| Series-parallel power conversion systems, in which multiple standardized converter modules are connected in series or parallel at the output and input sides, can reduce the cost, shorten the design process, and improve system reliability. This dissertation is dedicated to the DC/DC and DC/AC series-parallel power conversion systems where the DC/DC converters and DC/AC converters are taken as the basic modules, respectively.This dissertation is composed of three main parts. The first part includes chapters 2 and 3, and it focuses on the improvement of the full-bridge converter. With phase-shifted control, the full-bridge converter achieves soft-switching with constant switching frequency. Hence, it is very suitable to be the basic module of the DC/DC series-parallel power conversion system. In order to eliminate the voltage spike across the rectifier diodes, two clamping diodes can be introduced to the full-bridge converter. However, the conduction of the clamping diodes causes relatively larger primary side conduction losses. In order to overcome this problem, a full-bridge converter with reset winding is proposed in the Chapter 2. The clamping diode current can be decayed to zero rapidly by the reset winding which is series with resonant inductor, reducing the primary side conduction losses and improving the conversion efficiency. Under light-load condition, it is found that the reset winding can not decay the clamping diode current to zero and the clamping diodes may be turned off hardly with reverse recovery loss. To overcome this problem, a full-bridge converter with auxiliary transformer is proposed further in Chapter 3. The clamping diode current can be decayed to zero rapidly by the auxiliary transformer at any load, leading to reduced primary side conduction losses and natural turn off of the clamping diodes.The second part of this paper includes chapters 4 and 5, and it focuses on the control strategies of the DC/DC series-parallel power conversion system. Series-parallel conversion systems can be classified into four basic architectures on the basis of the connection forms, namely, input-parallel output-parallel (IPOP), input-parallel output-series (IPOS), input-series output-parallel (ISOP), and input-series output-series (ISOS). Tthe existing studies on DC/DC series-parallel power conversion system only focus on one or two of the four connection architectures and do not reveal the inherent relationships between the four connection architectures. In chapter 4, the inherent relationships between the sharing of input voltages/currents and the sharing of output voltages/currents are analyzed systematically from the view of power conservation. It is pointed out that the control strategies of controlling the sharing of the input currents or output currents (IPOP) or output voltages (IPOS) can be used for input-parallel connected systems, however; only the control strategy of controlling the sharing of input voltages can be adopted for input-series connected systems. Based on the analysis, a general control strategy is proposed for DC/DC series-parallel power conversion systems, which not only achieves equilibrium between the constituted modules, but also decouples the output voltage control loop and the sharing control loop. Furthermore, modularization architecture for DC/DC series-parallel power conversion systems is proposed, achieving full modularization where all modules are self-contained and standardized, and no extra control is needed to achieve sharing of voltage and/or current at the input and output sides.For ISOP DC/DC system, chapter 5 reveals the essential reason for system instability when output current sharing control strategy is used from the view of input impedance. It is pointed out it is the negative resistance characteristics of the module's input impedance that results in system instability. The concept of the positive resistance is proposed to ensure the stability of ISOP system. Based on the concept, the derivation of three-loop control strategy, consisting of a common output voltage loop, input voltage sharing loops, and individual inner current loops, and the corresponding minimum gain of input voltage sharing loop is also presented.The third part of this paper includes chapters 6 and 7, and it focuses on the control strategies of the DC/AC series-parallel power conversion system. The existing studies on DC/AC series-parallel power conversion system only focus on one of the four connection architectures. In chapter 6, the inherent relationships between the sharing of input voltages/currents and that of output voltages/currents are analyzed systematically from the view of power conservation. It is pointed out that the output active powers of the constituted inverter modules are ensured to be identical if the sharing of input voltages/currents is achieved, however, the output reactive power of the constituted inverter modules may be not equal; and the constituted inverter modules will achieve the sharing of input voltages/currents if the sharing of output voltages/currents is ensured, however, the control strategies to ensure the sharing of output voltages/currents are unstable for input-series connected inverter systems (ISOP and ISOS). Therefore, for input-parallel connected inverter systems (IPOP and IPOS), the control strategies of controlling the sharing of the output currents/voltages are effective as well as the hybrid input current sharing control strategy, and the later one is very complicated. For input-series connected inverter systems (ISOP and ISOS), only the hybrid input voltage sharing control strategy can be used, i.e., controlling the magnitude or phase of output current (ISOP) or output voltage (ISOS) to be equal based on controlling the input voltage sharing. Based on the above conclusion, a control strategy with output current feedback is proposed for ISOP inverter system in chapter 7. With the proposed control strategy, the ISOP inverter system can achieve output current sharing effectively no matter the output filter capacitors of the constituted inverter modules are matched or not. In the meanwhile, the input voltage sharing loops and system output voltage loop are decoupled in the proposed control strategy. The control loop design of the first-stage DC/DC converter is discussed. In order to reduce the current stress of the power devices in the first-stage DC/DC converter, it is pointed out that the crossover frequency of the output voltage loop is better to close to 1/5 of the inverter output frequency. The input voltage sharing loop of the proposed control strategy is designed based on the concept of positive resistance presented in chapter 5, and it is pointed out that compensator gain of the input voltage sharing loop must be larger than a specific value, otherwise the system is unstable.
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