Online Condition Monitoring For Bond Wire Degradation Of IGBT Modules In Three-Level Neutral-Point-Clamped Converters

Neutral-point clamped（NPC）three-level converter has been widely used in various emerging applications,such as renewable power generation,railway traction,aerospace.The power ratings of these applications reach tens or even hundreds of megawatts,where the higher reliability requirements of power electronic systems are requied remarkably.According to the related industrial investigation,the failure rate of power devices in power electronic systems is the highest,which is the key factor restricting the reliability of power electronic system.Meanwhile,insulated-gate bipolar transistor（IGBT）has been the most widely used fully controlled power device in medium-and high-power applications because of its excellent switching performance and lower switching loss.During the operation of power converter,IGBT module is repeatedly impacted by the internal and external thermal stress,which makes the temperature of IGBT chip and packaging materials fluctuate violently.Because of the different thermal expansion coefficients between materials,the temperature fluctuation would produce irreversible fatigue damage accumulation thus leading to package reliability issues（i.e.bond wire failure,solder fatigue）.The fault of power converter would cause huge economic loss,and even life loss.Therefore,it is of great significance for improving the reliability of power electronic systems to online monitor the health status of IGBT modules,find and replace the modules with low health level in time.In this paper,an online condition monitoring method for IGBT module bond wire degradation applied in three-level NPC converters is proposed,which has the ability to detect all the IGBTs in NPC converters.The influence of online monitoring on load current is analyzed,and the best insertion areas for condition monitoring is proposed.At the same time,a bond-wire condition monitoring module is developed,which can produce the test current with different amplitudes and sample the collector-emitter voltage of IGBT module.The main work of this paper is presented as follows:（1）The aging failure mechanism of bond wires and the composition of equivalent parasitic resistance and the expression of collector-emitter terminal voltage(V_CEON)of IGBT module are analyzed in detail.According to the expression of V_CEON,it is proposed that the collector-emitter saturation voltage at high current(V_CEH)is regarded as aging precuorsor of bond wires and the collector-emitter voltage at low current(V_CEL)is utilized as temperature sensitive electric parameter（TSEP）,then the current pulse trains with different amplitudes are injected into IGBT module,each stage lasts for 0.1ms and the terminal voltages V_CEH and V_CEL of each stage are sampled accurately to implement the online condition monitoring of bond-wire heathy level.（2）The load current paths of a leg in NPC converters are analyzed in detail.According to the analysis,it is found that under the specific switching sequences and load current directions,the load current flows through only one IGBT in a leg and the other one IGBT remains idle on-state,which can be injected the above-mentioned test current into it and sample its V_CEH and V_CEL to conduct the online test.Then,the influence of online condition monitoring on load current is analyzed.It is found that the influence of online monitoring can be completely ignored as long as the insertion areas for condition monitoring are selected during the operation period（0.2 ms）when the average output voltage of the leg equals or closes to zero.According to this principle,the selection of insertion areas for online condition monitoring are implemented in the single-phase three-level NPC inverter.（3）The condition monitoring system is designed,including the condition monitoring module,DSP control unit and condition monitoring system software.The condition monitoring module is the core part of condition monitoring system,which can generate the test current for condition monitoring and sample the collector-emitter saturation voltage.This module is highly integrated,accurately and reliably,which can be easily employed in actual industrial applications.（4）The experimental rig of single-phase three-level NPC inverter is established.The aging of bond wires is emulated by cutting off bond wires one by one.It is verified that V_CEL would not be affected by the aging of bond wires,so that it can be used as a reliable TSEP to measure the junction temperature.Meanwhile,the correctness of the proposed condition monitoring method in 3LNPC converters and the principle of selecting the best insertion areas for online test are verified.The experimental results shows that,the aging level of bond wires can be correctly distinguished,the increment of V_CEH reaches 34 m V when three bond wires are broken;in addition,the condition monitoring has no influence on the normal operation of converter when the online test are conducted when the average output voltage of the leg equals to zero.