Research On The Influence Mechanism Of Thermal Coupling On The Lifetime Of Electric Vehicle Full-bridge Power Electronics

Electric vehicles are an important means to solve the problems of environmental pollution and energy shortage.In recent years,electric vehicles have developed rapidly at home and abroad,and more and more enterprises have joined the automotive industry.Full-bridge power modules are widely used in electric vehicles.They have the ability to convert the electrical energy stored in the battery pack into three-phase alternating current for motors,and are key components in electric vehicles.The topology of the full-bridge power module is a three-phase full-bridge inverter circuit,which has the typical characteristics of multi-chip parallel connection and compact chip layout.There is thermal coupling.Thermal coupling causes the temperature of a chip not only to be determined by its own self-heating,but also affected by adjacent chips,which eventually leads to the uncertainty of the temperature distribution of the full-bridge power module,which leads to uncertainty about the reliability of full-bridge power modules.The unclear influence mechanism of thermal coupling on the reliability of power modules has become a bottleneck restricting the improvement of power levels and reliability of power modules.This paper explores the influence mechanism of thermal coupling on the lifetime of power modules through power cycling tests,optimizes the power cycling equipment considering thermal coupling,reveals the influence mechanism of thermal coupling on the lifetime of power modules,and proposes an optimal design method of chip layout considering the thermal coupling angle,which lays a foundation for the package design,test method and failure analysis of full-bridge power modules.Firstly,by studying the packaging characteristics of full-bridge power modules,a transient thermal impedance test method under thermal coupling is proposed.Based on this method,a current distribution test method under thermal coupling is innovatively proposed,and a power cycling equipment suitable for full-bridge power devices for electric vehicles is built,which lays the experimental foundation for the research of thermal coupling.The experimental results of junction temperature distribution considering thermal coupling show that the thermal coupling of full-bridge power devices can be divided into two forms:same-bridge thermal coupling and different-bridge thermal coupling.In the same-bridge thermal coupling mode,because the IGBT chip has a positive temperature coefficient,each IGBT chips have similar junction temperatures,and the junction temperature difference does not exceed 3K.In the different-bridge thermal coupling method,the junction temperature difference of IGBT chips increases with the increase of power loss,and can reach up to 18K.Combined with finite element simulation,the transient thermal impedance results of thermal coupling of different chips are analyzed based on heat flux,and a transient thermal impedance measurement method suitable for working conditions is innovatively proposed,laying a theoretical foundation for subsequent thermal coupling research.Then,by controlling the maximum junction temperature and junction temperature swing in the power cycling test,the power cycling lifetime and failure mode without thermal coupling are studied,and the influence mechanism of the chip surface temperature distribution on the lifetime of the power module is revealed.Combined with finite element simulation,a failure separation method for power modules is proposed,and an electrical measurement method for foot temperature was innovatively proposed.The influence of temperature gradient on lifetime under two failure modes is studied,and the key role of solder layer degradation on lifetime is clarified.The results show that under the condition of the same maximum junction temperature and junction temperature swing,different chip/copper area ratios lead to different maximum temperatures of the chip,and ultimately lead to different failure modes of the chip,devices with a lower chip/copper area ratio are more prone to bonding wire failure,and vice versa,solder layer degradation is more likely to occur.Furthermore,under the same chip surface temperature gradient,solder layer degradation will accelerate the bonding wire failure process and reduce the lifetime.The experimental results provide theoretical guidance and experimental methods for the subsequent study of the mechanism of thermal coupling on lifetime.Secondly,through the power cycling test of the full-bridge power module,the root cause of the lifetime difference with thermal coupling is studied,combined with the finite element simulation,the mechanism of the same-bridge thermal coupling improving the lifetime by reducing the maximum temperature and temperature gradient of the chip surface is revealed,and an effective method for packaging design of power modules is proposed,which greatly improves the reliability of packaging.The results show that with the same-bridge thermal coupling,the maximum temperature and temperature gradient of the chip surface are reduced by 5%and 50%respectively,so that the lifetime is doubled,meanwhile,the ability of thermal coupling to change the maximum temperature and temperature gradient of the chip surface is weak with the different-bridge thermal coupling,which increases the junction temperature difference of each chip connected in parallel with multiple chips,resulting in an increase in the thermal stress of some chips,but the impact on lifetime is small.Further research on the lifetime of different failure modes of FRD with negative temperature coefficient under the thermal coupling of the same bridge reveals the thermal coupling mechanism.Combined with finite element simulation,the positive feedback mechanism of negative temperature coefficient FRD under thermal coupling is clarified,and the inhibitory effect of thermal coupling on positive feedback in symmetrical package layout is clarified.The results show that thermal coupling under both bonding wire failure and solder layer degradation conditions ensures that the device lifetime with negative temperature coefficient fits the lifetime model.Finally,by studying the lifetime difference under different thermal coupling angles,the reason why the thermal coupling angle causes thermal coupling failure is revealed.Finally,by studying the difference in lifetime under different thermal coupling angles,the reasons for thermal coupling failures caused by thermal coupling angles are revealed,and an innovative method for power module chip layout optimization considering thermal coupling angles is proposed.This provides theoretical guidance for the package layout of full-bridge power modules.At the same time,innovatively proposed short-circuit power cycling test methods and applicable conditions considering thermal coupling.The results show that under the large thermal coupling angle,the asymmetric chip layout is due to the positive feedback effect of the negative temperature coefficient,the thermal coupling failure cannot reduce the temperature gradient on the chip surface,and the redistribution of the current at 60%of the life avoids the thermal resistance from reaching the failure standard,but the device lifetime will still be reduced to 85%of the expected lifetime,meanwhile,under the small thermal coupling angle,the influence of thermal coupling on the temperature gradient of the chip surface is reduced,and the lifetime expectancy is reduced to 50%of the expected lifetime.The most suitable thermal coupling angle is 26.3 degrees calculated by finite element simulation,which optimizes the packaging of power devices layout.