounted by witnesses to the TDC, these new practices call out rulers' shameful and contemptuous behavior (hchouma/ar) and decry corrupt and scornful governance (hogra and fasad). The politics of dignity has manifested over the last decade in the multiple daily protests for a wide range of quotidian causes such as decent housing, clean streets, garbage collection and access to water not only in the capital Tunis but also in Sfax, Bizerte, Gabes and Medenine. It includes local campaigns for clean and usable sidewalks, occasional protests against neighborhood gentrification, protests against the lack of water, mobilizations against street harassment such as AnaZada ("me too"), movements for land ownership such as the mobilization for the land of the Jemna Oasis and sit-ins against garbage trafficking between Italy and Tunisia. All these initiatives point to the emergence of a new form of moral activism that suggests dignity claims can be seen as claims to space. The grassroots campaign Winou El Trottoir (meaning "where is the sidewalk") launched in 2014, advocated for clean, walkable sidewalks. The NGO Velorution offers cycling classes to historically marginalized socioeconomic groups such as young women from poor neighborhoods on the outskirts of Tunis. In October 2016, the musician Lotfi Gharbi placed a piano in a public space in the city of Bizerte that was full of uncollected garbage. He performed with a mask to protect himself from the smell.[3] After his performance, officials from the city council sent trucks to clean up the area.

Participants in these initiatives cast their moral claims as spatial claims. Dignity is not an abstract idea, but a rooted project aimed at cleaning up and appropriating spaces. The transformation of major squares of Arab cities (such as Tahrir in Cairo) into places of resistance, and the politicization of symbols of the revolution and the counterrevolution, was a hallmark of the uprisings in 2011 and attracted significant scholarly interest. [START_REF] Dakhli | L'Esprit de la Révolte[END_REF] The practices of dignity referred to in this article belong to this broader set of struggles to assert citizenship through the occupation of squares. But more specifically, they build on the remarkable vitality with which rank and file citizens during and after the 2011 uprisings reappropriated streets, neighborhoods, coffeeshops and markets and turned them into fora for discussions, teaching and improvised artistic performances.

Citizens engaged in these initiatives do not necessarily see themselves as protesters engaged in acts of resistance. Instead, cleanliness, safety, integrity and wholeness are some of the key concepts that inspire their actions. The idea of wholeness is understood to be both metaphorical, as in a sense of integrity in contrast to state corruption, and literal. For example, the nascent movement around food sovereignty launched by the Tunisian Observatory for Food Sovereignty and the Environment (OSAE), directed by geographer Habib Ayeb, aims to raise awareness around the environmental damage done to agricultural lands, such as in the Gabes oasis. The Observatory criticizes the "industrial-dedevelopmentalist [model], which dumps the costs of production on the poor of the oasis" and seeks to re-introduce more sustainable and traditional ways of cultivating agricultural land. [START_REF] Ajl | What Lasted for 3000 Years Has Been Destroyed in 30:' The Struggle for Food Sovereignty in Tunisia[END_REF] These local and grassroots acts of reappropriation call out state corruption and contempt and resonate with similar practices of cleanliness throughout the Middle East and North Africa. As early as 2011, Egyptians who cleaned up Tahrir Square paved the way for this trend. As shown by Jessica Winegar, "The physical cleanup was highly symbolic of the larger drive to 'purify' the Egyptian government, ridding it of the kind of mismanagement, discrimination and corruption that characterized the sanitation crisis." [START_REF] Winegar | Taking Out the Trash: Youth Clean Up Egypt after Mubarak[END_REF] The revolution of staircases in Algeria (a series of grassroots initiatives aimed at cleaning and painting public staircases), the 2015 You Stink Movement in Beirut and local efforts to embellish public gardens in Istanbul are a few key examples of how this politics of dignity has unfolded as a project of cleaning spaces and fighting corruption, which is seen as encompassing a range of harmful economic, political and moral choices.

