ASIC Based Galvanically Isolated Driver Circuitfor the Use in Power Converters forPhotovoltaic Applications

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ASIC Based Galvanically Isolated Driver Circuitfor the

Use in Power Converters forPhotovoltaic Applications

Markus Niedermeier

To cite this version:

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ASIC Based Galvanically Isolated Driver Circuit

for the Use in Power Converters for

Photovoltaic Applications

Thesis in Joint Supervision

Submitted to:

the Faculty of Engineering of

the University Friedrich-Alexander of Erlangen-Nuremberg to obtain the degree of

DOKTOR - INGENIEUR

And to:

the Polytechnic Institute of Grenoble of the University of Grenoble

to obtain the degree of

DOCTEUR DE L’UNIVERSITÉ DE GRENOBLE

Presented by:

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As binational dissertation approved by: the Faculty of Engineering of

the University Friedrich-Alexander of Erlangen-Nuremberg And by:

the Polytechnic Institute of Grenoble of the University of Grenoble

Day of oral examination: 2014-10-15

Chairperson of Doctoral Examination Authority: Prof. Dr.-Ing. Marion Merklein President (University of Grenoble): Prof. Bertrand Girard

Thesis Supervisor (Germany): Prof. Dr.-Ing. L. Frey

Thesis Supervisor (France): Prof. J.-P. Ferrieux

President of Examination Board: Prof. Dr. R. Weigel

Reviewer (Germany): Prof. Dr.-Ing. L. Frey

Reviewer (Germany): Prof. Dr.-Ing. K. Helmreich

Examiner (Germany): Prof. Dr.-Ing. D. Fey

Reviewer (France): Prof. Bruno Allard

Reviewer (France): Prof. Stéphane Lefebvre

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ASIC basierte galvanisch getrennte

Treiber-schaltung für den Einsatz in

Leistungswand-lern für photovoltaische Anwendungen

Der Technischen Fakultät

Der Friedrich-Alexander-Universität Erlangen-Nürnberg

zur Erlangung des Grades

DOKTOR - INGENIEUR

vorgelegt von

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Als binationale Dissertation genehmigt von der Technischen Fakultät

der Friedrich-Alexander-Universität Erlangen-Nürnberg und

dem Polytechnischen Institut von Grenoble der Universität Grenoble

Tag der mündlichen Prüfung: 2014-10-15

Vorsitzende des Promotionsorgans: Prof. Dr.-Ing. Marion Merklein Präsident (Universität Grenoble): Prof. Bertrand Girard

Betreuer (Deutschland): Prof. Dr.-Ing. L. Frey

Betreuer (Frankreich): Prof. J.-P. Ferrieux

Gutachter (Deutschland): Prof. Dr.-Ing. L. Frey

Gutachter (Deutschland): Prof. Dr.-Ing. K. Helmreich

Gutachter (Frankreich): Prof. Bruno Allard

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THÈSE

Pour obtenir le grade de

DOCTEUR DE L’UNIVERSITÉ DE GRENOBLE

Spécialité : Génie Electrique

Arrêté ministériel : 7 août 2006 Présentée par

Markus NIEDERMEIER

Thèse dirigée par Jean-Paul FERRIEUX et Lothar FREY

préparée au sein du Laboratoire de Génie Electrique de Grenoble dans l'École Doctorale Electrotechnique Electronique Automa-tique et Traitement du Signal

Driver avec isolation galvanique basé sur

un ASIC destiné à des convertisseurs de

puissance pour les applications

photovol-taïques

Thèse soutenue publiquement le 15 octobre 2014, devant le jury composé de :

Prof. Dr. Robert WEIGEL Président (Allemagne)

Prof. Dr.-Ing. Lothar FREY Directeur de thèse (Allemagne) Prof. Jean-Paul FERRIEUX Directeur de thèse (France) Prof. Dr.-Ing. Klaus HELMREICH Rapporteur (Allemagne) Prof. Bruno ALLARD Rapporteur (France) Prof. Stéphane LEFEBVRE Rapporteur (France)

Prof. Dietmar FEY Examinateur (Allemagne)

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Abstract

The generation of electricity by photovoltaic sources plays a great role in the transition from fossil energy sources to renewable energy sources. It gained momentum especially since the introduction of affordable photovoltaic systems for private customers is accompanied by public subsidies. This leads to the application of photovoltaic systems in urban areas where conven-tional string topologies of photovoltaic arrays that are applied in solar parks are less efficient due to the effects of partial shading by the urban environment. This issue can be reduced by changing the photovoltaic array to a modular topology where each panel is treated individually and independently from the rest of the array. To achieve this modularity a converter topology needs to be developed that can be integrated into each solar module.

In the frame of this work a novel concept for a hybrid modular solar array was investigated. This concept included a converter technology that could be integrated into each solar module of the array. To reinforce the miniaturization aspect of the converter topology and with this to reduce the total volume of the converter, the increase of switching frequency and the use of high-electron-mobility-transistors (HEMT) was decided. For this application, the use of a HEMT based on gallium-nitride (GaN) that is commercially provided by EPC Corp. was selected. The focus of the thesis was to develop a novel integrated galvanically isolated driver concept that is able to drive the HEMTs included in the solar converter with a switching frequency in the megahertz range. A high-temperature CMOS technology provided by X-Fab Semiconductors AG was chosen for this purpose and a component library containing all cells required for the design of the driver ASIC was developed. For the driver ASIC, a core voltage circuit was de-veloped that provides a stable core voltage of 3.3V while being able to tolerate a wide range of supply voltages (5V - 18V) as well as temperature changes in the complete extended tem-perature range (-40°C - 175°C) of the high-temtem-perature ASIC technology (XA035 provided by X-Fab). To achieve the transmission of the switching signal over the galvanic isolation present in the module, several coupler circuits were evaluated and a novel modified Manchester coding algorithm was developed to reduce the complexity of the required coupling elements. This modified Manchester code enables the decoding and the rebuilding of the clock signal from just a single transmitted encoded signal and does not require the additional transmission of the original clock signal for the decoding process usually necessary in a great part of conven-tional Manchester decoder circuits. This allows the differential transmission of both signal and clock over the same two signal paths. Utilizing this code a galvanically isolated data transmis-sion circuit was developed that is able to transmit an encoded PWM signal at a frequency of up to 50MHz. Furthermore, the novel driver circuit itself was developed which is able to drive the used GaN HEMT at a frequency of 5MHz and was optimized to exhibit minimal delay be-tween input and output signal of the driver. This lead to the previously mentioned increase in switching frequency of the converter which was utilized to reduce its volume.

