Electrical Discharge Characteristics And Mechanism Of Press-Pack IGBT Cassette Packaging

High voltage and high power press-pack IGBT(Insulated Gate Bipolar Transistor)devices are self-commutating power semiconductor devices with the merits of increased current capability and a unique short-circuit failure mode,which makes it the preferred devices for the equipment of flexible HVDC(High Voltage Direct Current)technology.A comprehensive understanding of the unique technical challenges for the design and fabrication of the press-pack IGBTs(PPIs)is paramount,because the challenges are significantly different from those for wire-bond modules.In the challenges,electrical insulation capability of PPIs is one of the most critical challenges.To realize the full voltage capability of the dies,and to ensure the reliability of the device,it is essential to understand and control the high electric field effect inside the device.In this thesis,a comprehensive study on the electrical insulation of the PPI cassette packaging was performed to understand and control the electrical discharges in PPI cassette.Firstly,the experimental and analysis methods of the electrical discharges for PPI cassette packaging were investigated.An experimental platform,which has the capabilities to test the bare die,cassette and packaging material under arbitrary voltage waveform up to ± 10 kV at a temperature of 25?-150?,a gas pressure of 100 Pa-0.3 MPa,and a clamping force of 0-20 kN,was designed and constructed.The discharge current pulse detection system for PPI cassette packaging was built,and an efficient data collection method for the current pulses was proposed.A partial discharge measurement method for the PPI,which overcomes the shortcomings of the previous methods,was proposed.A photomultiplier detection system and an image measurement system based on the ultraviolet camera and digital camera for the study of electrical discharges in PPI cassette packaging were achieved.The current pulse waveform analysis technique and phase-resolved partial discharge(PRPD)analysis technique were developed,and the extraction methods of photocurrent amplitude and streamer stopping length were also developed.Then,the surface discharge characteristics of PEEK(Polyetheretherketone)in rod-plane electrodes driven by unipolar repetitive pulse voltage were investigated.The basic surface discharge characteristics were obtained by fast oscilloscopic techniques,photomultiplier and digital camera,and two types of surface discharge were observed:forward discharge and reverse discharge.Comparative analysis on the characteristics of reverse discharge and forward discharge under the opposite voltage was performed,and the homogeneity between reverse discharge and forward discharge under the opposite voltage was found.The surface discharge development characteristics driven by positive repetitive pulse voltage with different amplitudes were investigated,and the effects of voltage amplitude on the electrical and optical characteristics of the surface discharge streamers were obtained.The surface discharge inception characteristics driven by positive repetitive pulse voltage with different duty cycles were explored,and the surface discharge inception voltage decreases with the increase of duty cycle.Next,the surface discharge mechanisms under positive repetitive pulse voltage were investigated.The initiation mechanism of surface discharge under positive repetitive pulse voltage was analyzed and verified,and the electric field to initiate forward discharge is mainly determined by the applied voltage,while the electric field to initiate reverse discharge is mainly determined by the residual surface charges left by the forward discharge in the same cycle.The correlation between the electrical characteristics and optical characteristics of the streamer was analytically derivated,and the current rising rate of the streamer is proportionally correlated to the product of the streamer stopping length and the streamer velocity square.The mechanism behind the relationship between the measured surface discharge inception voltage and duty cycle was analyzed,and the relationship was successfully explained by the role of time-lag.It was found that the inverse power law between the mean stochastic time-lag and the applied voltage was the main reason for the relationship between surface discharge inception voltage and duty cycle.Finally,the partial discharge(PD)measurement,analysis and control in the PPI cassette were performed.The physical processes from partial discharge initiation to electric breakdown in the PPI cassette under DC blocking condition was obtained by fast oscilloscopic techniques,PD detector and ultraviolet camera,and PD induced insulation failure in the PPI cassette was observed.The electric field reinforcement in the cassette packaging,which occurs at the surface of the passivation layers,the edge of the silver plate and the triple region including the die,PEEK and the gas,was revealed by electric field analysis.The PRPD analysis method for PPI could discriminate the PRPD patterns in the elementary structures which were designed to simulate the PD characteristics in the electric field reinforcement areas.The PRPD patterns of the PPI cassette were compared with those of the elementary structures,and it was found that PRPD patterns of the PPI cassette were the same as those of the bare die,which indicated that the PD observed in the cassette was caused by the mismatch between the electric field on the passivation layers of the die and critical electric field provided by the gas.According to the failure mechanism of the previous packaging design,the design of the cassette packaging was improved.The effectiveness of the improved design was verified by electric field analysis and PD measurements,and the improved design was applied in the development of the PPIs.