The diverse socioeconomic groups participating in these mobilizations cut across traditional ways of segmenting the societies of the region along gender, racial, sectarian and ideological lines. For example, youth and women have played a prominent role in these initiatives, alongside the elderly, peasants and people with disabilities. While it is too early to determine whether these initiatives of solidarity-in-difference constitute a new shared identity that could potentially translate into mass movements, they do create shared norms and a new grammar of collective action.

These initiatives have contributed to imagining new possibilities of togetherness that are situated outside of the frameworks inherited from the postcolonial authoritarian state order. Practices of dignity as cleanliness have carved out a normative space independent from the Islamist-secularist divide that has organized most of the political competition since 2011. They are "a-secular" in the sense that Hussein Agrama gives to this notion: They fundamentally do not care about where to draw the line between religion and politics. The key divide in this grammar of dignity is the one between cleanliness and economic and political corruption. To the extent that making garbage and waste visible is an integral part of this politics of cleanliness, it creates a new understanding of the sacred that combines the claim of purity and the reality of filth, the strength as well as the destitution of the dwellers of such polluted neighborhoods. These initiatives constitute a form of messy sacred that includes pollution and impurity rather than erases it and that brings together participants from a variety of backgrounds. While these movements have a remarkable capacity to invent new tactics of cleaning, reordering and reappropriating spaces, they have left aside goal-oriented strategic planning for the medium and long term. They cultivate the uniqueness of the now, rather than strategize about the hypothetical transformation of the future. While governments have proven repeatedly unreliable, and the state seems to be collapsing, these movements have understandably prioritized concrete and immediate actions over a grand strategy of coalition building aimed at state capture. There is something post-apocalyptic about such movements. The participants see themselves as active witnesses affirming their citizenship rather than agents of revolution, and their initiatives are prophetic rather than strategic.

A Politics of Purification

The measures taken by Saied since July 25, 2021, such as suspending parliament and the government and freezing parliament members' immunity, represent a different type of reply to the popular dignity demands. A former law professor with no history of participation in opposition movements against the Ben Ali regime, Saied was elected in the 2019 presidential election thanks to a campaign in which he promised to better defend the people's will, to struggle against corruption and the pitfalls of shadowy politicking. He garnered the support of large segments of the youth-from disfranchised areas and from the educated middle class-that were disillusioned with the politics of rotten compromise that had governed alliance making up until 2019. He was also supported by businessmen, intellectuals and politicians who saw in him an alternative and an obstacle to their muchloathed enemy, Ennahda.

Saied argued that the reversal of the previous political order was the only way to address the peoples' growing frustration with elite politicking, parliamentary disorder and the dire economic situation. In his view, his predecessors had failed to properly fight corruption. As noted by Mohamed-Dhia Hammami, Saied had expressed his political vision on many occasions before the coup, in particular his distrust of political parties and his willingness to amend or suppress the 2014 Constitution. In June 2021, he called for "a new political system and a real constitution, because this constitution is based on putting locks everywhere and institutions cannot proceed with locks or deals." His ideal of concentrating power in the presidency and establishing locally elected councils to decentralize and fragment national representation overtly go against the efforts, enshrined by the 2014 Constitution, at building a representative democracy with a system of checks and balances between the parliament and the government. The myth of the people's will, for which Saied claims to speak, supplants the lexicon of the 2014 Constitution's phrase "we deputies of the people." Accordingly, in Saied's discourse, the concept of sovereignty that he claims to defend, has taken precedence over the idea of dignity. In a speech he made on August 24 to justify prolonging the measures of exception, Saied contended that not only the parliament, but also institutions in general are a threat to the state. He claimed that "The existing political institutions, as they used to function, are a persistent threat to the state. The parliament itself is a threat to the state." Saied's populist distrust of institutions and the plebiscite logic of ruling, based on referenda rather than election, introduces a grammar of dignity that is opposite to the local initiatives aiming at re-appropriating spaces. His discourse and policy express a willingness to purify politics that is very different from the form of messy sacred described earlier. In Saied's logic, the purity (as nathafa) of the political body is a precondition for people's dignity (as karama). For Saied, the entirety of political activity comes down to a practice of erasing corruption, rather than exposing and confronting it. Yet, when the very institutions tasked with documenting and investigating corruption scandals have been suspended, the struggle against corruption becomes a process of eliminating political pluralism and competition. Saied's personalization of the anti-corruption agenda can be seen in the arrest of MP's who criticized him and their trial in front of a military court as well as his decision to provisionally shut down the National Anti-Corruption Agency (INLUCC).

