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State Plane Analysis and Optimization of CLLC Resonant Converter for Bidirectional EV Charger

Rezayati, Mohsen | 2023

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  1. Type of Document: Ph.D. Dissertation
  2. Language: English
  3. Document No: 56154 (05)
  4. University: Sharif University of Technology and University of Grenoble Alpes
  5. Department: Electrical Engineering
  6. Advisor(s): Tahami, Farzad; Schanen, Jean-Luc
  7. Abstract:
  8. As more renewables step in, the ongoing change will be drastic for the electrical grid. The energy sector must find new ways to balance energy production and consumption. Electric vehicle (EV) batteries are by far the most cost-efficient form of energy storage since they require no additional investments in hardware. A bidirectional EV charger is a new technology that enables energy to be pushed back to the power grid and auxiliary loads from the battery. The electrical requirements of the EV charger are examined in detail to find an appropriate power electronics converter to meet all the requirements. The CLLC resonant converter is a promising candidate for high efficient, bidirectional power transfer applications such as vehicle-to-grid (V2G) on-board chargers and hybrid vehicle DC-DC converters. The fundamentals of the bidirectional EV charger based on the CLLC resonant converter and its main components including the transformer, resonant inductors, resonant capacitors, inverter and rectifier parts, etc. are studied in depth, where the advantages and disadvantages of the CLLC resonant converter and possible candidates are highlighted. A global design methodology including transformer design, inductor design, capacitor design, semiconductor losses, diode losses, magnetic core losses, and capacitor losses is studied to define an optimization problem corresponding to the objective function and constraints. An interpolation-based method is proposed for the GaN transistor switching losses with the different voltage and current levels, gate driver circuits, junction temperature, parasitic capacitors, etc. A conduction loss model is also presented based on the datasheet of GaN transistors. The objective function can be considered as maximizing efficiency and power density, as well as, minimizing mass, volume, and losses. The constraints are also highlighted, which are the battery charger operating region, maximum temperature, maximum RMS current of resonant elements, maximum voltage, soft-switching condition, and operating in continuous conduction mode (CCM). Most of these constraints and the CLLC resonant converter analysis have been calculated by the first harmonic approximation (FHA) method, which has low accuracy. Therefore, the analysis of CLLC still remains challenging because of its complex multi-resonant nature and several storage elements. In this dissertation, a circuit analysis method based on the change of variables is presented that maps the state space equations into two decoupled sets of equations. The analyses are carried out in two state-plane coordinate systems, then the results are mapped onto the original region. Two decoupled sets of equations are similar to the series resonant converter. The proposed method is then used to thoroughly analyze the CLLC resonant converter operating in the CCM and discontinuous conduction mode (DCM). The voltage and current stresses of components, zero voltage switching condition, output voltage gain, output characteristic diagram, and mode boundary of CCM/DCM are then obtained. The accuracy of the proposed approach is verified by the simulation on a 3.3 kW bidirectional CLLC resonant converter with GaN transistors as the primary and secondary side switches. The results confirm the accuracy of the proposed state-plane analysis of the CLLC resonant converter operating in either direction of power transfer. By completing the exact analysis of the CLLC resonant converter, the second aim of this dissertation is relative to the optimization of the CLLC resonant converter. The CADES Software is used as the platform of optimization and the state plane analysis is implemented on it using C++. Furthermore, the accuracy of the implemented method is verified by Maxwell simulation to be able to be used in the optimization process and check the value of magnetic components, saturation of magnetic material, etc. The common optimization of a resonant converter has been based on the single operating point. The selection of a single operating point leads the designed converter not to meet all the constraints when the converter operates in a different operating point. Based on the charging process of a Lithium-Ion battery, its operating region contains a wide range of voltages and currents. Therefore, there exist multiple operating points for a bidirectional EV charger. In this dissertation, a new algorithm for multiple operating points optimization is presented. The Sequential Quadratic Programming (SQP) algorithm is adopted to find the optimum results, where the second derivative of the objective functions must be available. However, there are some discrete parameters for optimizing a bidirectional EV charger, such as core sizes, number of turns, and number of turns in each layer. Therefore, in the proposed method, all the discrete parameters are considered as continuous parameters for the availability of the derivatives. This set of parameters is entitled to the imaginary world due to the fact that these parameters are not physically implementable. The proposed method optimizes the CLLC resonant converter in the imaginary world and with some steps tries to find the optimum solution in the real world (i.e. the physically implementable parameters). In each step, the discrete parameters will be fixed by putting a floor under them and setting a ceiling on them. At the end of the steps, the optimum charger is selected between 24 samples. Finally, the two aims of this dissertation, i.e. analysis and optimization, are verified by the experimental tests and comparison with the simulation results and FHA-based results. The accuracy of the proposed state-plane analysis is verified by the experiments on a 3.3 kW bidirectional CLLC resonant converter with GaN transistors as the primary and secondary side switches. The results confirm the accuracy of the proposed state-plane analysis of the CLLC resonant converter operating in either direction of power transfer. The optimization algorithm is also verified using two experimental setups, i.e. the optimum converter and a smaller in-size converter
  9. Keywords:
  10. Electric Vehicle Charger ; Resonant Converter ; Soft-Switching ; Optimization ; Power Loss ; Sequential Quadratic Programming (SQP) ; Multiple Operating Points ; Bidirectional Electric Vehicle Charger ; Magnetic Components Design ; Power Loss Modeling

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