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Kurzfassung

Die Erzeugung elektrischer Energie durch Photovoltaik spielt angesichts des angestrebten Energiewechsels weg von fossilen Brennstoffen und hin zu erneuerbaren Energien eine be-deutende Rolle. Die photovoltaische Energieerzeugung befindet sich seit der Einführung er-schwinglicher Solarmodule gepaart mit staatlichen Subventionen besonders im privaten Be-reich im Aufschwung. Dies führte dazu, dass es auch im urbanen BeBe-reich zum Aufbau von Photovoltaikanlagen kam, wo sich die herkömmlichen Reihenschaltungs-Topologien, die in großen Solarparks zur Anwendung kommen als nicht sehr effektiv erweisen, was besonders auf auftretende Teilverschattung durch die urbane Umgebung zurückzuführen ist. Dieses Problem kann durch die Verwendung von modularer Topologien verringert werden, da hier alle Solarpanels individuell und unabhängig voneinander behandelt werden können. Um diese Mo-dularität zu ermöglichen muss eine Wandlertopologie entwickelt werden, die in einzelne So-larmodule integriert werden kann.

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Résumé

La production d’électricité par des sources photovoltaïques joue un grand rôle dans les dé-marches qui visent à remplacer progressivement les combustibles fossiles par des sources d’énergie alternatives. La production énergétique photovoltaïque est en plein essor depuis l’in-troduction de systèmes photovoltaïques qui sont abordables pour des particuliers, une évolu-tion à laquelle les subvenévolu-tions de l’Etat ont contribuée. C’est la raison pour l’installaévolu-tion de systèmes photovoltaïques dans des zones urbaines où les topologies conventionnelles de panneaux photovoltaïques en couplage série, qui sont appliquées dans les parc solaires, sont moins efficaces en raison des effets d’ombrage partiel par l’environnement urbain. Ce pro-blème peut être réduit par l’application d’une topologie modulaire où chaque panneau est traité individuellement et indépendamment du reste des panneaux de couplage. Pour parvenir à cette modularité une topologie de convertisseur doit être développée qui peut être intégrée dans chaque module solaire.

Dans le cadre de cette thèse un nouveau concept d'une installation solaire hybride modulaire a été étudié. Ce concept englobe le développement d'une topologie de convertisseur associé, qui peut être intégrée dans chaque module solaire du système solaire. Pour améliorer encore l’aspect de miniaturisation pour cette topologie de convertisseur et réduire ainsi le volume glo-bal du convertisseur, une augmentation de la fréquence de commutation accompagnée par l'utilisation de transistors à haute mobilité d'électrons (HEMT) a été fixée. Pour cette applica-tion, un HEMT sur la base de nitrure de gallium (GaN), qui est disponible dans le commerce auprès de la société EPC a été sélectionné. L'objectif de la thèse est le développement d’un nouveau concept d’un driver intégré avec une séparation galvanique qui est capable de con-trôler les HEMT du convertisseur solaire avec une fréquence de commutation de l'ordre mé-gahertz. À cette fin, une technologie à haute température, fournie par X -Fab Semiconductors AG a été choisie et une bibliothèque de composants contenant toutes les cellules nécessaires à la conception du pilote ASIC a été développée. Pour le driver ASIC un circuit de tension de base a été développé qui génère une tension core stable qui est apte à supporter sans pro-blèmes une large gamme de tensions d'alimentation (5V - 18V) qu’ainsi que les changements de température dans la gamme de température étendue (-40°C -175°C) de la nouvelle tech-nologie ASIC haute température XA035 fourni par X-Fab. Pour réaliser la transmission du signal de commutation à travers l'isolation galvanique, plusieurs circuits ont été évalués et un nouvel algorithme de codage Manchester modifié a été élaboré pour réduire la complexité des éléments de couplage requis.

Cela a permis la transmission différentielle de signaux de données et d'horloge sur seulement deux voies de signaux à séparation galvanique. En utilisant ce schéma de codage, une trans-mission de données à isolation galvanique a été développée, qui permet la transtrans-mission d’un signal codé PWM avec une fréquence allant jusqu'à 50 MHz. En outre, le circuit de driver lui-même a été intégré ce qui permet de contrôler le nitrure de gallium (GaN) HEMT utilisé avec une fréquence de 5 MHz avec un minimum de retard entre les signaux d'entrée et de sortie. Ceci a permis l'augmentation susmentionnée de la fréquence de commutation du convertis-seur et la réduction concomitante du volume global.

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Acknowledgments

This thesis and project SmartPV on which this thesis is based were a joint effort between the research institutes of Fraunhofer IISB in Erlangen, Germany, CEA Leti, Grenoble, France and g2elab, Grenoble, France. It was based on a framework provided by the heterogeneous tech-nology alliance HTA which is an alliance between Fraunhofer (Germany), CEA (France), CSEM (Switzerland) and VTT (Finland).

My thanks go to all people that made the opportunity of participating in this project possible for me but especially to Dr. Vincent Lorentz and Dr. Charles Fort who had a big part in the for-mation and organization of this project.

Special thanks go to both of my PhD advisors Prof. Lothar Frey in Germany and Prof. Jean-Paul Ferrieux in France who always supported me with helpful advice and assisted me with organizational issues with both universities to keep both sides of this binational undertaking satisfied.

Further thanks go to all my colleagues in both countries at Fraunhofer IISB and CEA Leti, who supported me during the course of this thesis. I appreciate all inspiring conversations we had during this project. I also appreciate the warm welcome during my first arrival in Grenoble provided by all colleagues at the CEA Leti, especially Dr. Cyril Condemine and Dr. Sebastian Dauve who also provided support in finding a suitable accommodation for my time in Grenoble. I also thank all my friends for their understanding and support during this thesis and providing the necessary counterbalance to my life outside of scientific work. Thank you for always keep-ing your sense of humor even in times when I was about to lose mine.

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1 Introduction

1.1 Outline

Chapter 1 serves as an introductory chapter to the thesis. In this chapter the organization and conventions of the thesis are defined. Furthermore the motivation that drove this work is de-scribed.

Chapter 2 gives a review as well as an overview of the state-of-the-art technologies used in photovoltaic systems. This includes the solar cell technology, solar array structures, maximum power point tracking algorithms, solutions to achieve galvanic isolation of the modules, com-munication protocols usable in powerline comcom-munication, examples of suitable DC-DC con-verter topologies and an introduction to high-electron mobility transistors (HEMT). The chapter also serves as a line of thought that determines the best solutions for a smart photovoltaic array topology and describes the focus of the thesis. The design of the integrated version of a galvanically isolated driver circuit for the use in photovoltaic DC-DC converters.

