Research On The Topology And Characteristics Of The Unidirectional DC-DC Step-up Converter Based On Magnetic Coupling Components

With the development and mature of renewable energies and DC microgrids,high step-up unidirectional DC/DC converters have become a critical research topic,serving as a key link between renewable energy sources and the DC busbar.While traditional Boost converters can theoretically achieve any desired voltage gain,their actual gain is limited by parasitic parameters.Accordingly,there is currently a great deal of interest in exploring unidirectional high step-up DC/DC converters.Although many novel high step-up unidirectional DC/DC converters have been developed,their high voltage gains are usually achieved by stacking more devices at the topological level.As a result,the overall size and weight of the converter increase,limiting further increases in power density.Additionally,there is currently a lack of theoretical understanding regarding the steady-state characteristics of these converters,which makes it difficult to provide reliable design guidelines for these topologies and hinders their widespread adoption in industrial applications.In view of the above,the high step-up unidirectional DC/DC converter not only lacks topologies with high device utilization and high-power density,but also lacks a reliable and sound theoretical analysis method to provide design guidelines for these converters.Therefore,the existing high step-up unidirectional DC/DC converters are not suitable for the overall trend of power electronics converters towards high frequency and light weight.To address these problems and technical gaps in unidirectional high step-up DC/DC converters,this paper conducts research from two perspectives: topology and steady-state characteristics modeling analysis.The specific research contributions of this paper are summarized as follows:(1)A novel unidirectional high step-up DC/DC converter with improved device utilization is developed.Based on the boosting principle of traditional boost converters,by coupling the voltage of the output capacitor into the charging circuit of the boost input filter inductor using a switched-capacitor structure,a higher voltage boost factor can be achieved.Additionally,the filter inductor is replaced with a coupled-inductor-based voltage multiplier to achieve higher voltage output.This not only improves power density by reusing the magnetic core but also increases device utilization when combined with the switched-capacitor structure.Experimental results confirm the feasibility of this topology and demonstrate that the voltage stress on power switches can be significantly reduced by introducing a coupled-inductor-based voltage multiplier.(2)The voltage drop phenomenon of voltage multipliers based on magnetic coupling components(transformer/coupled inductor)is analyzed based on experimental phenomena.Through waveform analysis and simplified circuit modeling,it is identified that this phenomenon is the necessary condition for power transmission in these structures.By considering the reverse recovery process of leakage inductor current in characteristic analysis,the gain expression that accounts for voltage drop is quantified and reanalyzed.This lays a theoretical foundation for accurate characteristic analysis in the following research.(3)Based on the analysis conclusion of voltage drop,an accurate analytical method for the characteristic analysis of the transformer-based voltage multiplier is established.A more comprehensive mathematical model for gain characteristics is established by considering the reverse recovery process of leakage inductor current in model analysis.To describe the characteristics effectively,a quantitative relationship between it and the external electrical quantities of the converter is established since the duration of the reverse recovery process of leakage inductance current is uncontrollable.Accordingly,the output power characteristics of the transformer-based voltage multiplier and power-coupled-gain characteristics are obtained.Results show that the leakage inductor limits maximum output power and results in a strong coupling relationship between voltage gain and output power,and the voltage gain decreases with the increase in output power.(4)An improved transformer-based voltage multiplier without voltage drop is presented.To circumvent the voltage drop issue,the power transfer mechanism is modified by introducing a resonant capacitor.Specifically,a series resonant branch is formed between the resonant capacitor and the transformer’s leakage inductor,taking advantage of the characteristic that there is no voltage difference across the resonant branch during series resonance.This allows for lossless transmission of the primary winding voltage to the secondary winding voltage of the transformer,thereby avoiding the voltage drop phenomenon.Additionally,to prevent the occurrence of unexpected modes,a design method for the resonant capacitor and switching frequency is obtained based on the mode analysis.Finally,a comparative experiment is conducted to verify the feasibility of the proposed improvement method.(5)The frequency characteristics of the transformer-based voltage multipliers before and after improvement are analyzed.To achieve a high-frequency and lightweight design,frequency analysis is carried out based on the steady-state characteristics of the transformerbased voltage multiplier before and after improvement.Particularly,parasitic resistance is considered in the steady-state characteristics analysis of the improved structure to quantify the voltage loss during the resonant process.The changes in maximum output power,output voltage gain,voltage stress,current stress,circulating power,and power losses before and after improvement are analyzed during the process of the increase in switching frequency.The results show that the improved structure not only has a higher maximum output power limit in highfrequency situations but also has a more stable output voltage gain.Moreover,because there is no circulating power in the improved structure,higher efficiency can be achieved.Thus,the improved structure is more suitable for high-frequency operation to achieve the lightweight of the overall converter.