Tuning and Alignment Analysis of Capacitive Coupling 

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Chapter 2 General Structure of CCPT System


Like IPT, CCPT is designed to transfers power from a stationary electric power source to one or multiple movable loads (pickups) without direct electrical contacts. However unlike an IPT system, a CCPT system uses electric field coupling as the energy transfer medium so metal surroundings have almost no effect on the system power transfer capability due to the metal penetration ability of the electric field. However, electric coupling in CCPT systems is usually very weak compared to that in a normal capacitor due to the existence of the air gap between the primary and secondary coupling plates. The equivalent coupling capacitance of a CCPT system is usually designed to be as high as possible, where dielectric materials with high relative dielectric constants help greatly. Since the capacitive coupling is voltage driven,it is necessary to generate a high frequency ac voltage from a primary converter. To meet the load requirement, series and/or parallel tuning inductors are often used to compensate for the reactive power generated by capacitive coupling.This chapter describes a general structure of CCPT technology at a system level, including the primary power converting network topologies, electric field/capacitive coupling,secondary compensation, and power flow controller. As CCPT technology is relatively new with very limited publications, the contents in this chapter are mainly from the findings and publications based on this research. Nevertheless, the structure introduced can be regarded as a summary and guide for the rest of this thesis and future research. Detailed theoretical analysis and practical tests will be presented in the following chapters.
A high quality primary power supply is essential in a CCPT system because it generates high frequency ac voltages, thus the electric field enables contactless power transfer through the coupling of metal plates. In this section several primary converters suitable for CCPT technology are introduced.

Voltage‐fed resonant converter

Fig. 2-1(a) shows the circuit diagram of a typical full-bridge voltage-fed resonant converter used in CCPT systems. A dc voltage supply Vd is directly used as the input power source, and four semiconductor switches (usually MOSFETs) S1 to S4 form a switching network. The output of the switching network is connected to a series-tuned resonant tank LP and CP. Then the high frequency ac voltage VCp is generated to drive the two primary coupling plates P1 and P2. The advantages of this structure are that the load can always obtain a constant output current regardless of the coupling variations, and output voltage can be boosted with the proper design of the primary resonant tank. However, the major drawback is if the pickup plates are suddenly removed from the primary plates, a high voltage surge will be generated across the coupling plates because the discharge loop of the equivalent coupling capacitor is disconnected. Even under normal coupling conditions, because the coupling capacitance is usually very small (less than nF level), the voltage across the coupling plates will be quite high, which may cause dielectric breakdown unless carefully designed. Another structure is proposed as shown in Fig. 2-1(b), where the capacitive coupling is modelled as a series capacitor which can act as the primary resonant capacitor, and the compensation inductor LS acts as the primary resonant inductor. The whole system becomes very simple as it only contains the switching network, resonant circuit, and load. If the resonant tank is fully tuned, i.e., ω2 LSCS=1, where CS=CS1CS2/(CS1+CS2), the output voltage VO will be equal to the ac voltage expressed in (2.1). Therefore this structure outputs a constant voltage source. The disadvantage is that the output voltage does not have any boostup magnitude capability.

List of Figures 
List of Tables
Chapter 1 Introduction 
1.1. Background 
1.2. Capacitively coupled power transfer technology 
1.2.1. Basic structure and operating principle
1.2.2. Advantages of CCPT technology
1.3. State-of-the art and challenges of CCPT
1.3.1. Previous achievements
1.3.2. Present challenges
1.4. Objective and scope of the thesis
Chapter 2 General Structure of CCPT System
2.1. Introduction 
2.2. Primary power converter 
2.2.1. Voltage-fed resonant converter
2.2.2. Current-fed resonant converter
2.2.3. Other converters
2.3. Capacitive coupling 
2.3.1. Equivalent coupling capacitor model
2.3.2. Coupling structures
2.3.3. Dielectric material
2.4. Secondary pickup 
2.4.1. Tuning of capacitive coupling
2.4.2. Impedance matching
2.4.3. Multiple couplings
2.5. Power flow control 
2.5.1. Fixed frequency operation
2.5.2. Variable frequency operation
2.6. Summary 
Chapter 3 Modelling and Analysis of a Soft-switched CCPT System
3.1. Introduction 
3.2. ZVS frequency analysis of CCPT system with ac load 
3.2.1. System modelling
3.2.2. Determination of ZVS operating frequencies
3.2.3. System performance analysis
3.2.4. Simulation and experimental results
3.3. Steady-state analysis of CCPT system with dc load
3.3.1. System modelling
3.3.2. Determining ZVS operating frequency
3.3.3. Flow chart of the iterative algorithm
3.3.4. Steady-state analysis
3.3.5. Experimental verification
3.4. Summary 
Chapter 4 Capacitive Coupling Analysis 
4.1. Introduction 
4.2. Proposed capacitive coupling 
4.2.1. Coupling structures
4.2.2. Dielectric materials
4.2.3. Coupling design examples
4.3. A generalised model of capacitive coupling 
4.3.1. Proposed coupling model
4.3.2. Capacitive coupling factor
4.3.3. Generalised coupling model for multiple pickups
4.3.4. Simulation and experimental results
4.4. Summary
Chapter 5 Tuning and Alignment Analysis of Capacitive Coupling 
5.1. Introduction 
5.2. Simple circuit model
5.3. Analysis under ideal conditions 
5.3.1. Voltage transfer gain
5.3.2. Voltage across the tuning inductor
5.3.3. Displacement power factor
5.4. Non-ideal conditions
5.4.1. Voltage transfer function
5.4.2. Voltage across the tuning inductor
5.4.3. Displacement power factor
5.4.4. Total harmonic distortion
5.4.5. Power efficiency
5.5. Effect of parameter variations
5.5.1. Transfer gain of the output voltage
5.5.2. Maximum voltage across the tuning inductor
5.5.3. Displacement power factor threshold
5.6. Further explanation of two different tuning inductor positions 
5.7. Experimental results 
5.8. Summary
Chapter 6 Power Flow Control and Output Voltage Regulation 
6.1. Introduction 
6.2. Integration of voltage rectification and power flow control 
6.2.1. System structure
6.2.2. Control algorithm
6.2.3. Control operation analysis
6.2.4. Simulation and experimental results
6.2.5. Discussion
6.3. Dynamic tuning control through soft-switched transformer
6.3.1. Simple system using the proposed control method
6.3.2. Control switching waveforms and equivalent inductance
6.3.3. Control algorithm
6.3.4. Simulation results
6.4. A capacitively coupled contactless matrix charging platform with soft-switched transformer control 
6.4.1. System overview
6.4.2. Transfer function of output voltage
6.4.3. Power flow control algorithm
6.4.4. Simulation and experimental results
6.5. Summary
Chapter 7 Conclusions and Suggestions for Future Work 
7.1. General conclusions 
7.2. Contributions of this thesis work 
7.3. Suggestions for future work 
Appendix A: MATLAB code for calculating ZVS frequencies
Appendix B: MATLAB/PLECS schematic set-up for simulation of CCPT matrix
charging platform 


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