Chapter 3 describes in depth the process of the ASIC design. All cells necessary for the driver ASIC are defined in this chapter. Simulation results and information about the layout are pro-vided for each cell. Furthermore, included in the description of the signal transmission unit, a novel modified Manchester encoding algorithm is presented. The cells described in this chap-ter include the core voltage unit (consisting of the voltage-independent bias current circuit, the temperature-independent voltage reference circuit, an operational amplifier and a linear volt-age regulator circuit), the signal transmission unit (consisting of an encoder circuit for the mod-ified Manchester encoding algorithm, a decoder circuit and a signal conditioning circuit) and the driver circuit itself. The chapter concludes with the design process of the first prototype ASIC that was manufactured by X-Fab semiconductor AG.

Chapter 4 presents the results from the characterization of the first prototype of the driver ASIC. The chapter describes the test environment and setup which includes the packaging of the ASIC die and the design of a printed circuit board used for the measurements of the ASIC. The measurement results are compared with the expected results of previous simulations. Simul-taneously solutions for the increase in performance and redesign of underperforming cells are determined.

Chapter 5 provides information on the redesign process of underperforming cells and the cre-ation of additional or modified cells for the use in a second prototype ASIC. The design and redesign process of each cell is presented which includes simulation results and information about layout process. The chapter is concluded with the design process of the second proto-type ASIC again manufactured by X-Fab.

Chapter 6 presents the characterization of the second prototype ASIC analog to the process already described in chapter 4.

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1.2 Conventions

• In this thesis the decimal separator is represented by the dot (.) Example: 0.5 is the half of 1.

• The [number] given inside the square brackets represents a reference to the bibliog-raphy presented at the end of the thesis.

Example: [1] refers to an article by Gil Knier of the NASA Science division about the principle of photovoltaic energy conversion.

• The term “solar cell” within this work refers to a basic unit using the photovoltaic princi-ple to generate an electrical potential by utilizing solar radiation.

The term “solar panel” is used for a circuit consisting of a combination of several “solar cells”.

The term “solar module” is used for the unit of one “solar panel” including the electronic systems such as the DC-DC converter.

The term “solar array” refers to a system consisting of two or more solar modules con-nected according to a certain topology.

• The abbreviation ASIC stands for Application Specific Integrated Circuit and refers to the manufactured semiconductor die that contains the complete driver unit including driver circuit, core voltage circuit, and signal transmission circuit.

1.3 Motivation

Due to the decline of fossile energy resources, the search for alternative renewable energy sources is on the rise. One of these nearly limitless renewable energy sources is solar power. The energy of the sun can be utilized in different ways. One concept captures the heat that is produced by solar irradiation and uses it in different ways ranging from thermal processes to the generation of electrical energy with the help of a generator. This thesis however, will con-centrate on the application of another concept that converts the energy provided by solar irra-diation into electricity directly. This concept of energy conversion is called photovoltaics. Pho-tovoltaics are the direct conversion of light into electricity at the atomic level. Materials exhibit a property known as the photoelectric effect that causes them to absorb photons of light and in turn release electrons. When these free electrons are captured and controlled in a suitable semiconductor structure an electric potential difference will occur which in turn generates an electric current that can be used as a source for electricity. [1] The smallest part of a photovol-taic system is called a “solar cell”, in it the radiation of the sun is directly converted into electrical energy (see Figure 1.1).

Figure 1.1: Photovoltaic effect

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consist only of a singular large solar cell). A photovoltaic array consists of one or more solar panels that are connected to each other in different variations and one or more converters which are utilized to adjust the operating point and output voltage of the array. These photo-voltaic arrays can exhibit several different topologies that are dependent on the application field and surrounding environment of the solar array. This solar array is usually outfitted with one or more electrical converters to connect the solar arrays to the power grid or an electrical storage unit. One goal of this work was to determine a topology that would enable a solar array to be deployed regardless of environmental conditions and with solar panels that operate in-dependent from the condition of other panels in the same array. After further research it was shown that these requirements could be satisfied with a modular solar array topology. To en-able the use of this modular concept of a solar array, each solar panel would be outfitted with its own miniature converter unit to enable the required independence of the solar panels. Ad-ditionally, individual maximum power point tracking (MPPT) would become possible for each individual solar module. To achieve this miniaturization of the converter the chosen option was to increase the switching frequency of the converter and therefore reduce the overall size by reducing the required value and size of the passive electrical components. The high switching frequency that was needed for this miniaturization is made possible by utilizing a new high electron mobility transistors (HEMT) such as gallium-nitride (GaN)-based switching elements. These GaN HEMTs exhibit a low on resistance RDS(ON) as well as a low maximum gate charge QG which enable a fast switching frequency and high current capability. To maximize the po-tential of these new high frequency switching elements, a novel kind of driver circuit had to be developed that is able to be applied in the environment of the solar converter which includes a high temperature range as well as the ability to tolerate high voltages. Furthermore a galvanic isolation concept of the driver needs to be developed that isolates both the energy and signal transfer to the driver circuit. This enables the reuse of the standard low-side driver topology for the possible application in multi-level converter topologies. Due to the miniaturization aspect and the high signal frequencies the proposed galvanically isolated driver circuit would benefit greatly from an integrated realization of the concept. This thought process that lead from the idea of a modular solar array topology to the development of the driver ASIC is further illus-trated in Figure 1.2.

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2 Components of photovoltaic systems

To achieve an overview of possible topologies for a converter, preliminary research was done in the frame of this work. This preliminary research includes the following subjects:

• Type of solar cells

• Suitable and efficient solar array topologies

• Maximum power point tracking algorithms (to be applied in the converter)

• Means of galvanic isolation (between the primary side consisting of the control elec-tronic and the secondary side where the electrical power is treated)

• Communication between the independent solar modules (which will be used to com-municate the state of health, efficiency or shading of modules in the array)

The results of this preliminary research were used to evaluate a suitable solar converter topol-ogy and with it also the converter’s components.

2.1 Solar cells

Today, the available solar cell technologies can be divided in three generations which devel-oped over time due to the need to increase the efficiency of the solar cells while at the same time reducing the productions costs to increase the economic viability of photovoltaics [48]. First generation:

First generation cells consist of large-area, single junction devices. The materials used for these types of cells are usually monocrystalline, polycrystalline or amorphous silicon. The pro-duction of first generation solar cells involves high energy and work effort which prevents sig-nificant progress in reducing the overall production costs [49].

While the monocrystalline solar cells provide the highest efficiency of the first generation solar cells (up to 24%), they also have the most expensive production process as they are cut from large monocrystalline silicon single crystals that have to be grown, which is an elaborate and energy-intensive process. Additionally it has to be noted that the efficiency of monocrystalline solar cells decreases when exceeding a certain temperature (around 25°C) and, therefore, additional effort has to be taken into account when installing solar panels consisting of these cells to ensure proper air circulation under the cells for cooling purposes [49].

