Parallel Technology Of Si-IGBT With SiC-MOSFET In TNPC Inverter

The power semiconductor devices made from Silicon Carbide(SiC)materials have higher blocking voltage,lower on-resistance and wider range of operating temperature.However,due to the limitation of manufacturing technology,the rated current of SiC-MOSFET(Silicon Carbide Metal-Oxide-Semiconductor Field-Effect Transistor)is relative small.Thus,the Si-IGBT(Silicon Insulated Gate Bipolar Transistor)can not be replaced directly by SiC-MOSFET in the high voltage and large current application.In addition,the cost of SiC devices is still high,which will give rise to a great cost on a full SiC-MOSFET based inverter.Since the SiC-MOSFET has the advantage of low switching loss and the Si-IGBT has excellent ability to handle large current.The two devices can be connected in parallel to reduce the switching loss of the Si-IGBT and the conduction loss of the SiC-MOSFET under large current,so as to improve the efficiency of the inverter and reduce the manufacturing cost.This thesis focuses on the application of SiC-MOSFET and Si-IGBT paralleled device in TNPC inverter.The main work is as follows:Firstly,the structure and working principle of Si-IGBT and SiC-MOSFET are introduced.The switching and conduction characteristics of the two devices are compared and analyzed.The results show that the switching performance of SiC-MOSFET is better than Si-IGBT,but Si-IGBT has excellent conduction performance,which gives it the ability to handle a large current.Secondly,in order to reduce the switching loss of the parallel devices and enhance the overload capacity of devices,two switching modes for different ranges of load current are designed.The mode of SiC-MOSFET turns on first and turns off last is utilized to realize the high inverter efficiency when the inverter operates under the range of rated power.And the mode of Si-IGBT turns on first and turns off last is used during the operating range beyond the rated power to improve the overload ability of the inverter and protect the SiC-MOSFET at the same time.Posteriorly,the conduction characteristics in first and third quadrant of the paralleled device are analyzed and the conduction models are established.The switching process of the paralleled device is analyzed and the effects of the parasitic inductance on the SiC-MOSFET's switching characteristics and the dynamic current sharing process of the paralleled device are also studied theoretically with the help of LTspice simulation tool.The results show that the parasitic inductance will lead to the oscillations in gate voltage,drain current and drain source voltage in the switching process which will be intensified with the increasement of the parasitic inductance.At the same time.In order to reduce the total loss of paralleled devices in the switching process,the equivalent switching loss model of paralleled devices is established,and the relationship curve between equivalent switching loss and switching delay is obtained by using double-pulse test data.The optimal value of switching delay is selected,which further reduces the loss of shunt devices.Finally,the working process of the paralleled device based TNPC full-bridge inverter and the modulation method are introduced.The semiconductor loss models of the inverter are built and the theoretical curves of conduction loss and switching loss under specific working condition are calculated.In order to verify the effect of paralleled devices on improving the efficiency of inverters,a 2.5kW paralleled device based TNPC full-bridge inverter prototype is designed and built.The inverter efficiency results under different output power,switching frequencies and dc voltages are tested.The emciency results are also compared with these of the pure Si-IGBT based inverter.It is proved that the paralleled device has the great potential of efficiency improvement for the inverter.