Farnaz Nassiri Nia Aarnoud Tjerk Sluimer CONVERTER DESIGN for Nuna Maximum Power Point Tracker TU Delft Faculty of EEMCS July 2, 2012 2 Abstract he world is rapidly moving towards a greener future, a mainstay of which is solar energy. Hence, maximum power point trackers, or T MPPTs for short, are an indispensable component, since these devices provide one with the capability to fully maximize the achievable power from solar cells in a system. A symbol of the greener world that is evolving around us is the biannual World Solar Challenge, where state-of-the-art solar powered cars race against one another through the deserts of Australia. This thesis forms the link between these two elements: to design an MPPT optimally suited towards the requirements of the Nuna Solar Car. Commercially available MPPTs are not particularly well suited for the specific demands of such a bleeding edge race. They are mostly designed for high power or low voltage systems. The MPPT designed during this thesis is optimized for high efficiency and other requests stated by the Nuon Solar Team members. The design of the MPPT has been split into several parts. This thesis is focused on the converter within the MPPT. It presents the result of research, design exploration, simulation and testing. The results obtained with the prototype that has been built show the designs correctness and conformance to the requirements, with an efficiency of up to 98,5%. 3 4 Preface s part of the Bachelor program Electrical Engineering at the EEMCS faculty of Delft University of Technology, a research project has been A performed for the course Bachelor Afstudeer Project, or BAP for short. The subject of the thesis report is to design a Maximum Power Point Tracker for the Nuon Solar Team. This report is a summary of the work that has been done. Our dissertation would not have been possible without the guidance and the assistance of several individuals who in different ways contributed and put their precious time in the preparation and completion of this study. Special thanks to: • Jelena Popovic for being our supervisor during the project. • The Nuon Solar Team for making this project possible • Willem Zwetsloot and Javier Sint Jago for being our contacts within the Nuon Solar Team. • Milos Acanski for giving advice during the project. • Kasper Zwetsloot for helping us with building the prototype • And last but not least we would like to thank our supportive family for making this all possible, friends for their feedback and anyone who spent time reading this thesis. Delft, July 2, 2012 Farnaz Nassiri Nia (1544179) Tjerk Aarnoud Sluimer (1518712) 5 6 Contents Preface 5 List of Figures 10 List of Tables 11 Glossary 13 1 Introduction 15 1.1 Thesis Structure . 16 2 Problem Definition: The Maximum Power Point Tracker 17 2.1 Problem Description . 17 2.2 Brief of Requirements . 18 3 Literature Study on MPPT Hardware 21 3.1 Introduction to MPPT Hardware . 21 3.2 Different Converter Topologies . 24 3.3 Possible Switching Elements . 26 3.4 Possible Rectifier Diodes . 27 3.5 Converter Output Capacitor . 28 3.6 Converter Inductor . 31 4 Converter Design 39 4.1 Topology Decision . 39 4.2 Boost Converter Principles . 40 4.3 Assembling Converter Inputs . 42 4.4 Choosing the Rectifier Diode . 48 4.5 Choosing the Capacitor . 49 4.6 Inductor Design . 49 5 Concept Design of the MPPT 55 5.1 Concept Simulation . 55 6 Evaluation of the Prototype 59 6.1 Building Process of the MPPT . 59 6.2 Test Setup of the Prototype . 61 6.3 Test Results of the Prototype . 62 6.4 Proof of Principle . 64 7 Conclusion and Recommendations 69 7 Appendix 75 A. Matlab and Simulink Code & Files . 75 B. Brief of Requirements . 79 C. Measurements on the System . 82 D. Inductor Core Dimension and Calculation . 88 8 List of Figures 1 I-V and P-V characteristics of solar panels [1] . 17 2 Classification of power supply technologies ( [2], pp. 2) . 21 3 PWM regulator topologies ( [2], pp.3) . 22 4 Hard switching versus resonant switching ( [3], pp. 2) . 23 5 A basic schematic of a boost converter . 24 6 A basic buck-boost converter schematic . 25 7 Circuit of a flyback converter . 26 8 Preferred operating regions for MOSFET and IGBT [4] . 27 9 Voltage versus capacitance for different types of dielectrics ( [5], pp.5-2) . 30 10 Core loss versus frequency for different types of magnetics ( [5], pp.6-5) . 32 11 Saturation flux density versus temperature for different types of magnetics ( [5], pp.6-5) . 33 12 B-H characteristic of a transformer core having hysteresis and hence magnetic losses . 34 13 Temperature dependence of core loss ( [6], pp.9) . 36 14 Magnetization curve of the core ( [6], pp.6) . 37 15 Dimensioned diagram of (a) a double-E core (b) bobbin, and (c) assembled core with winding ( [7], pp.750) . 