The production of the polycrystalline silicon solar cells is not as elaborate as the monocrystal-line production process as the conditions for the growth of the polycrystalmonocrystal-line silicon has not to be controlled as harshly. As a trade-off the efficiency of these polycrystalline silicon solar cells is slightly less (up to 19%) than that of the monocrystalline solar cells [50].

The amorphous silicon solar cells use a different production process where a thin layer of silicon is deposited on a substrate material. These solar cells exhibit a conversion efficiency of only around 9% and are mainly used for their flexibility and wide application field even on uneven surfaces [51].

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14 Second generation:

The second generation of solar cells focused on the energy requirements and productions costs of solar cells and the development of new materials to achieve these requirements. The second generation solar cells consist of layers of semiconductor materials that are only several micrometres thick. These “thin-film” solar cells may allow a low cost manufacturing process combined with less used material which greatly reduces the production cost [52].

The most successful materials that were established for the second generation solar cells have been cadmium telluride (CdTe), copper indium gallium selenide (CIGS), amorphous silicon and micromorphous silicon. These materials are applied in a thin film to a supporting substrate such as glass or ceramics, reducing material mass and therefore the overall costs [52]. Furthermore the introduction of these thin film materials enabled the development of stacked thin cells with different bandgaps which would not be possible when using crystalline silicon. These multijunction cells or “tandem cell” structures are capable of utilizing light with greater energy than the bandgap of silicon which lies at about 1.13 eV. Any energy that is greater than these 1.13eV which corresponds to the photon energy of red light would normally be lost. However with these new structures additional different bandgaps can be utilized which are optimized for light with higher photon energy and, therefore, allow the efficient utilization of a wider solar spectrum [52].

Third generation:

The goals of the third generation of solar cell technologies are to enhance the electrical per-formance of second generation (thin-film) technologies while maintaining low production costs. Current research is focused on increasing the conversion efficiency and tries to achieve target conversion efficiencies of 30-60% while retaining the low cost materials and manufacturing techniques established in second generation solar cells.

There are a several approaches to achieve conversion efficiencies of this magnitude, for ex-ample the use of multijunction solar cells, the concentration of the incident spectrum by optical means [96], the use of thermal generation caused by UV light to enhance voltage or carrier collection [94], or the use of the infrared spectrum for possible operation even during night-time [95].

The most promising of these approaches is the multijunction solar cell which at the time is able to achieve a conversion efficiency of up to 51.8%. This efficiency was achieved with a 3-layer multijunction cell consisting of InGaAs (bandgap 0.94eV), InGaAsP (bandgap 1.39eV), and InAlAs (bandgap 1.93eV). This proves to be a significant increase in conversion efficiency to a single-junction silicon solar cell which is only able to efficiently use the red spectrum of light corresponding to the silicon bandgap of 1.13eV and has a theoretical maximum conversion efficiency of 34%. Theoretically, the maximum multijunction solar cell conversion efficiency calculates to about 87%, but this theoretical maximum could in reality only be reached with an increase of junctions and the optimal semiconductor materials with optimized bandgaps to uti-lize a wide portion of the solar spectrum [2].

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is one way to increase the efficiency of a single solar cell, there exist additional methods to increase the efficiency of a whole solar panel or a solar array that will be elaborated in the following chapters.

2.2 Solar array topologies

This chapter gives an overview over currently used solar array topologies and their advantages and disadvantages before choosing a suitable topology. The main requirements for the selec-tion of a topology were the modularity of its components and the independence of the solar modules from the performance of other modules in the solar array.

2.2.1 Centralized string inverter topology

Figure 2.1: Centralized string inverter topology

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16 Figure 2.2: Grid voltages around the world

2.2.2 AC-module topology

Figure 2.3: AC-module topology

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and are interchangeable. Because of this, an expansion of an already installed solar array using this topology is simple, as additional panels can be added without further changes to the rest of the array. The disadvantages of this topology would be the higher cost for the additional inverters and the lower peak-efficiency of the solar array [56][57][59].

2.2.3 Hybrid technology combining modular panels with a centralized inverter and an energy storage system

Figure 2.4: Hybrid modular topology with energy storage system

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18 2.2.4 Conclusion

While achieving a voltage high enough to supply the power grid can be achieved easiest by the string technology, the modular technology is supposed to be more reliable and better suited for areas with frequent irradiation changes. While both of the modular concepts provide inde-pendence of each module to be able to control each module for its highest power output, the “hybrid” technology additionally incorporates the idea of a future-proof concept where the solar power can be stored easily and be mainly supplied to the connected household. Furthermore studies have shown, that the own power consumption of a solar array with this hybrid concept can be increased from about 30-35% to 60-70% [3]. Since this hybrid modular topology is deemed the most promising topology for the future, it was selected for the project. Furthermore, to avoid misinterpretations on behalf of the used terminology, Figure 2.5 clarifies the terms used for the system components. To enable the use of this solar array topology, each solar panel has to be outfitted with its own independent converter. To reduce the cost and space requirement of this converter, the use of a higher switching frequency is contemplated which enables a size reduction of the required passive electrical components of the converter. How-ever, the increase of switching frequency creates further requirements for the converter topol-ogy and the switching elements that are to be applied by the converter.

Figure 2.5: Terminology used for system components

2.3 Maximum power point tracking

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19 Figure 2.6: P-V characteristic of a solar panel

Therefore in order to locate this special operating point depicted in an example P-V diagram in Figure 2.6, a calculation model or a search algorithm has to be implemented into the con-verter that is dedicated to control the output of solar panels. Several different MPPT algorithms exist and will be further examined in this section. The MPPT algorithms can be divided into two main categories. One category defines the maximum power point based on lookup tables or simple calculations based on open circuit variables and is called passive MPPT. The other category is called active MPPT and constantly monitors the operating voltage and current and tracks the MPP based on these variables and a complex tracking algorithm [4][5].

To maximize the potential of the modular solar array topology an MPPT algorithm had to be found that could be easily implemented into and was compatible with the final converter topol-ogy and high switching frequency.

2.3.1 Passive MPPT methods

2.3.1.1 Constant Voltage (CV) method

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2.3.1.2 Short Current Pulse Method

The short current pulse method controls the MPP by giving commands to a current-controlled converter. Because the current at the MPP is proportional to the short-circuit current, it can instantly be determined by measuring the short-circuit current. This poses an adequate ap-proximation because the short circuit current of the PV panel is directly proportional to the irradiation level and relatively unaffected by temperature changes. To obtain this measure-ment, a static switch, which is in parallel with the PV panel, is utilized to create a short-circuit condition. However, at this time, no power is supplied by the PV panel. Additionally, as in the CV method, the measurement of the PV panel voltage is necessary to control the duty cycle of the converter [4].

