Dynamic analysis and QFT-based robust control design of switched-mode power converters

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Doctoral thesis (monograph)
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Verkkokirja (7599 KB, 145,[9] s.)
The use of switched-mode power converters is continuously growing both in power electronics products and systems, e.g. in Telecom applications, commercial grid systems etc. The switching converters are required to provide robust behavior and to operate without instability under a variety of operation conditions. Hence the converter system may be subject to disturbances due to load, input voltage, and system parameter variations. In the thesis a robust control design procedure based on the QFT method (Quantitative Feedback Theory) is applied successfully for switching-mode DC-DC converters in order to achieve robust output in spite of different uncertainties. Simulation results are presented to demonstrate and validate the control design, showing good dynamic performance of the QFT controller. When designing large-scale systems it is often impractical to analyze and design the system as a whole. Instead, it is desirable to divide the system into manageable subsystems which can then be designed independently. The subsystems may then be connected together to form a complete integrated system. One of the major difficulties in integrated subsystems is the stability performance degradation due to the interaction between the subsystems. A formalism to analyze the interaction between subsystems using the unterminated two-port small-signal representation is derived. Two-port models are first defined as unterminated models, where the effect of load is excluded but may be easily included using the developed reflection rules. The use of the impedance ratio as a minor loop gain, which can be used to check system stability, is outlined. Recently, there has been increasing interest in the parallel operation of DC-DC converters for reasons of increasing system reliability, facilitating system maintenance, allowing for future expansion, and reducing system design cost. However, paralleled DC-DC converters require a systematic modeling methodology and a categorical current-sharing mechanism to improve a performance of the overall system. In order to achieve desirable characteristics when operating converter modules in parallel, a unified systematic approached for modeling of parallel DC-DC converter with current-sharing control, is proposed, developed, and analyzed.
switched-mode converters, QFT-based robust control, subsystem interaction, current-sharing control