Abstract
Three-phase switching converters are broadly used in high power electrical systems. However, because of the nonlinear operation of switching components and the coupling between the three phases, the modeling work of a three-phase switching converter is very difficult. Therefore, a graphic modeling tool, namely switching flow-graph (SFG) modeling technique, for three-phase switching converters is proposed in this dissertation. This technique extends the familiar linear signal flow-graph theory to model the nonlinear switching circuits and greatly simplifies the analysis of nonlinear dynamics of the multi-switch circuits. One of the major contributions of this dissertation is that the proposed SFG technique can be applied to model the three-phase switching converters. The large-signal, steady-state and small-signal models of the switching converters can easily be developed simultaneously. At the same time, the proposed SFG model is able to provide the cause-effect phenomena of the switching converters, which is helpful to the controller design and system analysis. Another major contribution of this dissertation is proposing the concept of the virtual switch and virtual switching function which are very powerful for modeling multi-switch converter systems and possess the following merits: (a) By defining the virtual switch and virtual switching function, the difficulties of modeling the three-phase switching converters which include three-phase full-bridge inverters and three-phase full-bridge rectifiers can be obviated successfully. (b) Based on the concept, the effect of ON-resistance and blanking-time of the active switches can easily be taken into account. It is very valuable for actual application. (c) As the virtual switch and virtual switching function are defined, the six active switches and six diodes of the three-phase switching converters can easily be integrated and simplified to three virtual switches. According to the equivalent circuit with the virtual switches, the switching flow-graph of each phase is almost decoupled. Therefore, the complexity of the modeling work between phase and phase is automatically diminished. (d) Only three virtual switches and three virtual switching functions are required during the modeling process. The switching flow-graph for three-phase switching converters can be developed very easily. (e) The corresponding large-signal, steady-state and small-signal models can be obtained straightforward from the simple switching flow-graph. (f) The virtual switching function can be obtained easily by using the logical operators to combine the switching functions of active switches and the judgment of current directions. Simultaneously, the switching function of the diode can also be found. Moreover, the proposed SFG model can be implemented easily with MATLAB/SIMULINK facilely to carry out the time-domain simulation. The simulation results generated from the proposed SFG model are well confirmed with that generated from PSPICE. The computer execution time required by using the proposed SFG model is only about one tenth of that required by PSPICE under the same simulation conditions. It is more computationally efficient for system-level simulation as compared with the PSPICE model. In fact, the proposed SFG technique can also be applied to model various converters such as DCM DC-DC converters, three-phase diode-rectifiers, soft-switching circuit and parallel VSI, etc.