Analysis of VCE_on as a Health Monitoring Approach Applied to IGBT Power Modules in Wind Power Converters
1, Ng Chong2, Volker Pickert1, Paul McKeever2
1Newcastle University, Newcastle upon Tyne, UK, 2Offshore Renewable Energy Catapult, Newcastle upon Tyne, UK
Knowledge about power converter health in wind power applications is vitally important as any wear-out failure leads to costly unscheduled maintenance. IGBTs are the most common power modules in wind power converters today and it is well-known that devices fail due to solder fatigue and bond wire failure. Recently, techniques have been proposed to monitor solder and bond wire structure changes; the most common technique is to monitor the collector and emitter voltage during the on-state, VCE_on. VCE_on is a temperature sensitive electric parameter (TSEP) and as any growth in the solder fatigue or bond wire lift-off results in an increased junction temperature, the voltage changes proportionally. This is well documented in various literature. However, the challenge in using VCE_on as a health monitor indicator is the voltage measurement circuit. The circuit must deal with: high blocking voltages when the device is off; common mode issues associated with high and low operating voltage levels; measurement accuracies and EMI noise levels.
This paper provides an assessment of circuits published so far. The circuits’ pros and cons are listed supported by analytical work and simulation results. The full paper includes the following sections: an analytical description and empirical approaches to derive VCE_on expressions including a discussion on equations and their impact on the temperature and voltage relationship; an overview of different measurement circuits proposed so far including discussions on operation; a comprehensive circuits’ comparison and their operation in the context of accuracy, reliability, practical implementation and cost; and finally suggested improvements.
Accurate VCE_on measurement is difficult due to large voltage disparity between on and off states. Isolation must be embedded in these circuits; a discussion is taking place at which stage isolation should be added. In addition, common mode rejection ratio(CMRR) is a problem thus the correct amplifier selection depends on circuit design. Data handling and data processing must be fast and methods are proposed where information handling is carried out; either within the measurement circuit or the main converter controller. The measurement circuits are temperature dependent and the temperature dependency is discussed. The accuracy of voltage measurements for each circuit will be analyzed in depth. All of the analyses are based on literature reviews, analytical work and simulations carried out in Saber.
VCE_on, result in both the junction temperature and the electric resistance of the bond wire are measured. Consequently, measurements can cause incorrect readings, e.g. the case where the impact of VCE_on changes caused by bond wire lift-off is greater than that caused by temperature rise due to solder cracks. This is especially the case where bond wire count and semiconductor chip count representing one IGBT are both high. Therefore, VCE_on circuits must be selected first on the grounds of the semiconductor chip and bond wire counts. Another result highlights practical implementations of some proposed circuits for single-phase inverters, i.e. they do not work for three-phase inverters due to grounding issues; the two failures (solder fatigue and bond wire lift-off) cause a different temperature signature.
This paper provides an overview about VCE_on measurement circuits for health monitoring. Various circuits have been compared based on literature reviews, analytical work and simulations. Advantages and disadvantages of different methods are presented which provide a useful guidance for practical applications.
The work presented here results in new observations that are not highlighted in previous work. The selection of the VCE_on measurement circuit is first of all driven by the number of chips and the number of bond wires used. These, however, vary from manufacturer to manufacturer. Thus, there is not a single solution that can be applied to all wind power converters for health monitoring. The challenge now is to ensure that the solutions required are optimized.