2.3.1.3 Open-Voltage (OV) Method

It has been observed that the MPP voltage is always close to a fixed percentage of the open-circuit voltage. The open-voltage method utilizes this relation to control the converter open-circuit. Spreads in production, temperature or solar irradiation levels can change the position of the MPP within a 2% tolerance region. Therefore the determined voltage is very close to the actual voltage at the MPP. Usually a value of 76% of the open-circuit voltage is used for this calcula-tion. This method requires the measurement of the open-circuit voltage and like before the PV panel voltage to control the converter. To measure the open-circuit voltage, a static switch has to be included in series with the PV panel to create an open circuit condition. At the time of this measurement, no power can be supplied by the PV panel [4] [5].

2.3.1.4 Temperature Method

The temperature method works in analogy to the short current pulse method. Whereas, in the short current pulse method, the short circuit current is proportional to the irradiation level, in this method the open-circuit voltage is proportional to the temperature and can be described by the following equation [4][5]:

dT

dV

)

T

(T

V

ov STC ovSTC ov

=

+

−

⋅

, (eq. 2.01)

where VovSTC is the open-circuit voltage and TSTC is the cell temperature under standard test conditions. T is the cell temperature and dVov/dT is the temperature gradient. This equation can be used to determine the open-circuit voltage of the system, while the MPP voltage is then determined as in the open-voltage method.

Another different temperature method determines the MPP voltage directly by another equa-tion [4][5]: ) ( ) (u S v T w S y VMPP = + ⋅ − ⋅ + ⋅ , (eq. 2.02)

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21 2.3.2 Active MPPT methods

2.3.2.1 Perturb and Observe (P&O) Method

The P&O MPPT method operates by continually decrementing or incrementing the panel volt-age, while comparing the current panel output power with the output power of the previous perturbation cycle. If the output power increases in that interval, the P&O algorithm will further move the operating point in that direction. Alternatively, if the output power decreases, the operating point is moved in the opposite direction [4][5]. This operation is repeated in each perturbation cycle. In short, the P&O algorithm can be summed up by the following table: Table 2.1: Summary of P&O algorithm function

Perturbation Change in output power Next perturbation

Positive Positive Positive

Positive Negative Negative

Negative Positive Negative

Negative Negative Positive

The major problem with this method is that the voltage is perturbed in every cycle, even if the MPP has already been reached. In that case the output power continually oscillates around the MPP, which results in power losses to the system. This flaw is especially noticeable in constant or slowly changing atmospheric conditions. However another problem is bound to arise under rapidly changing atmospheric conditions. Illustrated in Figure 2.7 is the behaviour of a P&O controlled solar panel under these rapidly changing conditions [4][5]:

Figure 2.7: Behaviour of P&O under rapidly changing atmospheric conditions

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repre-22

sents an increase in power, the following perturbation will again be in the same direction, mov-ing the operatmov-ing point further away from the MPP. If the irradiance continues increasmov-ing, this faulty behaviour will also continue [4][5].

The P&O method can be further divided into several subcategories. The most important of these are the classic, the optimized and the three-point P&O algorithms. The classic P&O algorithm works with a fixed step size for each perturbation. This magnitude is usually around 0.4% of the open-circuit voltage of the PV system [4][5]. In the optimized algorithm, the pertur-bations work with a dynamic step size, which is determined by an average of samples of the measured output power. Therefore, when the MPP is approached, the step size is scaled down to reach the MPP more accurately and minimize the problems by oscillating around the MPP. In the three-point weight comparison method, the direction of the perturbation is decided by comparing three points of output power on the P-V curve. These points are usually the current operating point, another point perturbed in one direction and a third point, perturbed two times in the opposite direction from the second point. With the use of this three point algorithm, the faulty behaviour in times of sudden increases of irradiance levels can be avoided. All of these algorithms require the measurement of the PV array voltage and current [4][5].

2.3.2.2 Incremental Conductance (IncCond) Method

The most effective of the active MPPT method is the IncCond method. It is based on the fact that the slope of the output power curve is zero at the MPP (dP/dV=0), positive on the left side of the MPP (dP/dV>0) and negative on the right side (dP/dV <0). With the help of the following equation [4][5]: V V I dV dI V I dV IV d dV dP ∆ ∆ + ≅ + = = ( ) I , (eq. 2.03)

this relation can be rewritten as follows: ∆I/∆V = -I/V at the MPP

∆I/∆V > -I/V on the left side of the MPP ∆I/∆V < -I/V on the right side of the MPP

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23 Figure 2.8: Flowchart of IncCond algorithm

If the operating voltage V reaches the MPP, the operation of the PV system is maintained at this point, unless a chance in current (∆I) is detected, which indicates a change in atmospheric condition and a dislocation of the MPP. The algorithm then increments or decrements V in order to track the new MPP. The size of the increments determines how fast the new MPP can be tracked. A bigger increment size provides a faster tracking ability, but the system may not be able to exactly locate the MPP and oscillate around it like in the P&O method. Therefore the IncCond algorithm is often combined with a faster tracking algorithm into a two-model MPP tracking method that combines the advantages and at the same time tries to eliminate the shortcomings of the individual tracking methods. Usually the IncCond algorithm is combined with the CV or OV method. In the first stage, the operating point is brought near the MPP with the CV or OV method, while in the second stage the MPP is exactly tracked by the IncCond algorithm with a relatively small increment size. This two-model approach also ensures that the real MPP is located under the circumstances of multiple local maxima. Another effective way of performing the IncCond method is to use instantaneous and incremental conductance to generate an error signal [4][5]:

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24

Subsequently, a PI-control element can be used to drive the error e to zero. For the IncCond tracking algorithm, the measurements of both solar panel current and voltage are necessary. With an additional sensor installed for the measurement of solar irradiation it is additionally possible to switch from IncCond to another passive tracking algorithm in the case of low irra-diation conditions (ca. 30% of nominal irradiance conditions), in order to save power losses by the constant tracking and instead just approximate the MPP by the passive tracking method [4][5].

2.3.2.3 Fuzzy Logic Method

The fuzzy logic control is a lesser known MPPT method. It generally consists of three stages: Fuzzification, rule base table lookup and defuzzification. During the first stage the numerical variables from the input are converted into linguistic variables based on a membership function such as in figure 2.09 [5].

