Design and analysis of high frequency matrix converters for induction heating

Nam Nguyen-Quang High-frequency matrix converters for induction heating Summary This thesis describes the development of novel high-frequency (>150 kHz) matrix converters for induction heating applications The primary goal has been to explore the possibility of making high performance direct converters, allowing more compact and reliable power converters to be realised, by removing the bulky energy storage components of the DC-link found in traditional approaches High input power quality, i.e unity input power factor and very low input total harmonic distortion (THD), and good efficiency are demanded A single-phase matrix converter has been developed to firstly explore the possibility of performing high speed commutation of output current, without any DC-link between the input and the output A novel single-step voltage commutation strategy, implementing soft-switching condition over a wide power control range, has been developed and experimentally verified The high performance of the converter has been confirmed by comparison to a benchmark reference converter, which is a modified H-bridge converter with an unsmoothed DC-link A pulse density modulation scheme has also been developed and preliminarily verified, for use in the development of the three-phase to single-phase matrix converter A novel topology of three-phase direct converter, in the form of a 3×2 matrix converter, featuring high input power quality, and soft-switching operation, has been proposed, along with a few novel modulation strategies A basic rectifying algorithm has been used to investigate the impact of unavoidable disconnection of one input phase (in a 3×2 matrix converter), for an interval of up to a few tens of switching cycles Following that, a constant output pulse density modulation (CPDM) method has been developed i Nam Nguyen-Quang High-frequency matrix converters for induction heating and tested, showing improvements on the input current waveform, but further reduction of THD is still required Three variable output pulse density modulation (VPDM) methods, utilising different pulse patterns, have finally been proposed, to create high input power quality and good efficiency, with results supported by measurements on an experimental laboratory prototype It is notable that this high performance has been achieved with a very simple controller, requiring no on-line calculations for the synthesis of three-phase input current system Finally, some methods of improving converters’ efficiency, namely reducing on-state resistance of power devices, and application of synchronous rectification, have been investigated Since the on-state resistance of power devices has been reduced to a value that is not yet realistic in commercial devices, the investigation has therefore been carried out by simulations Switching patterns for both the single-phase and the threephase to single-phase matrix converters have been modified to accommodate synchronous rectification action where appropriate, with supporting results from experiments on laboratory prototypes Unlike other matrix converters, the matrix converters described in this thesis are the first direct converters to supply high frequency (>150 kHz) output current, mainly for induction heating applications, featuring the following advantages: • Single-step voltage commutation, allowing high speed soft-switching, at low implementation cost • Very high input quality, i.e unity power factor and very low input current THD • Very good efficiencies (at least 92% at full power) • PWM and PDM methods of power control, helping reduce EMC problems associated with frequency modulation methods ii Nam Nguyen-Quang High-frequency matrix converters for induction heating • Very simple control algorithm, independent of output frequency and requiring no online calculations Published material based on the research presented in this thesis include: N Nguyen-Quang, D.A Stone, C.M Bingham, & M.P Foster: ‘Single phase matrix converter for radio frequency induction heating’, SPEEDAM 2006, Taormina, Italy CD-ROM Proceedings N Nguyen-Quang, D.A Stone, C.M Bingham, & M.P Foster: ‘Comparison of singlephase matrix converter and H-bridge converter for radio frequency induction heating’, EPE-2007, Aalborg, Denmark CD-ROM Proceedings N Nguyen-Quang, D.A Stone, C.M Bingham, & M.P Foster: ‘A three-phase to single-phase matrix converter for high-frequency induction heating’, accepted for publication in EPE-2009, Barcelona, Spain iii Nam Nguyen-Quang High-frequency matrix converters for induction heating Acknowledgements First, I would like to thank my supervisors, Dr David Stone and Dr Chris Bingham, for their invaluable guidance and support during the entire length of my PhD study I also thank Prof Geraint Jewell for acting as internal examiner, and Dr Suleiman AbuSharkh of the University of Southampton for acting as external examiner I would also like to thank the Vietnamese government, in particular the “Vietnamese Overseas Scholarship Program”, for their financial support, without which my PhD study would be impossible My thanks to all members of the Electrical Machines and Drives research group, where the work for this research was conducted This has been my second home thanks to your hospitality My wife, Lien Cao-Bich, for her love, understanding and patience over the last years Thank you for being a great companion I would like to express my appreciation to my family and close friends In particular, I thank my mother, my father and my sister iv Nam Nguyen-Quang High-frequency matrix converters for induction heating To my mother v Nam Nguyen-Quang High-frequency matrix converters for induction heating Table of Contents Summary i Acknowledgements iv Introduction .1 1.1 Basic principles of induction heating 1.1.1 Excitation frequency vs heat penetration 1.1.2 Background to induction heating 1.2 Resonant inverters 11 1.2.1 Current-fed inverter .11 1.2.2 Voltage-source inverter 17 1.2.3 Alternative resonant load circuit .34 1.3 Methods used for analysing resonant-mode inverter systems .35 1.4 Power factor correcting rectifiers 40 1.5 Other converters 42 1.6 Proposed system 45 1.7 References .46 Benchmark reference converter and state-of-the-art matrix converter technologies .54 2.1 Introduction .54 2.2 Design of the reference system .56 2.2.1 Resonant output circuit 56 2.2.2 MOSFET-based