Medium Voltage LLC Converter With Series- Connected Sic Mosfets

With the increasing demand for energy conversion in the international community,the high-power power electronic converter system has developed rapidly in recent years.In data center,electric traction,medium-voltage distribution network and other applications,with the improvement of power level,the limitations of the traditional scheme based on low frequency transformer become more and more serious.However,the appearance of Solid State Transformer(SST)further improves the performance of the high-power converter system.At present,SST mostly adopts the multi-cell structure of input-series output-parallel(ISOP)architecture,which is easy to realize,but its power density does not have obvious advantages compared with the traditional scheme.In order to further improve the performance,it is necessary to build SST with single-cell structure.The appearance of the high voltage SiC MOSFET devices make it possible to build the single-cell SST,but limited by the cost and manufacture,there is no commercial high voltage SiC MOSFET products.As an alternative solution,using low voltage SiC MOSFETs in series to replace high voltage devices has advantages in some aspects,but the voltage unbalance problem brought by the series connection need to be addressed well.Although SiC MOSFETs perform better than traditional silicon devices,they are more susceptible to crosstalk voltages due to their faster switching speed and lower turn-on threshold voltage and negative voltage tolerance.For this reason,this paper analyzes the cause of crosstalk voltage in half bridge application,and establishes the impedance model of drive circuit considering parasitic inductance.A parallel capacitor branch which can be used for adjusting the gate loop impedance is added into the drive circuit.Based on the spectrum characteristics of the actual crosstalk voltage waveform,a crosstalk suppression drive based on the impedance coordination of the gate loop and power loop is proposed.After experimental verification,compared with the traditional method,this method can improve the crosstalk suppression effect by 35%.In order to realize the voltage balance of SiC devices in series,the cause of the unbalanced voltage of devices in series is analyzed,and a voltage balance method combining the resonant soft switching mode and adding snubber capacitor is proposed.This method not only improves the voltage sharing of devices,but also does not bring in extra losses.Based on the linear model of MOSFET,the unbalanced voltage model is established,and the unbalanced voltage sensitivity S_v is defined.The model points out that when the device is turned off with a large snubber capacitor,the current flowing through the channel is approximately zero during the voltage rise stage.The device realizes a ZVS turn off,and S_v is only related to the load current and the snubber capacitor.The conclusions obtained are verified by experiments.A 9.6kV half-bridge power module based on eight series-connected SiC MOSFETs is built,and a medium-voltage LLC converter based on series-connected SiC MOSFETs is constructed.Due to the addition of parallel snubber capacitors for voltage balance,the working characteristics of the medium-voltage LLC are significantly different from those of the traditional LLC.Therefore,a converter model is established in the time domain and the similarities and differences between the LLC and the traditional LLC are compared on this basis.A compromise principle for parameter design of snubber capacitor is proposed,and an optimal design case is given.Finally,a prototype of 5kV/400V 30kW is built and verified by experiments.The results show that the voltage balance effect is good,the devices realize soft switching and the converter realizes high conversion efficiency,reaching 99%at the rated power point.