Figure 2.9: Membership function for input and output of a 5-level fuzzy logic controller [5] In the example seen in Figure 2.9 five different fuzzy levels are used: NB (negative big), NS (negative small), ZE (zero), PS (positive small) and PB (positive big). If a greater accuracy is necessary, more fuzzy levels can be added. The variables a and b are based on the range of values of the numerical variables. While the membership function in figure 2.4 is symmetric, this is not required. Sometimes the membership function can be made less symmetric to em-phasize the importance of specific fuzzy levels. The inputs of a MPPT fuzzy logic controller are usually an error signal e and a change in error ∆e. How the error functions are computed is up to the user. For example, because dP/dV becomes zero at the MPP, the following functions could be used [5]: 1) -e(n -e(n) e(n) ) 1 ( ) ( ) 1 ( ) ( ) ( = ∆ − − − − = n V n V n P n P n e (eq. 2.05)

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25 Table 2.2: Example of a fuzzy rule base table [5]

∆E E NB NS ZE PS PB NB ZE ZE NB NB NB NS ZE ZE NS NS NS ZE NS ZE ZE ZE PS PS PS PS PS ZE ZE PB PB PB PB ZE ZE

The assigned linguistic variables are based on the power converter being used and the pre-liminary knowledge of the user. As an example for a boost converter: If the actual operating point is to the far left of the MPP, this corresponds to e being PB and ∆e being ZE, then the duty ratio of the converter has to be highly increased to reach the MPP, which correspond to a PB. After the fuzzification and this lookup stage, the linguistic fuzzy logic controller output has to be converted back to a numerical value by using a similar membership function like the one displayed in figure 2.4, in order to provide an analog signal that is able to control the power converter. This stage is called defuzzification. While the fuzzy logic MPPT method has proven to perform well even under varying atmospheric conditions, its overall effectiveness depends on the choice of the correct error computation and the creation of an effective rule base table [5].

2.3.2.4 Neural network

Another more complex MPPT method is the neural network. Neural networks usually consist of three layers: an input layer, the hidden layer and the output layer. The number of different nodes in each layer is variable and depends on the user and the application. In case of a PV array, the input variables are parameters like voltage and current of the PV array or even en-vironmental parameters like temperature and irradiance. The output parameter usually is a signal to control the duty cycle of the power converter connected to the PV array. The accuracy of this MPPT method depends on the algorithms working in the hidden layer and how advanced the neural network has been trained. The specific links between nodes are all weighted and therefore have a varying degree of importance. For example in Figure 2.10 the link between nodes A and B has been assigned the weight wAB [5].

Figure 2.10: Example of a neural network [5]

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26

months or even years to record the pattern between input and output layer. However, since most PV arrays have varying characteristics, a neural network has to be trained specifically for each PV array for which it is intended to be used. Since the characteristics of a PV array can also change over time caused by pollution or aging, the neural network must constantly be trained to assure an accurate tracking of the MPP [5].

2.3.2.5 Summary

Table 2.3: Summary of MPPT methods

Method Advantages Disadvantages

Constant voltage

(CV) + no input necessary + low computation level + especially effective in low ir-radiation conditions

- inaccurate

- real MPP is not reached

Open voltage

(OV) + low computation level + more effective than CV - better than CV but still inaccu-rate - real MPP is not reached - no power harvested during measurement of OV

Short-current pulse + low computation level + more effective than CV

- better than CV but still inaccu-rate

- real MPP is not reached - no power harvested during measurement of SC

Temperature + Influence of temperature is considered

+ Improvement to the OV method

- additional temperature sensor necessary

- Lookup table for optimal equa-tion parameters has to be es-tablished

- real MPP is not reached Perturb and observe + Real MPP is tracked actively - Oscillations around the MPP

- power losses Incremental

conduct-ance (IncCond)

+ most effective MPPT algo-rithm

+ Real MPP is tracked actively + Can be combined with an-other passive tracking method for additional efficiency

- high computation complexity

Fuzzy logic + Real MPP is tracked actively - effective computation and rule base table have to be estab-lished

- high computation complexity Neural network + Real MPP is tracked actively - great effort for training the

neural network

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27 2.3.3 Conclusion

To effectively track the MPP an active MPPT method has to be selected. Out of the active methods, the incremental conductance algorithm has proven to be the most effective in track-ing and maintaintrack-ing operation at the MPP. Additionally the speed of the incremental conduct-ance algorithm can be varied to enable fast tracking or precise location of the MPP which also benefits from the increased switching frequency proposed for the converter that is to be utilized in the solar module. Furthermore the IncCond method can be combined with an additional passive method like the “constant voltage” to guarantee a high efficiency even in low irradiation conditions. Therefore, for this project, realizing the the MPP tracking by means of the IncCond algorithm was chosen for it will grant the most benefits.

2.4 Galvanic Isolation

To ensure a galvanic isolation of a solar module or a converter, the supply of energy, as well as the signal transmission has to pass over galvanically isolated coupling elements. There are several possibilities of realizing this isolation which will be further elaborated in this section. 2.4.1 Coupling elements

The analysis of readily available coupling elements for isolated signal and energy transmission was based on the research done in the framework of [28] Table 2.4 shows an overview of galvanically isolated coupling elements.

Table 2.4: Overview of galvanically isolated coupling elements [28] Principle of the

isolat-ing couplisolat-ing element State-of-the-art for energy transmission State-of-the-art for signal transmission Examples

Magnetically coupled possible possible - transformer, transponder

- inductive data coupler - magnetoresistive data coupler

Optically coupled 1 _possible _possible _{- optocoupler}

- optically ignitable thyristor - solid-state relay

Electrically coupled possible ² possible - flying capacitor

- capacitive data coupler

Mechanically coupled possible possible - piezo-transformer

- ultrasonic emitter/receiver Electromagnetically

coupled possible ² possible ² - solar cells (receiver) - wireless switch

1_{Even though the optically coupled elements is based on an electromagnetic coupling (wave)} it is listed as a separate element for better understanding

² not used for trigger circuits

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28

Additionally, the thickness of the piezo ceramic is responsible for a delay between input and output of the coupling. The maximum speed at which the signal is able to travel through the ceramic is limited to the speed of sound which means that the delay is then only influenced by the thickness of the ceramic. The minimum processable thickness of a piezo-ceramic that still provides reliable electrical isolation limits the maximum transmittable frequency to about 2MHz.

Optical coupling elements also provide a reliable isolation with only a small delay of the trans-mitted signal. However the materials used for these optical coupling elements are prone to the effects of aging and their reliability decreases over time.

A galvanic isolation of the signal transmission using an electrically coupled capacitive coupling element seems optimal as it combines a high possible transmission frequency with small delay and very small effects of aging when using ceramic capacitors.

For a detailed analysis of these coupling elements an in-depth analysis performed by S. Zeltner in [28] can be referred. Table 2.5 shows a commercially available selection of coupling ele-ments provided by [28].

