| Implementing the”Dual Carbon”strategy is an important measure for sustainable development in the new era.In recent years,the proportion of new energy represented by”wind,solar,hydro,and hydrogen”in the national energy structure has rapidly increased.Photovoltaic(PV)power generation,as a renewable and clean energy source,is an important part of a clean,low-carbon,safe,and efficient new energy system in China.DC/DC converter and DC/AC inverter are the core units of PV DC microgrid system,whose efficiency,power density and reliability will directly affect the comprehensive performance of the whole system.Improving the conversion efficiency and power density of the two converters by all means has been the goal pursued in the field of PV power generation.At present,the DC bus of PV microgrid shows the development trend of high voltage and high current,and the traditional silicon-based(Silicon,Si)devices can no longer meet the needs of the high power density of the system.Silicon Carbide(Silicon Carbide,SiC)devices,especially Metal Oxide Semiconductor Field Effect Transistor(MOSFET)have become a hotspot in the application of high-voltage and high-power electrical equipment due to their higher switching frequency,high thermal conductivity,and high blocking voltage.However,the fast switching rate of SiC MOSFET will further increase the switching losses with the increase of frequency and also aggravate the level of electromagnetic interference(Electromagnetic Interference,EMI).These issues have somewhat affected the rapid promotion and application of SiC MOSFET.In view of the above problems,this paper takes PV power generation systems as the application background,reveals the mechanism among SiC MOSFET switching modes,switching losses,and efficiency of converters through theoretical analysis and modeling;optimizes the system performance,power density,and conversion efficiency of the two converters by utilizing optimized topologies and soft-switching techniques.The main innovative research results are as follows:(1)This paper proposes a SiC MOSFET switching loss model suitable for soft switching techniques.Traditional SiC MOSFET loss model uses drain-source current instead of channel current to calculate the switching loss,this makes the loss model only suitable for hard switching but not for soft switching mode.This paper reclassifies the switching modes based on the physical mechanisms of junction capacitance charging and discharging and changes in channel current.Except for traditional soft and hard switching,a new mode-partial soft switching,is introduced,in which the drain-source voltage is still not fully charged when the channel current drops to zero due to the light load.By solving time-domain differential equations,an analysis model of SiC MOSFET-based soft/partial soft switching is proposed.The model incorporates nonlinear characteristics such as transconductance and junction capacitance,making the channel current independent of the drain-source current,and provides methods for solving the key processes in switching.The generation mechanisms of different special modes are studied,and criteria for their classification,boundary conditions,and influencing factors are provided.A SiC MOSFET switching loss model based on soft switching/partial soft switching modes is proposed.Finally,experiments and simulations verify that the improved model effectively reduces switching loss distortion in SiC MOSFETs compared to traditional models.(2)The high step-down buck DC/DC converter brings insulation challenges between high voltage(750～1500V)and low voltage(21～48V)sides,and issues such as high voltage overshoot,high switching losses,and severe EMI caused by frequent switching of switching devices at a very small duty cycle.Referring to this,this study proposes a SiC/Si hybrid T-type three-level LLC soft-switching DC/DC power converter.This topology fully utilizes the high blocking voltage and fast switching characteristics of SiC MOSFET,it could avoid switches operating in the saturation area under a small duty cycle and high-frequency applications,thus reducing high switching losses.Meanwhile,due to the asymmetry of T-type topology,the auxiliary branch will stand low voltage stress and low turn-off current,allowing for traditional Si MOSFETs to reduce costs.Different from the traditional LLC modulation strategy that only operates at the resonant frequency,this study proposes a Varied Frequency Adoptable Phase Shift(VFAPS)modulation strategy with high efficiency and wide gain range.A new operation mode is established,which makes full use of the fr2<fs<fr1 and fr1<fs two frequency ranges.By adjusting the phase angle to reduce the proportion of magnitizing current in the resonant current,the reactive loss is minimized,thereby achieving the demand for high efficiency and wide gain;Based on the aforementioned modes,a time-domain differential expression is established and solved,the analytical expressions of the gain range and resonant current at different switching frequencies are obtained,the precise frequency and phase angle required for the target gain is calculated;The converter gain characteristics,and soft-switching conditions are analyzed;Finally,the effectiveness of the proposed topology and modulation strategy,as well as the accuracy of the time-domain model,are verified through simulation and experiments.(3)Traditional soft-switching grid-tied PV inverters used to adopts soft-switching techniques to improve the overall efficiency,however,this worsens dead-time effects and injects more harmonics into the output.Regarding to this,this study proposes a full SiC MOSFET-based Active Neutral Clamped(ANPC)three-level single-phase string grid-tied PV inverter.Based on the concept of partial soft-switching mode proposed in Chapter 2,a novel inverter operation mode of partial soft-switching is proposed.Combined with the advantages of small junction capacitance and fast switching rate of SiC MOSFET,the resonant energy of load inductor and junction capacitance is fully utilized to realize partial soft switching of SiC MOSFET without adding additional resonant circuit or prolong the dead time.This effectively reduces the excess cost and loss.An improved Level-shifted PWM(LS-PWM)is proposed,which enabled the dual resonant tanks to simultaneously operate in zero-state periods,thus achieving smaller output harmonics,lower on-state loss,lower heat loss,and higher power density.A loss model-based dead time optimization algorithm is proposed.By calculating the dead time loss,switching loss and conduction loss,the mathematical model of loss V S.dead time is established,and the dead time that both meets IEEE519 grid-tied harmonic standard and the lowest loss is derived,which can optimize the conversion efficiency and reduce the low-order harmonics generated by the dead time effect.The dead time mechanism based on soft-switching techniques is analyzed,and the configurable range of dead time and the realizable range of soft switching are deduced according to the stage transitions and input power changes of inverter.Finally,the effectiveness of the proposed inverter topology and modulation strategy,as well as the accuracy of the loss model,are verified through simulation and experiments.In the application of PV power generation,this paper thoroughly investigates the impact of switch losses of SiC MOSFETs on the efficiency of converters under soft-switching techniques.Through optimization of topology and modulation strategies,efficiency improvements are made to both DC converters and photovoltaic inverters in microgrids,thereby achieving an overall upgrade in the performance of photovoltaic power generation systems. |
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