From Transitional Justice to Transactional Justice

A significant factor underlying the distortion of the grassroots politics of dignity into a populist politics of purification is the curtailing of the project of transitional justice. In lieu of the comprehensive process of reparation and accountability after the revolution necessary for transitional justice, ruling coalitions and political parties have agreed on a politics of transactional justice that prioritizes political settlements with the same corrupt business elites and perpetrators of violence. [START_REF] Hibou | l'État d'Injustice au Maghreb[END_REF] The rationale behind this choice was that maintaining stability and national unity is too important and cannot be jeopardized by a divisive and emotional series of trials. The high hopes and expectations Tunisians expressed when the TDC organized its first hearings in November 2016 have remained largely unmet. Compromise and pragmatic coalition building have allowed for a peaceful and widely celebrated political transition after the ouster of Ben Ali, but it has also weakened the willingness of key political players to defend a process of transitional justice that would hold accountable corrupt economic and political leaders and violent perpetrators of torture.

The Economic Reconciliation Act adopted in the summer of 2017 illustrates the shift from transitional to transactional justice. The law granted amnesty to corrupt civil servants as long as they had not personally benefited from embezzling public funds. The law triggered a wave of protests and critiques from grassroots organizations such as Manich Msamah ("I will not forgive") as a step backward in the Tunisian democratization process. [START_REF] Miller | I Do Not Forgive!' Hope and Refusal in Tunisia's Democratic Transition[END_REF] The amnesty law was deeply offensive to the memory of the 300 dead and more than 2,000 wounded of the 2011 uprisings. Despite these critiques, then-president Essebsi contended that the law was necessary to create a context of trust that might help bring back investors. The 2017 reconciliation law not only made the public relations campaign against corruption launched by the government in August 2016 less credible, it also caused further disappointment due to the lack of strong opposition by Ennahda to the underlying approach of the law (31 of their 117 deputies at the time voted for the law). Although there are many factors at play in the current political stalemate in Tunisia, the massive disappointment created by this shift toward transactional justice facilitated the acceptance of, and even increased support for, Saied's discourse of purification. This context partly explains why he managed to garner the support of wide segments of disfranchised youth alongside that of the anti-Islamist establishment. Former members of Manich Msamah themselves-who had mobilized and organized against the various drafts of the amnesty bill since 2015-expressed solidarity with the anti-parliamentary discourse of Saied. Their attitude is not one of blind obedience, but strategic support for measures seen as disqualifying Islamists and the parliament that they blame for mismanaging the economic crisis since 2011 and the Coronavirus health crisis since 2019. Saied's nationalist rhetoric about the defense of the nation's sovereignty against foreign politicians who call for a return to the rule of law also resonates with the anti-imperialistic and anti-colonial arguments of some segments of the leftist youth.

Ironically, it is unclear whether the concrete measures taken by Saied against corruption will depart from the pragmatism of his predecessors and if the groups that aligned behind him will be disappointed, yet again. On August 16, 2021, he issued travel bans for businessmen and parliament members suspected of corruption to show his commitment to the politics of purification. But he also announced a legislative initiative aimed at approving a political quid pro quo in which businessmen accused of corruption receive amnesty in exchange for a promise to invest in historically marginalized regions. Adopting a personalized logic of blaming and shaming, he also encouraged them to reduce prices of goods and help Tunisians during these exceptional circumstances. In other words, it is unclear whether Saied will be able to break with the practice of compromise, patronage and deal making against which he manufactured a coup.