Table 2.5: Examples of commercially available coupling elements suitable for isolated data transmission

Principle Type

(exam-ple) Technology Setup / Packag-ing Volume (H x W x L in mm) Manufac-turer Reference Optically cou-pled

HCPL-3120 Optocoupler IC, Gull

Wing 10 x 7 x 4 Avago [75] [76] Magnetically coupled 1ED020I12-F Integrated impulse-transmitter IC, PG DSO 16-15 11 x 8 x3 Infineon [77], [78], [79] Magnetically coupled ADUM1200W Integrated impulse-transmitter

IC, SO8 5 x 4 x 2 Analog

Devices [80], [81] Magnetically coupled IL710 HCPL-9030

GMR effect IC, SO8

IC, SO8 5 x 4 x 2 5 x 4 x 2 NVE Avago [82], [83] [84] Magnetically coupled Skyper32 2SD106AI Impulse-transmitter Discrete Semikron Concept [85], [86] [87] Electrically coupled ISO7220M Integrated capacitor

IC, SO8 5 x 4 x 2 Texas

In-strument

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29 2.4.2 Trigger circuits for coupling elements

As both the inductive and capacitive galvanic isolation have their own requirements on how to handle the transmission of energy and signal, specific trigger circuit have to be applied to en-able the transmission over the galvanic isolation as well to ensure its robustness and reliability.

2.4.2.1 Circuit for energy transmission over inductive isolation

The circuit for the transmission of energy over the galvanic isolation consists of an inductive coupling element a chopper circuit to generate an alternating voltage on the primary side and a rectifier circuit on the secondary side. Figure 2.11 shows a basic topology of this energy transmission circuit that can be utilized with an inductive coupling element.

Figure 2.11: Energy Transmission circuit using an inductive coupling element

The chopper and rectifier circuit have to be further modified to fullfill the requirements on the value of energy to transmit and the switching frequency that the inductive coupling element is able to handle with a compromise between efficiency and size of the passive components utilized and parasitic capacitive coupling.

2.4.2.2 Circuit for signal transmission over capacitive isolation

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30

Figure 2.12: Capacitively coupled differential signal transmission

A simple form of a signal conditioning circuit is depicted in Figure 2.13. This impulse transmis-sion circuit is located after the galvanic isolation and consists of two comparators and an SR-Flipflop and ensures a well-defined and reliable signal at its output. More information on the function of this circuit is located in section 3.2.3.

Figure 2.13: Impulse transmission circuit for signal transmission over capacitive isolation [28]

2.5 Communication between modules / Transmission of Data

To fully utilize the potential granted by the use of a modular topology of the solar array, re-search was done on the possibility of integrating a communication and monitoring system into the modules. With this, several questions arise, like the possible ways of transmitting data, or the suitable communication protocol for this task. Furthermore it has to be evaluated which parameters have to be monitored and how these will be further computed. This research was done in the preliminary research phase to generate an overview of communication possibilities and although it was not further researched in-depth in the frame of this work, a possible follow up work of realizing a micro DC-DC converter and the modular solar array proposed in Figure 2.25 will benefit from these preliminary evaluations.

2.5.1 Monitored parameters

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31

to determine a singular failure of one panel. If additional information is needed about the envi-ronment, a temperature sensor or even an irradiance sensor could be added to the module. Although these parameters could also be determined from the actual voltage and current as well as the information about typical operating values of the solar panel, these data sheet values usually assume ideal operating conditions an don’t considers pollution or other environ-mental changes. Therefore it would only be of advantage to the monitoring system if such additional sensor would be used.

2.5.2 Possible solutions for transmission of data

Because the monitored data is generated at each single module and a central monitoring sys-tem is to be used, this data has to be transmitted from the modules to the central unit. In this section, several ways of transmitting this data are presented.

2.5.2.1 By additional data cable

The most obvious way of data transmission would be the addition of a separate cable bus used solely for this purpose. While this seems to be the easiest solution, several problems arise from it. While in a non-modular topology, only the data from the central inverter unit have to be processed, in a modular topology each module should be monitored and controlled separately and therefore has to have its own connection for data transmission. With growing module count, the complexity and cost of wiring would rise dramatically, therefore this solution is not feasible for the modular technology approach.

2.5.2.2 By wireless data transmission

Because of the disadvantages mentioned in section 2.5.2.1, the excessive use of wiring should be avoided. The next step would therefore be the use of a wireless data transmission technol-ogy. However the possibility of a wireless data transmission with this number of modules is mostly rejected due to economic reasons. Furthermore the aspect of electromagnetic compat-ibility (EMC: e.g., reliability and robustness against interferences) has to be evaluated as well.

2.5.2.3 By power line communication (PLC)

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32

2.5.2.4 Conclusion

To reduce cost and cabling complexity, power line communication would be the method of choice for the use of data transmission in the modular solar topology. Each module should have an integrated PLC communication device that is able to transmit data about its current status to a central master control unit for surveillance, monitoring, and optimization purposes. To transmit this data on the DC-bus, several modulation schemes are available. Annex 9.1 provides a further evaluation of these modulation schemes.

2.6 DC-DC Conversion

To enable a modular technology to convert its output to levels, at which the power grid or a DC-bus can be supplied, converter topologies with a high amplification factor have to be used. Generally, to increase a voltage, a boost topology, like the one depicted in Figure 2.14, is used.

Figure 2.14: Standard boost converter topology

The output voltage of the boost converter is controlled by the duty cycle of switch S. To achieve high voltage amplification, a very high duty cycle has to be used. This poses a problem, be-cause at those duty cycle rates, the output rectifier conducts only in a small time frame during each switching cycle and suffers from serious reverse recovery problems. Therefore, the standard boost converter technology has to be modified to achieve higher voltage gain and efficiency.

In general, because the transmitted power is defined by the equation

P

₌

V

_⋅

I

, a relatively high voltage should be used on the DC-Bus to reduce the current and with it the ohmic losses in the wiring. This in turn enables the use of wiring with smaller diameter. A voltage of 400V DC has been established as a reasonable value for the DC-bus. Therefore the MPP voltage output of a common solar panel used in domestic applications of around 30-40V has to be amplified by a factor of at least 10. This high amplification factor is usually achieved by multi-stage converter designs combining a boost converter with another voltage amplifying topology (e.g. a transformer).

2.6.1 High boost rate converter using a voltage multiplier

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33

Figure 2.15: High boost converter topology using a voltage multiplier [6]

As seen in Figure 2.15 the circuit consists of a pair of coupled inductors, where the secondary side is rectified using a voltage multiplier. If the inductor current is continuous, the voltage at capacitor C3 can be expressed by the following equation [6]:

D V V_C _S − = ₁1 3 , (eq. 2.06)

where D represents the duty cycle of the switch S. Furthermore, the voltages at capacitors C1 and C2 are defined by the following equations [6]:

S C S C

V

n

V

D

n

V

⋅

=

−

⋅

=

2 1

₁

_{(eq. 2.07)}

The variable n represents the turn ratio N2/N1 of the coupled inductances L2/L1. By using these equations, the output voltage comes to [6]:













−

+

=

+

=

D

n

V

_OUT _C _C _C _S

1

3 2 1 (eq. 2.08)

If additionally the voltage multiplier stages are increased, the equation changes to the following one [6]:













−

+

=

+

=

D

nk

V

_OUT _C _C _Cn _S

1

1 ...

