Modeling strategy for EMI filter and flyback transformer

“The switch-mode power supply is key to miniaturizing power adapters. However, the switching nature of the circuit introduces issues in conducted emissions. In a flyback converter, the transformer serves as the path for common mode current flowing from the primary side to the secondary side. Different winding technologies have been invented and implemented to reduce the capacitance between the primary side and the secondary side. But the repeatability of the winding is still poor due to the fluctuations of the winding machine. Thus, the resulting conducted emission has a fluctuation that can lead to failure in the compliance tests. EMI filter is another module implemented to reduce the conducted emissions. Due to the miniaturization, the components inside a filter are closely placed, therefore, strong mutual parasitics. These parasitics degrade the performance of the EMI filter. Overall, it would be beneficial if the performance of the EMI filter and the fluctuation of the transformer can both be analyzed through pre-design simulation. In this dissertation, a model strategy for EMI filters is developed and validated through comparison with measurement. The strategy covers different types of film capacitors, common mode chokes, and circuit topologies. This dissertation also provides an approach to asserting the parasitic capacitance of transformers through 2D analysis. Contradictory to the existing models that relate the parasitic capacitance and conducted emissions, the best-performance capacitance is found not zero. A simplified circuit model is developed to associate the conducted emissions with the parasitic capacitance of the transformer. This circuit model leads to an analytical formulation for evaluating the best-performance parasitic capacitance of the transformer, and its prediction matches with the observed relationship in the measurement. In conclusion, the research in this dissertation clarified the procedure for utilizing computer-aided simulation to guide the design of EMI filters and flyback converters in compact designs”--Abstract, page iv