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«Master of Science Thesis GaN Enabled OLED Driver for Automotive Lighting Application Foivos Palaiogiannis Supervisor: Dr. Jelena Popovic February, ...»

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Delft University of Technology

Faculty of Electrical Engineering, Mathematics and

Computer Science

Department of Electrical Sustainable Energy

Master of Science Thesis

GaN Enabled OLED Driver for Automotive

Lighting Application

Foivos Palaiogiannis

Supervisor:

Dr. Jelena Popovic

February, 2015

To my parents and sister...

Abstract

The impressive features, both in a design and technical aspect, of the Organic LED

(OLED) lighting technology have attracted the interest of the research and commercial world and have recently be in the spotlight of the automotive industries, like the Lighting Department of Audi. Some of the most exciting features of these lights are the flexibility, transparency and the very small thickness. It is reasonable, therefore, that in order to take full advantage of this new technology the electronics that drive these lights, that is the dc/dc converter, should also be characterised of low profile, flexibility and small size. A boost towards the direction of the converter minimization and high power density operation could be the recently commercialised power switching technology of Gallium Nitride (GaN) devices. This technology, which exploits the advantages of the wide band gap semiconductors, offers high frequency, high power density, low switching losses operation and low profile design, as well. As every newly commercialised technology, the areas of application that at most exploit the advantages of these switches are still to be found, but it is expected that applications that require high power density or low profile features, like the OLED applications, could benefit the most from the GaN technology.

This area is the exact topic of the current master thesis. At this project a dc/dc converter based on GaN switching devices is designed and built. The converter is purposed for the driving of Organic LED lights that belong to the tail light system of a vehicle. As such, the electrical, mechanical and thermal specifications of the converter are based on the guidelines of the Lighting Department of the Audi automotive industry and the nature of the Organic LED lighting. At this thesis, the required dc/dc converter is designed, built, measured and assessed for its adequacy to the defined requirements.

During the design part of this project the necessary simulations are conducted. For the purpose of estimating the losses of the GaN device a detailed analytical model for the switching transients is used. Also, both the possibilities of using a planar and a discrete coil are investigated during the simulations and the two components, which were built in the lab, are compared experimentally. A final prototype of the converter is also built in the lab and the experimental and simulated results are then compared and assessed.

The assessment of the results showed that the features of the GaN device can be fully exploited at this application and can offer the low profile and high power density requirements. In order, however, to achieve the minimization of the magnetic component more advanced and wider range of core materials are required, especially if a planar coil is desired. Finally, full exploitation of the detailed analytical GaN loss model requires specialised software tools or accurate analytical models in order to determine the values of the various parasitics and the thermal resistance of the component, both strongly related to the PCB layout. This, also, means that during the design procedure in order to achieve better accuracy -which is required at applications which push the frequency to the limits - the PCB design layout parameters should be included in the iterative process of the parameter specification.

iiiAknowledgments

First of all, I would very much like to thank Dr. Jelena Popovic for her invaluable help, consistent guidance and thought provoking discussions. Our excellent communication all this time helped me significantly. I would also like to thank Ing. Mark Gerber because his advice, experience and clear way of thinking during my internship proved to be very helpful during this project. A great thanks, also, goes to the Phd students Milos Acanski, Ilija Pecelj and Wenbo Wang whose help during the practical part of my project was extremely important.

Thanks,finally, to my colleagues and fellow master students - Udai, Ralino, Nikolas, Joost, Einar, Andreas, Vasilis, Sotiris, Didier, JK - for the great time that we had all this and the past year.

–  –  –

Introduction

1.1 Problem Statement The Organic LED lighting technology has recently attracted significant attention for its research interest but mainly for its commercial opportunities as it offers thin, flexible, shapeable, transparency features for light applications, allowing for new impressive and futuristic designs for various areas including automotive applications. The Audi automotive industry more specifically has shown special interest in this new technology and it tries to exploit its features for the lighting of its products, a very strong sector for this specific brand. Apart from the various bottlenecks of this still young technology mainly related to temperature and lifetime issues, an aspect that requires special attention is the driver of the OLED components. In order to fully exploit the geometry-related advantages that this technology offers it is very important for the dc/dc converter that drives the lights to be low profile and flexible.





A recently commercialized and innovative power switching technology, the GaN switches, could be a boost towards the direction of the minimization of the converter’s size. The new switching components exploit the advantages of the wide band gap semiconductors and they promise high frequency, high power density and high efficiency operation of the switching converters. The utilization of these switches is generally expected to push forward the capabilities of the power electronics applications, especially as regards the reduction of the size, but in order to do so the design and built approaches, used for Si-based converters up till now, need to be carefully reconsidered. Similar to any new commercialized technology, it is a very important aspect to locate the areas of application that this technology could be more useful and the advantages that could be the most exploited. In general, GaN based switches are expected to help significantly at areas where high power density or low profile features are crucial parameters, such as the aerospace industry, the lighting technology and so on.