"We Are Choking Under All This Garbage"

Ten years after Tunisians ignited an unprecedented wave of democratic protests throughout the Middle East and North Africa, their attempts to institutionalize a politics of dignity, notably through the work of the TDC, have led to unfinished outcomes and have been largely hijacked by the transactional deals of the former regime and the Islamist Ennahda party. At the grassroots level, the politics of dignity has translated into a wide range of initiatives and mobilizations. Yet, to the extent that the grassroots politics of dignity is not primarily about alliance building and state capture, it leaves open the possibility for its own instrumentalization and distortion into a politics of purification in the service of a populist agenda.

The path from karama to nathafa is not, however, inevitable. The multiple manifestations of the politics of dignity at the grassroots level suggests that there is not a single path but a polyphony of possible arrangements around these two moral grammars of dignity and purification. While the non-institutional aspect of the politics of dignity may have helped the emergence of the populist discourse of purification, it is also this very aspect that has the potential to empower mobilizations that could topple the populist and anti-constitutional regime that Saied seeks to establish.

One of the most vivid challenges to Saied's rule has been the mobilization against the reopening of the El Guenna toxic landfill in Agareb, a small town in the Sfax region. The landfill had been closed due to complaints of environment pollution and negative health effects from the local population. On November 9, 2021, angry protesters burnt the local national guard station. "We are choking on all this garbage!" was one of the main slogans of the day's protests. Saied's first response was to deploy the military to tame the protests. Yet, the protests continued. The UGTT called for a one-day general strike on November 10. In response, Saied changed course and agreed to meet with a delegation of environmental activists from Agareb calling themselves "I am not a landfill." The Agareb uprisings are just one set of protests, which alongside other mobilizations from Sousse to Bizerte make up a loose, ongoing #Iamnotalandfill ش( ي ان )#مصب_م campaign. These initiatives suggest that citizens' concern for a clean and sustainable environment still drives dignity protests, which may have the power to counter the populist agenda of purification.

While Tunisia has been alternatively praised as a model of successful democratization and dismissed as a failed one, it continues to be an intriguing laboratory for the invention of new forms and meanings of political life. The versatility of the politics of dignity, with the multiplicity of political outcomes it potentially enables, is one of the most humbling and intriguing sites of investigation for the analysis of future political developments in Tunisia. While it renders obsolete the overstudied paradigm of the Islamist-secularist opposition, it encourages observers to think beyond the main divide of the present moment-between populist and rule-of-law approaches to democracy-and to suspend the social scientific obsession with measurable process and outcomes. The history of the Tunisian politics of dignity, with the shift from karama to nathafa, suggests that the affirmation of citizens never means the complete disappearance of subjects being subjected to power. The tensions between multiple appreciations of purity and impurity give shape to competing and sometimes overlapping projects of citizenship or subjectification. [START_REF]Citizenship After Orientalism, an Unfinished Project[END_REF] Remaining aware of this layered temporality and the inherent multiplicity of the politics of dignity provides supporters of the revolution hope for the next chapters of the perilous Tunisian transition.


Cédric Mathieu De Vienne
email: cedric.mathieudevienne@supergrid-institute.com

Pierre Lefranc
email: bpierre.lefranc@g2elab.grenoble-inp.fr

Besar Asllani
email: cbesar.asllani@supergrid-institute.com

Pierre-Olivier Jeannin
email: dpierre-olivier.jeannin@g2elab.grenoble-inp.fr

Bruno Lefebvre
email: ebruno.lefebvre@supergrid-institute.com

Experimental investigation of a 10 kV-70A switch with six SiC-MOSFETs in a series-connection configuration

Keywords:

In this paper, a prototype of 6 series-connected SiC-MOSFETs for double pulses test purpose is presented. This prototype represents many challenges amongst which the measurements of voltage sharing and the EMI issues due to high dv/dt. High voltage tests are performed up to 8 kV at 65A and voltage balancing, losses and switching speeds are evaluated. Switching speed up to 520 V/ns are recorded.