H-bridge .64 2.2.3 Transistor gate sequencing 65 2.2.4 Gate drive module 67 2.3 Simulation and experimental results .68 2.4 State-of-the-art of matrix converter technology 74 2.4.1 Introduction .74 2.4.2 Background to matrix converter technology 75 2.4.3 Single-phase AC-AC converters .78 2.4.4 Three-phase to single-phase matrix converters .81 2.4.5 Three-phase to three-phase matrix converters 89 2.4.6 Practical bidirectional switch realisation 100 vi Nam Nguyen-Quang High-frequency matrix converters for induction heating 2.4.7 Commutation methods 102 2.4.8 Input filter design 107 2.4.9 Protection issues 109 2.4.10 Driving circuit designs 111 2.5 Summary .113 2.6 References .115 Single-phase matrix converter .123 3.1 Introduction 123 3.2 Fundamentals of high frequency single-phase matrix converter 124 3.2.1 Resonant output circuit (load) .124 3.2.2 Structure of the single-phase matrix converter .124 3.2.3 Switching control pattern and operating principle 125 3.3 Design of single-phase matrix converter .131 3.3.1 2x2 matrix converter design 131 3.3.2 Gate drive module design 132 3.3.3 Input filter design 134 3.4 Simulation and experimental results .135 3.5 Performance comparison of single-phase matrix converter and H-bridge converter 140 3.5.1 Topology comparison 140 3.5.2 Input quality comparison .142 3.5.3 Controllability comparison 144 3.5.4 Efficiency comparison 145 3.6 Pulse density modulation 148 3.7 Conclusions 151 3.8 References .153 Three-phase to single-phase matrix converter 154 4.1 Introduction 154 4.2 Basic principles of the three-phase to single-phase converter 156 4.3 Input rectifier algorithm 160 4.4 Constant output pulse density modulation 162 4.5 Variable output pulse density modulation 169 4.5.1 Interlaced pulse density modulation .172 4.5.2 Non-interlacing pulse density modulation 178 vii Nam Nguyen-Quang High-frequency matrix converters for induction heating 4.5.3 Hybrid pulse density modulation 181 4.5.4 Line frequency synchronisation and output current circulation 183 4.5.5 Performance evaluations .187 4.6 Conclusions and discussions 204 4.7 References .207 Performance improvement for matrix converters 209 5.1 Introduction 209 5.2 Influence of on-state resistance 209 5.3 Modified switching algorithms .211 5.4 Performance evaluation of matrix converters with new switching algorithms .214 5.5 Conclusions and discussions 219 5.6 References .222 Conclusions and Future Work .224 6.1 Conclusions 224 6.2 Future work 227 Appendices 230 7.1 Hardware schematics 230 7.2 FPGA configurations (VHDL code) .232 7.3 PIC and dsPIC programs (Basic and C code) .255 viii Nam Nguyen-Quang High-frequency matrix converters for induction heating Introduction The research and development of radio-frequency matrix converter technology, for use in induction heating applications, is described This chapter introduces related technologies and the current state-of-the-art in the field A review of resonant inverters used in induction heating systems, and the analysis methods associated with those inverters, is also given 1.1 Basic principles of induction heating The underlying principles of induction heating are based on the creation of an alternating electromagnetic field that is used to induce current in a load and in so doing heat the load (via the ‘skin effect’) The energy transfer mechanism is similar to ‘transformer action’, whereby energy is transferred from a primary winding to a secondary winding through induction In the case of induction heating systems, the primary is the heating coil, known as the ‘work-head’, and the secondary (which is effectively a short circuit) is the object to be heated up, termed the ‘work-piece’ The work-head and work-piece are therefore isolated, giving a non-contact heating method, which is very important for improving the product quality in metallurgy and semiconductor industries, for instance The method also provides localised heat treatment, allowing very accurate heat profiles to be realised When designing transformers the coupling between the primary and secondary is usually good, however, the coupling between the heating coil and the load is normally poor as a consequence of the mechanical clearance needed for loading and heat isolation This means only a small fraction of the power from the work-head ultimately dissipates as heat in the load, and therefore, a loaded coil normally has a low power Nam Nguyen-Quang High-frequency matrix converters for induction heating factor The situation becomes worse in high frequency coils since the effect of the leakage inductance is more pronounced than in low frequency counterparts This also implies that the loaded work-head acts like an inductive load In the induction heating industry, the quality factor, QL, is usually used to denote the effect of the poor coupling in work coils QL is defined as in (1.1), where the coil has the inductance of L (Henrys), and RL (Ohms) represents the sum of the coil and reflected load resistances fs is the frequency of the electromagnetic field (which is usually the switching frequency) In this definition, the coil inductance L and the total resistance RL are in series The quality factor, in fact, describes the ratio between the reactive power and the active power of the coil QL = 2πf s L RL (1.1) The magnetic coupling between the work-head and the work-piece depends on the magnetic properties of the material of the work-piece, with ferrous materials having lower QL than non-ferrous counterparts However, after heating above the Curie point, at which point the ferrous material looses its ferromagnetic properties, QL increases significantly, implying that much lower active power can be delivered to the load for a given VAr Table 1.1 shows some examples of quality factor for different materials and operating conditions [1.1] Radio-frequency loaded work coils usually have Q values in the range 5-15 for ferrous loads and between 10-25 for non-ferrous loads [1.2], implying that much more power is dissipated in the coil than into the load, which is a common case Therefore, coil losses ultimately limit the Q However, coil loss is usually increased by the skin effect, which increases the AC resistance of the coil By silver-plating the coil, or replacing the coil with cooled Litz wire, the range of effective Q can sometimes be increased [1.1], [1.3] Nam Nguyen-Quang High-frequency matrix converters for induction heating if (cnt > (power + 10)) then Sayr