It seems, therefore, that the requirements of the OLED technology for low profile, small and flexible power converters could be possibly fulfilled to a great extend with the appropriate exploitation of the advantages that the GaN technology offers. Considering the fact that in power supplies the passive components are the main bottlenecks as regards the size minimization of the converter, the possibility of high frequency operation and low losses that GaN switches allow, along with the availability of the appropriate inductor core materials, could help towards that direction.

This thesis project focuses exactly on this aspect as its purpose is to built an OLED driver for automotive lighting using the advantages of GaN switches.

1.2 Thesis Objectives As already stated, the main objective of this thesis project is to exploit the advantageous features of the GaN switching devices and build a low-profile OLED driver purposed for automotive applications. The project is carried out in collaboration with the Audi Lighting Department and the basic electrical and mechanical specifications for the driver are imposed by the technical standards of the company and will be presented in detail at Chapter 3, where the design procedure is described.

At Figure 1.1 a general schematic of the desired configuration is presented:

Figure 1.1: General scehmatic of the configuration.

There are two limitations regarding the design: the size, and especially the thickness of the converter (related to frequency) and the maximum component operating temperatures (related to frequency and ambient temperature - the range of which is shown at the previous figure). Due to these limitations the following aspects are also investigated

during this project:

- The possibility of using a planar inductor, as compared to a discrete one, for enhancing the low profile feature of the converter.

- The maximum operating frequency that will allow operation into the temperature limits of the converter.

Added to these, an analytical model of the losses of the GaN switch is required to be developed.

1.3 Thesis Layout The report of this thesis is structured in the same way as the project was conducted during time. Initially, a literature review on the main aspects of the topic took place.

Later on, the design process was conducted and, finally, the converter was built and the experimental results were taken and evaluated. Here, a short description of the content of the chapters is presented.

At chapter 2 the appropriate theoretical background regarding the two main aspects of this thesis is developed. At the first part of this chapter the basic concepts of the organic LED technology are developed along with the electrical and mechanical properties and the advantages and disadvantages of this new technology. At the second part, a thorough review of the GaN technology is presented. Initially, their physical structure and concept is briefly presented and then their basic inherent characteristics are discussed. The influence of parasitics, some thermal issues and a review on their application in the power electronics area follows at the end.

At Chapter 3 the most important parts of the design procedure are presented and discussed. Main axis of this chapter is the construction of an analytical model for simulation purposes. Initially, the reasons for selecting the appropriate topology (buckboost) are explained and then the boundary conduction operating mode with valley switching is discussed and the corresponding analytical equations are given. Special emphasis is given on the analytical model of the GaN switch losses that is used for the simulation. After that, the design of the magnetic component and the selection of the inductance value is discussed. Note that the value of the inductance defines the operating frequency of the converter and, therefore, is an important parameter. Finally, some other considerations regarding the design procedure are briefly addressed.

Chapter 4 contains the experimental results of the project. Initially, the converter prototype and the experimental setup is presented along with some output waveforms and thermal images. After that, a more accurate calculation of the GaN junction-toambient thermal resistance is described. Finally, the experimental results are presented and are compared with simulation results.

Finally, at Chapter 5 an assessment and an evaluation of the final proposed converter is attempted and some conclusions are extracted.

Chapter 2 State of the art At the previous chapter the problem outline and the project goals and objectives of the current master thesis were presented. It became clear that two recently commercialized components, the organic LED lights and the GaN switches, are the focal points of this project but with the former being treated exclusively from the perspective of an electric load, which means that only the electrical characteristics (and the geometrical which affect the electrical in that case) are of interest, and the latter being, eventually, a very important and fundamental component of the design, as regards its behaviour and contribution.

Because of the importance of these two elements at this project, it is considered necessary to present the theory behind both of them at this chapter. Initially, the OLED technology, physics and some design aspects are presented. Since the OLED device is treated at this project solely as an electric load, similar to a common LED but with its specific electrical properties (and mechanical properties, because they define the electrical ones), the theoretical background for it is not as detailed as the one for the GaN device, which follows and includes thorough descriptions for its physics, electrical characteristics, design aspects and applications.

2.1 The Organic LED Lighting Electroluminescence in organic materials is known already from the 1950’s, however it is mainly the last twenty years that the organic light emitting technology has attracted much of the attention of the lighting research and industry. Because of the various advantages of this technology, which will be presented later at this paragraph, OLED lighting is expected to lead a paradigm shift in the lighting industry. Up to now, OLED technology is mainly used for displays and monitors. Since the early 2000’s OLEDs are used in small-screen devices such as cell phones or digital cameras while the last few years large OLED TV screens have been released to the market. However,the spreading of these devices at large area light emitting elements for general application is slower and up to now there is a lack of availability of a broad array of off-the-self products.

At this paragraph, the basics of the organic electroluminescence and the organic devices are initially presented. The main advantages and bottlenecks of this technology follow and, finally, the design aspects of the OLED devices, such as thermal and driving considerations and their electrical characteristics, are demonstrated.



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