Nam Nguyen-Quang High-frequency matrix converters for induction heating i 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 Nam Nguyen-Quang High-frequency matrix converters for induction heating ii 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 three- phase 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 Nam Nguyen-Quang High-frequency matrix converters for induction heating iii • 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 single- phase 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. Nam Nguyen-Quang High-frequency matrix converters for induction heating iv 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 Abu- Sharkh 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 3 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. Nam Nguyen-Quang High-frequency matrix converters for induction heating v To my mother Nam Nguyen-Quang High-frequency matrix converters for induction heating vi Table of Contents Summary i Acknowledgements iv 1 Introduction 1 1.1 Basic principles of induction heating 1 1.1.1 Excitation frequency vs. heat penetration 4 1.1.2 Background to induction heating 6 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 2 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 Nam Nguyen-Quang High-frequency matrix converters for induction heating vii 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 3 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 4 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 Nam Nguyen-Quang High-frequency matrix converters for induction heating viii 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 5 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 6 Conclusions and Future Work 224 6.1 Conclusions 224 6.2 Future work 227 7 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 Nam Nguyen-Quang High-frequency matrix converters for induction heating 1 1 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 2 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, Q L , is usually used to denote the effect of the poor coupling in work coils. Q L is defined as in (1.1), where the coil has the inductance of L (Henrys), and R L (Ohms) represents the sum of the coil and reflected load resistances. f s is the frequency of the electromagnetic field (which is usually the switching frequency). In this definition, the coil inductance L and the total resistance R L are in series. The quality factor, in fact, describes the ratio between the reactive power and the active power of the coil. L s L R Lf Q π 2 = (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 Q L than non-ferrous counterparts. However, after heating above the Curie point, at which point the ferrous material looses its ferromagnetic properties, Q L 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]. [...]... principle limit of 50 kHz to 150 kHz and beyond with power ratings per device of 4 MW at 50 kHz [1.6] 6 Nam Nguyen-Quang High-frequency matrix converters for induction heating In the 1980s, a trend of developing MOSFET-based inverters to replace valve-based systems for radio frequency induction heating began to emerge [1.7] – [1.9] ‘Transitorized power supplies for induction heating [1.7] is one of the first... magnitude of the current density is 1/e of its value at the surface (where e is the base of natural logarithms) is called ‘depth of penetration’ or ‘skin depth’ [1.2] ρ πµf s δ= 4 (1.3) Nam Nguyen-Quang High-frequency matrix converters for induction heating The depth of penetration depends on the resistivity and permeability of the material, and the frequency of the magnetic field, as shown in (1.3) [1.2] For. .. primary variable to control the skin depth (permeability being relatively constant), for induction heating, standard operating frequency bands have been defined, as shown in Table 1.2 [1.2] 5 Nam Nguyen-Quang High-frequency matrix converters for induction heating 1.1.2 Background to induction heating Induction heating of objects has now become a relatively mature technology In 1927 [1.2], the first medium... transformer due to its low impedance at resonance 3 Nam Nguyen-Quang High-frequency matrix converters for induction heating The efficiency of the coil is also important when designing work pieces made of different materials For a tubular coil with a work piece inside it, the efficiency of the coil can be estimated from (1.2) [1.2], where ρc and µc are the resistivity and permeability of the coil, and. .. half-cycle of the load current, S1 and S2 are switched off As the inductive impedance of the load circuit tends to maintain the load current, C1 and C3 now support the current of S1, and C2 and C4 support the current of S2, as shown in Fig 1.14 The load current charges C1 and C2, whilst discharging C3 and C4, hence lowering the voltage at the node between S1 and S3, and raising that at the node of S2 and. .. S4 on; S1, S2, S3, and S4 on The paper does discuss the presence of ‘ringing’ due to the parasitic lead inductance and the drain-to-source capacitance of the MOSFETs, and 9 Nam Nguyen-Quang High-frequency matrix converters for induction heating emphasises the importance of a good component layout and routing to reduce the problem Many of the concepts pioneered in this paper remain in use, despite the... load power factor For sinusoidal voltage/current circuits, λ is the cosine of the phase angle between the voltage and the current Pload = Vin ( RMS )I in ( RMS )λ 21 (1.4) Nam Nguyen-Quang High-frequency matrix converters for induction heating If the voltage and the current are non-sinusoidal, then a Fourier series can be used to obtain a series of sinusoidal voltages and currents, and the total power... Using a variable ratio transformer between the inverter and the resonant circuit can partially solve the impedance mismatch problem 24 In this case, the normalized Nam Nguyen-Quang High-frequency matrix converters for induction heating impedance and power of the resonant load can be represented as functions of the normalized frequency, with different curves for different values of the quality factor (QT)... changes polarity, when S1 and S4 are in the on-state, as shown in Fig 1.19, as the load current is in its positive half-cycle and S2 is open, the current circulates through S1 and D4, effectively short-circuiting the load voltage This state remains until S4 is switched off (and S2 is complementally 25 Nam Nguyen-Quang High-frequency matrix converters for induction heating switched on), and there is no power... inverter has been used for most of the induction heating systems due to its simple and straightforward approach, however, it has detrimental effects on the utility supply, and low efficiency in high frequency systems 1.2.2 Voltage-source inverter Voltage-source inverters were an emerging topology at the time of the publication of the review in [1.1] Historically, the performance of the voltage-source . father and my sister. Nam Nguyen-Quang High-frequency matrix converters for induction heating v To my mother Nam Nguyen-Quang High-frequency matrix converters for induction heating vi Table of. Nguyen-Quang High-frequency matrix converters for induction heating i Summary This thesis describes the development of novel high-frequency (>150 kHz) matrix converters for induction heating. single-phase matrix converter for high-frequency induction heating , accepted for publication in EPE-2009, Barcelona, Spain. Nam Nguyen-Quang High-frequency matrix converters for induction heating