In recent years, DC micro-grid in commercial street, data centers, residentialhouses and other occasions have get more and more attention. Among them, the distributed generation system mixing energy by photovoltaic, fuel cells and energy storage, has become an indispensable network unit.High voltage gain non-isolated DC/DC converters with low input current ripple have attracted much attention for photovoltaic cells, fuel cells and other renewable energy system applications. Also, in order to improve the efficiency of power transmission, the converter with high efficiency and low cost has became the goal of industrial and research scholars. In this thesis, high-gain converter and soft switching converter is selected and researched.Part I:Coupled-inductor High voltage gain convertersA coupled-inductor-Boost high voltage gain converter with a nondissipative LC snubber is presented in this thesis, steady state analysis of the converter and operating characteristics is developed, and also voltage gain of the converter, voltage stress of switching devices and characteristics of input current ripple is analyzed. The analysis results indicate thatleakage inductance energy is recycled and voltage spike stress of the power switch is suppressed by utilizing a snubber circuit composed of capacitor, inductor and diode. Therefore, a low switch-on-resistance low-voltage-rated MOSFET for reduction of the conduction loss and cost can be employed and efficiency of converter is improved. Furthermore, the proposed converter maintains the characteristics of continuous input current, increasing magnetizing inductance and decreasing coupled-inductor ratio can further reduce the input current ripple, and therefore makes it easy to design electromagnetic interference (EMI) filter. Meanwhile, in order toresolvevoltage oscillation of the output diode, a high voltage gain converter with coupled-inductor voltage-doubler cell and nondissipated LC snubber circuit is also presented, and steady state analysis of the converter and operating characteristics is analyzed.Based on zero-ripple circuit cell, a zero input current ripple high voltage gain non-isolated converter,zero-Ripple non-isolated high voltage gain converter based on coupled inductor voltage-doubler cell and coupled-inductor Boost integrated Flyback converter is proposed, which combines a zero input current ripple boost converter and a coupled-inductor cell to achieve high voltage step-up. Steady state analysis of the converter and operating characteristics is developed and Compared with existing high voltage gain converter, the corresponding performance comparison analysis is also given. It is shown that this converters can achieve high step-up voltage gain by utilizingcoupled-inductor ratio without extreme duty cycle.In addition, ripple-free input current can be achieved, which makes the design of electromagnetic interference (EMI) filter easy.Furthermore,leakage inductance energy of the transformer can be absorbed by intermediate storage capacitor and release to load when switch is turned-off. And also, voltage stress of the switch is clamped to storage capacitor voltage and thus a low-voltage-rated MOSFET with low Rds_on for reduction of the conduction loss and cost can be chosen to improve the efficiency of converter.By incorporating coupled-inductor into the boost cell of boost-flyback converter, the voltage stress across the output diode is effectively reduced compared with existing coupled-inductor converter.Part II:Secondary resonance soft switching techenique for isolated DC/DC converterBased on operating characteristics of traditional phase shift full bridge(PSFB) soft switching converter, this thesis analysis the relation between voltage gain ratio and duty cycle losses, ZVS condition of the switch, circulating loss and voltage oscillation of the output diode. In additon, full-bridge secondary dual-resonant DC/DC converter using phase shift pulse-width modulated (PSPWM) strategy is proposed, and zero-voltage switching for the lead leg and zero-current switching for the lag leg are obtained. Also, full bridge converter utilizing the asymmetrical pulse-width modulated (APWM) strategy is proposed, and zero-voltage switching for the power switches and zero-current switching for the rectifier diodes in the whole load range without the help of any auxiliary circuit are achieved. Given the use of the APWM strategy, a circulating current that exists in a traditional phase-shift full-bridge converter is eliminated. The voltage stress of secondary rectifier diodes in the proposed converter is also clamped to the output voltage. Thus, the existing voltage oscillation of diodes in traditional PSFB converters is eliminated, and improve the efficiency of the converter.Moreover, the circuit configuration of the proposed converter and its operating principle was analyzed in detail, and design example is given.This thesis also show steady state analysis of the asymmetrical half bridge(AHB) converter and operating characteristics is developed, and ZVS condition for the power switches, voltage stress of power devices, and DC-Offset Current in Transformer are given.Base on the AHB converter topology, a novel unregulated isolated DC/DC converter, called as DC/DC Transformer(DCX), is proposed in this thesis. Steady state analysis of the proposed converter and operating characteristics are developed, and also voltage gain of the converter, voltage stress of switching devices and ZVS condition for the power switches are analyzed. It is indicated that magnetizing current is provided to achieve ZVS turn-on for all switches. Meanwhile, Series-resonant circuit at second side of transformer is utilized to achieve ZCS turn-off for the diodes and also eliminates the diode reverse recovery losses and voltage spike stress of diodes, while the voltage stress of diodes are clamp to output voltage.Comparing to the traditional AHB converter topology, the output diode voltage stress is reduced without effect of varied duty cycle.Furthermore, asymmetric half-bridge secondary-side-resonant PWM switching converter was proposed and steady state analysis of this converter and operating characteristics are analyzed. Meanwhile, ZCS condition for secondary diode and voltage stress of power devices are also given. The analysis results show that zero-voltage switching (ZVS) of the switches was achieved by using asymmetric pulse width modulation (APWM) strategy and clamps the voltage of the switch to the input voltage. Also, Zero current switching (ZCS) of the output diode was achieved. The output diode voltage stress is independent of the duty cycle, and the voltage gain is almost linear, similar to that of the isolation Buck-type converter.Furthermore, the output filter inductor in the proposed converter reduced the output voltage ripple.In order to verify the correctness of the theoretical analysis, a mass of experimental results and experimental data are provided, and corresponding prototypes in lab is established. Experimental results have a good agreement with the theoretical analysis presented in this dissertation. |