Introduction

In the context of the development of converters in the range of mega-Watt for grid applications with voltage ratings up to tens of kV, SiC-MOSFETs are considered as a promising option. Seriesconnection of MOSFETs has similar conduction resistance at on-state compared to single High-Voltage MOSFET for the same breakdown voltage and the same active chip area [START_REF] So | 15-kV/40-A FREEDM Supercascode: A Cost-Effective SiC High-Voltage and High-Fr q Pow r Swi[END_REF]. Switching performances are expected to be high. Unfortunately, some solutions applied to reach accurate voltage balancing decrease the switching speeds. Switching performances remain highly better than highvoltage Si-IGBT allowing switching frequency up to hundreds of kHz.

To reach a breakdown voltage of 10kV, 6 SiC-MOSFETs (1.7 kV 45mΩ from Wolfspeed) are connected in series. These MOSFETs have a breakdown voltage above 2.2 kV allowing to safely work at 8 kV with 6 MOSFETs. In order to evaluate performances of series-connected SiC-MOSFET with the simplest and reliable drain-source voltage balancing, a prototype is developed with passive snubbers. The focus is made on voltage balancing, gate-drivers supplies and measurement issues. This prototype is designed for 10 kV and tests are performed up to 8 kV. Fast switching speeds are obtained as expected with SiC-MOSFET devices leading to significant common mode current through insulation barriers and measurement equipment.

Voltage Balancing

Snubber circuits. Snubber circuits (Erreur ! Source du renvoi introuvable.) are commonly used to optimize switching trajectory and damp the voltage and current ringing but also to balance voltage across devices connected in series. Snubbers are simple, robust, cheap and simple to implement. In some cases, it is relevant to use a resistor-capacitor-diode (RCD) snubber to charge the capacitor through the diode reducing losses in the resistor at turn-off and then discharging the capacitor at turnon into the resistor. RCD snubber allows dynamic voltage balancing of series-connected switches since it creates a capacitor voltage divider bridge. Static voltage balancing is then obtained thanks to parallel connected resistors that compose a voltage divider.

Selection of Csn capacitance is different depending on the snubber objective. If the objective is to damp overvoltage then Csn values is selected to be large enough to store the energy of the stray inductance [START_REF]A i v s ig io i o ff s of g riv resistance on the losses of the MOSFET-snubber-io o fig r io[END_REF]. Otherwise, if the goal is to balance voltages, the capacitor value is usually selected to be 5-10 times larger than output capacitance value at high voltage of the switching devices [START_REF] Al Oiz | P rform v l io of s ri s o 1700V Si MOSFET vi s[END_REF].

Prototype. The series-connection of MOSFETs is built on a PCB that allows fast and cheap manufacturing. MOSFETs and snubber diodes are soldered as surface mounted components by bending pins in this way there is no holes breaking epoxy isolation. Regarding gate-driver circuits, push-pulls with gate resistors are placed as close as possible to MOSFETs on the same PCB. Gate driver power supplies need high voltage insulation. Bespoke 15kV insulated flyback (with 9pF of isolation capacitance) are used to supply multiple gate drivers as described on Fig. 3. Each gate driver has small commercial isolated DC-DC converter. Gate signals are insulated thanks to Plastic Optical Fiber (POF). Gate-driver signals can be manually synchronized with nanosecond precision on the optical fiber emitter board that has been specifically designed.

Double pulse test. The test bench is designed to perform short circuit and double pulse tests up to 10kV. The modularity of this platform means that the stray inductance is not optimized. Total loop inductance is estimated at 400 nH. The half-bridge is built with one or two SiC 10 kV PiN diodes from SuperGrid Institute [START_REF] Asllani | Pl so , "10 kV sili o r i PiN io s-from sig o p k g ompo r riz io[END_REF] (used for freewheeling) on high side and the series connected MOSFETs on low-side. The load is a bespoke air-inductor with a value of 1mH.

Measurement. Current measurement is performed with a 25mΩ shunt resistor. Middle point voltage measurement is measured thanks to high-voltage passive probes P6015A (Tektronix) that have a 75 MHz bandwidth. However, these passive probes cannot be used to measure drain-source voltages sharing since they impact voltage balancing. Parasitic elements have a significant impact on switching behaviors [START_REF] Bl J Rg | mp -Oriented Transient Instability Mo li g of Si mosf r M s r m Pro s[END_REF] with high switching speed. Connected probes add small capacitance and common mode current path that impact deeply voltage balancing for this high switching speed applications.

