Research On Low Ripple Two Stage DC/DC Converter

Renewable electricity generation increases year by year,so the power electronic converters used in renewable electricity generation system become a research hotspot.Based on the research of DC/DC converters in photovoltaic and fuel cell electricity generation systems,this dissertation compares the characteristics and applied ranges of several domestic and foreign DC/DC converters.A Boost-LLC two-stage DC/DC converter is proposed in consideration of wide input voltage range,low current ripple,high step-up ratio and high efficiency.Interleaved Boost converter can effectively reduce the ripple of input current and stabilize the output voltage by adjusting the duty cycle when the input voltage changes.In this dissertation,the working principle of interleaved Boost converter is analyzed in depth,the expression of the input current ripple with the number of parallelled phases and inductance value is obtained.The small signal model is carried out using state space averaging method.The step-up ratio of the front-end stage is limited,considering energy efficiency and voltage stress.As the back-end stage,LLC resonant converter works at a fixed frequency.It works like a DC transformer,that is,when the load changes,its step-up ratio keeps basically constant.In this dissertation,the working principle of LLC is analyzed,the expression of DC gain in resonant network is obtained by fundamental wave analysis.Inductive resonance interval is selected in consideration of soft switch.In order to improve the efficiency and simplify the control scheme,it works at a fixed frequency in open-loop mode,and the small signal model of LLC resonant converter in this mode is established.According to the design requirements and theoretical analyses,circuit parameters are determined,such as the distribution of the step-up ratio between the two stages,the phase number in the front-end stage,the design of the magnetic elements and so on.The loss of the main circuit at rated operating point is calculated,and the open-loop simulation is carried out with Saber.The transfer function(from the front-end stage’s duty cycle to the back-end stage’s output voltage)is obtained by combining the small signal models of the two stages.The Bode diagram of the open-loop transfer function is drawn and verified by comparison with the scan result in Saber.Comparing the characteristics of analog controller and digital controller,considering the complexity of the control signal in this design,digital controller is finally selected,and the limit cycle in digital controller is also analyzed.Thecharacteristics of two design methods of digital controller(Digital Redesign and Direct Digital Design)are compared,and the control parameters are designed by direct digital design method.The digital closed-loop circuit is simulated in Matlab.The results show that the digital controller can guarantee the loop stability in the whole input voltage range.Finally,the principle prototype is designed and the theoretical analysis is verified by experiments.