Research Of Suppressing Bridge-leg Crosstalk And Oscillation Of SiC MOSFET For NPC Three-level Converter

In order to improve the working frequency of power switch and reduce its switching loss,it is usually done by adding soft-switch techniques such as resonant circuits,and then the efficiency and power density of Neutral-Point-Clamped(NPC)three-level inverter are improved.However,it will increase the voltage and current stress of the power switch,increase the circuit complexity and cost,and is not suitable for high-voltage and high-power applications.Therefore,domestic and foreign researchers began to study the application of wide bandgap devices in NPC three-level circuit.Through the advantages of low switching loss and high switching frequency of wide bandgap devices,the performance of NPC three-level inverter was improved,and already applied to photovoltaic,uninterruptible power supply,electrically propelled aircraft and other occasions.Above all,SiC MOSFET has the advantages of fast switching speed,low on-resistance and high temperature resistance,and gradually replaces the traditional Si IGBT in the applications of high efficiency,high power density and high temperature.However,the switching speed of SiC MOSFET is severely restricted by the bridge-leg crosstalk and gate-source voltage oscillation caused by the parasitic parameters of SiC MOSFET and the parasitic inductance of power circuit and drive circuit during high-speed switching.Therefore,it is of great significance to design the SiC MOSFET driver which can effectively suppress the bridge-leg crosstalk and gate-source voltage oscillation at high operating frequency in NPC three-level circuit where the power switch is SiC MOSFET.The following contents are studied in this paper.(1)Since NPC three-level bridge-leg circuit needs to consider the switching condition and interaction of four SiC MOSFET at the same time,it is difficult to directly design the SiC MOSFET driver.Therefore,this paper firstly simplifies the analysis of the bridge-leg crosstalk and gate-source voltage oscillation in two-level bridge-leg circuit containing only two SiC MOSFET.Then,the SiC MOSFET driver based on push-pull-capacitor auxiliary circuit(PPCAC)are studied,and put forward a kind of driver parameter designing method.The mathematical model is set up respectively on the push-pull capacitor charge and discharge time,the relationship between bridge-leg crosstalk and push-pull capacitor,the LCR resonance of driver circuit and the gatesource voltage oscillation,which can effectively suppress the bridge-leg crosstalk and gate-source voltage oscillation,and improve the switching speed of SiC MOSFET.The experiment verifies the rationality of the PPCAC driver parameters designing method on the double-pulse test platform of two-level circuit.(2)In order to test the effectiveness of PPCAC driver and its parameter designing method in suppressing the bridge-leg crosstalk and gate-source voltage oscillation in NPC three-level circuit,this paper firstly expands the traditional double-pulse test method to obtain the two-switch action double-pulse test and four-switch action doublepulse test of NPC three-level circuit.In high-speed switching process of SiC MOSFET,the drain-source voltage of the internal and external switch of NPC three-level bridgeleg circuit will be unbalanced,and due to the parasitic inductance in the power loop,the drain-source voltage of each switch will also produce overshoot.These change of the drain-source voltage will be through the miller capacitor coupling to driver circuit,seriously affected the SiC MOSFET gate-source voltage and driver design.Therefore,a capacitor is connected in parallel between the drain-source terminals of each switch of NPC three-level bridge-leg circuit to suppress the imbalance of drain-source voltage between the internal and external switch of the bridge-leg circuit,and an RCD buffer circuit is connected in parallel to suppress the drain-source overvoltage.Finally,this paper analyzes the effect of PPCAC driver and its driver parameter designing method on the suppression of the bridge-leg crosstalk and gate-source voltage oscillation of NPC three-level bridge-leg circuit,and the switching speed of traditional driver and PPCAC driver in the four-switch action double-pulse test.(3)Finally,NPC three-level inverter platform using only SiC devices is designed and built,and a four-switch action double-pulse test is carried out on the platform.It is verified that the PPCAC driver and the driver parameter designing method can also effectively suppress the bridge-leg crosstalk and gate source voltage oscillation of NPC three-level brdge-leg circuit.