A Study On An IGBT’s Condition Assessment Method Based On The Measurement Of Saturation Voltage

With the increasingly serious situation of energy crisis, wind power, as a kind ofclean and renewable energy, is attracting more and more attention, and it is of vitalimportance to solve the current energy crisis. Wind power converters, which arecomprised of Insulated Gate Bipolar Transistor （IGBT） power modules, have taken therole of transporting energy in the whole wind power generating system. However, dueto the great random fluctuations of wind power, the junction temperature of IGBTpower modules will suffer huge fluctuations, causing reliability problems such as lift-offof bonding wires and aging of solder layers and etc. Further, it may even lead to largearea off-grid phenomena of wind power system. Thus the reliability of IGBT powermodule is closely related to the safe and stable operation of wind power system. For thepurpose of improving the reliability of IGBT power modules and saving the cost ofoperation and maintenance fees of converters, the concept of state maintenance basedon state assessment is extended in this paper. That is to say, the current state of IGBTpower modules or the whole system should be assessed to judge its health condition andthen decide whether maintenance is needed now. This maintenance strategy is differentfrom that adopted in the past, where regular maintenance is taken. Regular maintenancestrategy suggests that equipment be maintained regularly even though it is in goodcondition, which is not only a waste of money but may also do harm to the equipmentwhen it is dismounted. Based on the perspectives above, following jobs have beencarried out in this thesis, and corresponding results have been got, centering on the stateassessment of IGBT power modules:①Detailed discussion about the aging and failure mechanism is covered and agingof bond wires and solder layers is emphasized. Also, changes of the device statecharacteristic parameters against aging process in previous literatures are expounded.The comparative analysis of the advantages and disadvantages of these parameters iscarried out; getting the conclusion that saturation voltage V_cesatis more suitable forstatus assessment as a state characteristic parameter. Additionally, explain the influenceof junction temperature on the measurement of V_cesat.②The significance of junction temperature to state assessment of IGBT module isclarified. Based on previous methods of junction temperature measurement, the thesistries to use saturation voltage drop V_cesatof IGBT to directly reflect the junction temperature by design single pulse temperature cabinet experimental platform. Thencombined with experiment and simulation in Saber, the thesis demonstrates thecorrectness of the method it applied.③To accelerate the changing of IGBT’s state, the thesis designed an acceleratedageing experimental platform. The procedure is that at first large current is applied todevices to ensure a prompt rise of junction temperature to reach its maxim value causedby the conduction loss of IGBT and then compulsory cooling to devices is conductedafter interrupt of power supply. As a result, junction temperature will fluctuate rapidly ina large range to bring about great thermal stress to the module, which realizesaccelerated aging of bond wires and solder layers. The platform lays solid foundationfor upcoming state assessment researches.④Because there is yet no standardized products in domestic market and longduration of manufacturing, the thesis adopted another method to imitate changes ofaccelerated aging. First, unseal the module and the cut off part off the paralleled bondwires on IGBT chip to approximate the changes led by normal accelerated aging. Threedimensional surfaces V_cesat=f（I_c, T_j） are compared before and after the cutting of bondwires. Based on the analysis of the difference between the two surfaces, the thesissummarized: changes of saturation voltage drop V_cesatat certain operating point can be atrue reflection of changes in the module’s state. After comparing this method to aprevious method, we got: this method can avoid a miscarriage of justice in on the healthstatus of the devices.