37 16 A basic schematic of a boost converter . 40 17 Equivalent circuit while the MOSFET is on . 41 18 Equivalent circuit while the MOSFET is off . 41 19 Waveform of the voltage over and current trough the inductor 42 20 Efficiency of the boost converter according to the switching frequency . 45 21 Voltage over the inductor and current through the inductor . 47 22 Specified power loss as a function of flux density with fre- quency as a parameter . 52 23 Simulink model used for testing the boost converter . 57 24 Output voltage steady state of the boost converter . 58 25 Output voltage of the boost converter including initialization . 58 26 Total setup of the prototype . 60 27 Boost converter of the prototype . 61 28 Final setup for testing . 62 29 Setup for measuring the efficiency of the converter . 63 30 Measured efficiency against input power . 64 31 Inductor current . 65 32 Output voltage and current of the boost converter . 66 9 33 Voltage ripple at 60V input . 67 34 Core dimensions 1 . 88 35 Core dimensions 2 . 89 36 Core dimension calculations . 90 37 Ferrite specifications . 91 10 List of Tables 1 Overview of different capacitor choices . 29 2 Capacitor application measurements . 29 3 Converter electrical specifications . 43 4 Database of core characteristics needed for inductor design . 50 11 12 Glossary Q Charge [C] DC Direct Current AC Alternating Current I Current [A] V Voltage [V] CCM Continuous Conduction Mode DCM Discontinuous Conduction Mode EMF Electromotive Force EMI Electromagnetic Interference ESR Equivalent Series Resistance [Ω] f Frequency [Hz] Il Inductor Current [A] LC Inductor-Capacitor IGBT Insulated Gate Bipolar Transistor ◦ Tj Junction Temperature [ C] RL Load Resistance [Ω] MPP Maximum Power Point MPPT Maximum Power Point Tracking MOSF ET Metal-Oxide-Semiconductor Field-Effect Transistor RDS(on) Mosfet on-state resistance [Ω] Voff Off-state Voltage [V] CO Output Capacitance [F] Vout Output Voltage [V] Ipeak Peak Current Rating [A] PV Photovoltaic P Power [W] PWM Pulse Width Modulation SiC Silicon Carbide fs Switching Frequency [Hz] SMP S Switch-Mode Power Supply t time [s] ZCS Zero Current Switching ZVS Zero Voltage Switching 13 14 1 Introduction he World Solar Challenge is a biannual event where solar-powered cars race 3000km straight through the Australian desert. Participating T teams come from all over the world, most of them fielded by university teams or companies. The goal of the race is to be the first one to arrive at the finish line, given only a limited amount of stored energy. The rest of the needed energy has to be obtained from solar power. A high efficiency is therefore key in finishing first. The Nuon solar team (Nuna) is a team, formed by TU Delft students, that participates in this competition. They have an impressive track record of four successive wins, and of achieving second place during the last event. The chase to reclaim the title is on. One of the improvements planned for the next car is to design a custom-made MPPT that is built specifically for this application. Previous Nuna cars utilized commercially available, off-the- shelf, MPPTs. However, using an MPPT that is optimized for the Nuna solar car allows for a faster, lighter, and thus better, car than previous models. Tracking the maximum power point of solar panels can be used to greatly improve their efficiency. This is especially desirable in implementations where maximum efficiency is critical like in the Nuna Solar Car, built by the Nuon Solar Team. This report covers the design of a maximum power point tracker that is specifically designed to suit the needs of the next version of the Nuna. In order to cope with the complexity of the subject, the design of the MPPT is split in several smaller parts. Each of these different parts is de- scribed, fleshed out and simulated in a separate thesis. The titles of the three theses are: • "Maximum Power Point Tracking: Algorithm & Software Develop- ment", describing a high speed algorithm that tracks the maximum power point, • "Maximum Power Point Tracking Topology, Sensor and Switch De- sign", an interface between the hardware and software, • "Converter Design for Nuna Maximum Power Point Tracker", a con- verter to transform the output voltage to the desired level. This thesis concentrates on the design of the power stage of the MPPT and the design of the converter is explored and simulated. The converter design can be broken down further into choosing a suitable topology, calcu- lating the values for the required components, and designing the customized components. After the design process, simulations have been performed to 15 evaluate the efficiency of these parts. Besides the theoretical part, a proto- type of the MPPT has been built and tested to verify that the requirements are reached.