One promising solution that is highlighted here is to use insulated differential probes. Such probes have no impedance between probe head and the oscilloscope because of the optical fiber isolation. Some parasites can remain by proximity of the probe head with the circuit. Measurement with high common mode voltage and dv/dt can be done in a non-intrusive way. The used probe is the Isovu differential probe by Tektronix with the reference TIVP02 (200 MHz bandwidth, ±2500 V differential voltage and ±60 kV common mode voltage).

Switching performances.

Balancing. Thanks to these measurements, the voltage balancing can be quantified. Voltage balancing percentage is defined as follows:

𝑈𝑛𝑏𝑎𝑙 𝑛 (%) = 𝑉 𝑑𝑠𝑂𝐹𝐹 𝑛 - 𝑉 𝐷𝐶 𝑁 𝑉 𝐷𝐶 𝑁 • 100 (1)
Where 𝑈𝑛𝑏𝑎𝑙 𝑛 (%) is the balancing at off state of the n th MOSFET. The most unbalanced MOSFET has the highest unbalancing percentage. Measurements of Fig. 4 show a maximum unbalancing percentage under 10%. The main voltage unbalance cause is parasitic capacitances though gate drivers supply insulation, other causes are minimized since components are selected, components are not heating and gate driver signals are synchronized.

Losses. An estimation of losses is made by post-processing of the voltage vds and current id. To get accurate results, delays between id and vds are compensated. At turn-off, the measured energy is Fig. 3 : Switching waveforms measurement at 8kV and 65A. EOFF = 13.5mJ. This energy is mainly due to snubber capacitors charging. With 2.2nF for each MOSFET, capacitance energy is:

𝐸 𝐶𝑠𝑛 = 𝑁 1 2 𝐶 * 𝑉 𝑑𝑠𝑂𝐹𝐹 2 = 6 2 2.2 • 10 -9 • ( 8000 6 ) 2 = 11.7 𝑚𝐽 (2)
The remaining 1.8mJ is lost in the snubber resistor and the MOSFET itself. It could be optimized by reducing the value of Rdsn. ECsn energy is dissipated at on-state.

Measured turn-on energy is EON = 62.5 mJ. Turn-on loss is then: ElossON = EON + ECsn = 74.2 mJ. EON value is mainly explained by the important current of the diode.

Switching speed. With ID = 65A at turn-off switching, voltage rising slope reach 170 V/ns which is well limited by snubber circuit. Contrary to turn-on switching, falling speed is not limited by snubber: voltage speed reaches 520 V/ns. At this speed, a significant common mode current passes through the insulation barrier and can potentially damage driver circuits. It could also have functional impact on gate driver circuit and they must be designed with a strong EMI immunity.

Conclusion

A prototype of 6 series connected SiC-MOSFETs is designed, built and tested in double test up to a bus voltage of 8kV. It represents a challenge on multiples aspects: voltage balancing, dielectric consideration, EMC immunity and measurements. Thanks to isolated differential voltage probes, it is possible to measure Vds of each MOSFETs in a non-intrusive manner. Balancing percentage is maintained under 10%, meaning that MOSFETs work in safe voltage condition with high switching speed up to 170V/ns at turn-off and 520V/ns at turn-on. Nevertheless, high switching speed represents an EMC risk. Electronic circuits must be designed with low parasitic capacitances (for insulation barriers) and strong EMI immunity and if not enough, switching speed must be slowed down, which unfortunately results in an increase of switching losses. Then snubber values can be optimized to get the best compromise between voltage balancing and switching losses. For this purpose, an optimization tool is under development to optimize the values of components of the snubber circuits based on accurate model of SiC-MOSFET and stray capacitances of the system.

Fig. 2 :

 2 Fig. 2 : Series connected MOSFETs with gate-driver supplies in a double pulse test bench.

Fig. 1 :

 1 Fig. 1: RC snubber and RCD snubber