2 1 , (eq. 2.09)

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34

high turn ratio deteriorates the coupling coefficient, which would result in low efficiency. Also a magnitude of multiplier stages increases the number of diodes and therefore the conduction loss. The equation used to solve this trade-off issue could be shown as the following [6]:

,

1 )

1 (

k

V

D

V

n

S O

−

⋅

−

=

(eq. 2.10)

With VO as the desired output voltage and the duty cycle D fulfilling the following requirements for greater efficiency [6]:

S O V V D D < < < 0,5 0

Also the number of multiplier stages k should be kept as low as possible to minimize the num-ber of components and conduction losses [6][41][42][43].

2.6.2 Optimized low voltage boost converter

The circuit shown in Figure 2.16 allows a continuous adjustment of the transmission ratio M between the input voltage Vpv and the output voltage VL with a nearly continuous conduction of current on the input side. Therefore it is possible to withdraw a nearly constant current ipv form the solar panel, even with a relatively small filter capacity Cin. Furthermore, research done in [7] has shown that with this topology it is possible to achieve efficiencies of more than 90% at input voltages down to 2V and input currents up to 100A.

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35

The switches S1 and S2 on the primary side work in full complementary operation. Therefore, the boost converter is able to withdraw a nearly continuous input current (aside from the very short periods of commutation compared to the value of the inductances L1 and L2). The great transmission ratio M is achieved in two stages. Because of S1 and S2 working in push-pull operation, the transmission ratio between UPV and UB is mostly defined by the turn ratio n2/n1 of the high frequency transformer. Subsequently, the continuous voltage adjustment is accom-plished in a second step by a boost converter consisting of the inductor LS, the switch SB, and diode DA. The switch SB works asynchronous to the push-pull stage and enables the control of the current ii in the inductor LS and, therefore, also the input current iPV. The switch SA is used as a commutation assistance for the switches S1 and S2. The commutation assistance switch is always activated shortly before a commutation change of the switches S1 and S2. Because of this, the voltage of the link circuit VL is applied to the secondary side of the transformer. This leads to a decrease of the current at the secondary side iS (ideally to zero), which enables a nearly zero-current-switching (ZCS) of S1 and S2. The placement of the commutation assis-tance SA on the secondary side and outside of the main current conducting circuit is advanta-geous in several aspects. Due to the low current stress, the transistor used for SA can be downsized. A further advantage is the ability to mostly eliminate the switching losses of the transistors used in the primary side (S1, S2) and, therefore, the rise of efficiency of the whole circuit. The control of the MPPT is executed by a change in duty cycle of transistor SB on the secondary side [7][44][45][46][47]. The switching cycle of this optimized low-voltage boost con-verter topology is illustrated in Figure 2.17.

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36

Figure 2.17: Switching cycle showing voltage and current waveforms of the optimized low-voltage boost converter topology [7]

2.7 High electron mobility transistors

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37

based on these materials have become commercially available. Table 2.6 compares the ma-terial properties of silicon, gallium nitride and silicon carbide [8].

Table 2.6: Material properties of silicon, SiC and GaN at 300K [8]

Property Si SiC GaN

Bandgap energy EG (eV)

1.12 3.2 3.4

Critical electric field for break-down EBR (MV/cm)

0.3 3.5 3.3

Saturated drift velocity VS (x 107 cm/s)

1.0 2.0 2.5

Electron mobility

µ (cm2_/Vs) 1500 650 990-2000

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38

Figure 2.18: Typical AlGaN/GaN HFET transistor structure [8]

In 2009 the first enhancement-mode gallium nitride in silicon (eGaN) field effect transistor was introduced to the market by Efficient Power Conversion (EPC) Corporation. This device was designed specifically as a power MOSFET replacement for application in the RF range [8]. Figure 2.19 shows the structure of the EPC eGaN transistor.

Figure 2.19: Structure of an eGaN transistor by EPC corporation [8]

The structure of the eGaN FET is built on a silicon substrate. On the substrate a thin layer of aluminium nitride (AlN) isolation is grown to provide a seed structure for the subsequent pro-cess of growing the GaN heterostructure. The GaN heterostructure consists of a thin AlGaN layer on top of a highly resistive GaN layer. Between these two layers the two dimensional electron gas zone is generated. The gate electrode is further processed by non-specified pro-cess steps to form an area under the gate where a depletion region is located. To turn on the eGaN FET a positive voltage has to be applied to the gate electrode to enhance the depletion region under the gate with free electrons. Therefore this eGaN FET behaves the same way as a regular n-channel enhancement mode power MOSFET [8].

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39

Figure 2.20: RDS(ON) vs. VGS at various values of ID for the eGaN evice EPC2010 [11]

Figure 2.21: RDS(ON) vs T for the eGaN device EPC2010 [11]

ASIC Based Galvanically Isolated Driver Circuitfor the Use in Power Converters forPhotovoltaic Applications

HAL Id: tel-01094380

https://hal.archives-ouvertes.fr/tel-01094380

ASIC Based Galvanically Isolated Driver Circuitfor the

Use in Power Converters forPhotovoltaic Applications

Markus Niedermeier

To cite this version:

ASIC Based Galvanically Isolated Driver Circuit

for the Use in Power Converters for

Photovoltaic Applications

Thesis in Joint Supervision

DOKTOR - INGENIEUR

DOCTEUR DE L’UNIVERSITÉ DE GRENOBLE

ASIC basierte galvanisch getrennte

Treiber-schaltung für den Einsatz in

Leistungswand-lern für photovoltaische Anwendungen

Der Technischen Fakultät

Der Friedrich-Alexander-Universität Erlangen-Nürnberg

DOKTOR - INGENIEUR

THÈSE

DOCTEUR DE L’UNIVERSITÉ DE GRENOBLE

Markus NIEDERMEIER

Driver avec isolation galvanique basé sur

un ASIC destiné à des convertisseurs de

puissance pour les applications

photovol-taïques

Abstract

Kurzfassung

Résumé

Acknowledgments

Contents

1 Introduction

2 Components of photovoltaic systems

dT

dV

)

T

(T

V

V

=

+

−

⋅

P

=

V

⋅

I

V

n

V

D

D

n

V

V

⋅

=

−

⋅

=

1













−

+

=

+

+

=

D

n

V

V

V

₌

_⋅

₁