| Due to the inherent advantages, including the ability of both voltage step-up and step-down, positive output voltage, and reduced number of passive components and voltage stress of the power devices, the two-switch buck-boost (TSBB) converter has been widely used in the applications with wide input voltage range. However, the TSBB converter does not achieve galvanic isolation, and its output voltage should be limited in the input voltage range. This dissertation is dedicated to isolated buck-boost converters, including the topologies and control strategies, to achieve high efficiency and satisfactory input voltage dynamic response. The proposed isolated buck-boost converters are attractive for the applications where input voltage is wide and galvanic isolation is also required, such as photovoltaic (PV) generation system, fuel-cell power supply, and distributed power system (DPS).This dissertation are mainly composed of three parts.The first part is Chapter II, where detailed analysis on the two-mode control method of the TSBB converter is given to prepare for the isolated buck-boost converter. Firstly, two-mode control, i.e., the TSBB converter operates in buck and boost modes in the high and low input voltage regions, respectively, is derived from the point of view on reducing the average inductor current. Compared with one-mode control, the two-mode control can reduce the conduction and switching losses effectively, achieving high efficiency over the entire input voltage range. In order to achieve automatic switching between buck and boost modes, the principle of two-mode control based on the two modulation signals and one carrier is discussed, and the condition for automatic smooth mode-switching is given. Besides, the small-signal model of the TSBB converter under the two-mode control is built, the corresponding closed-loop control is studied, and a250V-500V input,360V output and6kW rated power prototype is built and tested to verify the theoretical analysis of this chapter.The second part is Chpter III, in which a family of isolated buck-boost converters are derived on the basis of the TSBB converter, and taking full-bridge-boost (FB-boost) converter as an example, a high efficiency control scheme with automatic mode-switching is proposed. This control scheme includes:1) two-mode control:the FB-boost converter operates in full-bridge and boost modes in the high and low input voltage regions, respectively, to reduce the average inductor current, and thus reduce the conduction loss;2) two-edge modulation based on phase-shifted control:phase-shift control is adopted to achieve zero-voltage-switching of the switches of the full-bridge cell, and the reduction of the switching loss. Moreover, considering the duty-cycle loss resulted from the resonant inductor (including the leakage inductor of the transformer), only the leading-edge modulated FB and trailing-edge modulated boost cells can achieve mimimum inductor current ripple, reducing the conduction loss as a consequence;3) dual-frequency control:In the region where the input voltage refelected to the secondary side of the transformer is close to the output voltage, the inductor current ripple is very small, thus the switching frequency of the boost cell can be lowered to1/(2N+1)th (N is the positive integer) of the preset switching frequency, further reducing the switching loss and avoiding the saturation of the transformer. Furthermore, the high efficiency control scheme can achieve automatic smooth mode-switching by adopting the method with two modulation singals and one carrier. Besides, optimal design on the turns ratio of the transformer is discussed, the small-signal model of the FB-boost converter under this proposed control scheme is built, and the corresponding closed-loop control is studied in this chapter. In the same way, a250V-500V input,360V output and6kW rated power prototype is built and tested to verify the effectiveness of this proposed control scheme in Chapter Ⅲ.The third part is Chapter Ⅳ, where input voltage feedforward (IVFF) method for the FB-boost converter is proposed to improve the dynamic response to the input voltage. Firstly, the IVFF functions under different operating modes of the FB-boost converter are derived. Moreover, condsidering the small-signal and large-signal control laws of the derived IVFF functions, the automatic mode-switching control schemes with small-signal and large-singal IVFF compensation are proposed, respectively, in this chapter. With the two proposed control schemes, automatic selection of the operation modes and the corresponding IVFF functions can be achieved, simultaneously, and thus both high efficiency and satisfactory input dynamic response of the FB-boost converter are guaranteed. Besides, the proposed automatic mode-switching control schemes with IVFF compensation are also applied to the TSBB converter, and demonstrated on the prototypes of the TSBB and FB-boost converters, respectively. |
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