Modeling of parasitic elements in high voltage multiplier modules

It is an inevitable trend that the power conversion module will have higher switching frequency and smaller volume in the future. Bandgap devices, such as SiC and GaN devices, accelerate the process. With this process, the parasitic elements in the module will probably have stronger influence on circuit operations than before. In this thesis, modeling of parasitic elements in the HV multiplier module of the HV generator in medical X-ray machines is addressed. The modeling is studied in two cases, the steady and transient operation of the circuit. The study results in a thorough understanding of the parasitic elements at a modular level and their influences on the circuit operation. In addition, the approaches to the modeling of the parasitics contained in this thesis can also be applied in other power electronic modules those have high power density by utilizing new devices in the future. Parasitic capacitances in the multiplier module in the steady state: The HV multiplier module in a CT scanner, which is typically a symmetrical C.W. multiplier module, can have an output as high as 160kV. This results in large number of diodes and capacitors in a compact module. By utilizing SiC diodes, the volume of the module will become much smaller than before, and the switching frequency of the circuit will become higher. The structural parasitic capacitances in such a module can be significant to the steady circuit operation. However, they were never studied before. In this thesis, the parasitic capacitances in the multiplier are thoroughly studied at a modular level. The study follows a systematic approach, including definition of the role of the capacitances, extraction of the capacitances by 3D FE simulation, construction of the analytical model, analysis and validation of the model and minimization of the parasitic capacitances. With the complete analysis, the structural parasitic capacitances in the HV module are well known and minimized. The obtained knowledge can also be applied to different HV multiplier modules. Electric field in the multiplier module in the steady state: Apart from the influence of the parasitic capacitances, the HV multiplier module has another important issue --- insulation. The strong electric field induced by the high voltage should be well contained to avoid breakdown of the insulation oil. In this thesis, a simple shielding technique is introduced for use in the field reduction. The distribution of the electric field strength is analyzed based on the results obtained through 3D FE field simulation. The distribution can be expressed as a function containing parasitic capacitances. Then, a shielding technique is proposed to reduce the field based on the expression. Simulation results show that the shielding technique can well contain the strong electric field. Parasitics in the multiplier module in the fast transient period: Over time, X-ray tubes have a tendency to arc-over. This results in short-circuiting of the load of the HV generator and in turn fast transient current pulses in the circuit. The bandwidths of such pulses can enter upper MHz range. In such a system, high-order circuit model of the parasitic elements are required for accurate circuit analysis. In this thesis, a theoretical development is presented on the circuit modeling of an electrical component or system in the beginning of the intermediate frequency range, which is just above the quasi-static range. The power series approach is utilized to obtain the LME model of the parasitics. With this approach, the underlying reason why the kth-order circuit model is valid for a structure with given electrical size is well understood. The calculation of EM fields and the derivation of the LME model is mathematically simpler than that by the full wave approach. Besides, the extension of the power series approach to the continuous spectrum systems provides an extension from sinusoildally single frequency systems towards real cases involving pulsed quantities. The theory can be developed in the future and applied to various applications in which high-order circuit model is necessary.