Robustness and balancing of parallel connected power devices : SiC vs. CoolMOS

Differences in the thermal and electrical switching time constants between parallel connected devices cause imbalances in the power and temperature distribution thereby reducing module robustness. In this paper, the impact of electro-thermal variations (gate and thermal resistance) between parallel connected devices on module robustness is investigated for 900V-CoolMOS and 1.2kV-SiC MOSFETs under clamped inductive switching (CIS) and unclamped inductive switching (UIS). Under CIS, the difference in the steady-state junction temperature (ΔTJ) and switching energy (ΔESW) between the parallel connected devices for a given difference in the gate and thermal resistance (ΔRG & ΔRTH) is used as the metric for determining robustness to electrothermal variations i.e. how well the devices maintain uniform temperature in-spite of switching with different rates and thermal resistances. Under UIS conditions, the change in the maximum avalanche current/energy prior to device failure as a function of the ΔTJ and ΔRG between the parallel connected devices is used as the metric. Under both CIS and UIS, SiC devices show better performance with minimal negative response to electrothermal variations between the parallel connected devices. Finite element models have also been performed showing the dynamics of BJT latch-up during UIS for the different technologies

Electrothermal PSpice Modeling and Simulation of Power Modules

Integrated power electronics modules (IPEMs) represent an innovative typology of power electronics assemblies able to guarantee several advantages such as increasing of power density, better management of the thermal flows, and a significant reduction of the package sizes. Their characteristics make them suitable for applications like motor drives or power conditioning. IPEM usage in emerging fields like hybrid automotive traction and electric generation from renewable energy sources is continuously increasing. In this paper, we describe the implementation of a devised flow to generate the layer-based electrothermal PSpice model of an IPEM and the simulation flow of the model. The proposed modeling methodology allows reducing an electrothermal multidomain problem to an electrical single one. The general PSpice-like nature of the proposed model makes it suitable for a wide range of simulation frameworks where the integration of heterogeneous multiphysics models could be a difficult task. The outlining of both electrical and thermal PSpice layers is discussed, and the implementation into the final model, by the assistance of custom electronic-design-automation flow, is presented. Moreover, we describe the validation procedure of the proposed approach, and the results are compared with the ones obtained by a commercial finite-element-based package used as a benchmark. Two simulation approaches related to specific conversion systems, and related issues, are presented and discussed

Health Condition Assessment of Multi-Chip IGBT Module with Magnetic Flux Density

To achieve efficient conversion and flexible control of electronic energy, insulated gate bipolar transistor (IGBT) power modules as the dominant power semiconductor devices are increasingly applied in many areas such as electric drives, hybrid electric vehicles, railways, and renewable energy systems. It is known that IGBTs are the most vulnerable components in power converter systems. To achieve high power density and high current capability, several IGBT chips are connected in parallel as a multi-chip IGBT module, which makes the power modules less reliable due to a more complex structure. The lowered reliability of IGBT modules will not only cause safety problems but also increase operation costs due to the failure of IGBT modules. Therefore, the reliability of IGBTs is important for the overall system, especially in high power applications. To improve the reliability of IGBT modules, this thesis proposes a new health state assessment model with a more sensitive precursor parameter for multi-chip IGBT module that allows for condition-based maintenance and replacement prior to complete failure. Accurate health condition monitoring depends on the knowledge of failure mechanism and the selection of highly sensitive failure precursor. IGBT modules normally wear out and fail due to thermal cycling and operating environment. To enhance the understanding of the failure mechanism and the external characteristic performance of multi-chip IGBT modules, an electro-thermal finite element model (FEM) of a multi-chip IGBT module used in wind turbine converter systems was established with considerations for temperature dependence of material property, the thermal coupling effect between components, and the heat transfer process. The electro-thermal FEM accurately performed temperature distribution and the distribution electrical characteristic parameters during chip solder degradation. This study found an increased junction temperature, large change of temperature distribution, and more serious imbalanced current sharing during a single chip solder aging, thereby accelerating the aging of the whole IGBT module. According to the change of thermal and electrical parameters with chip solder fatigue, the sensitivity of fatigue sensitive parameters (FSPs) was analyzed. The collector current of the aging chip showed the highest sensitivity with the chip solder degradation compared with the junction temperature, case temperature, and collector-emitter voltage. However, the current distribution of internal components remains inaccessible through direct measurements or visual inspection due to the package. As the relationship between the current and magnetic field has been studied and gradually applied in sensor technologies, magnetic flux density was proposed instead of collector current as a new precursor for health condition monitoring. Magnetic flux density distribution was extracted by an electro-thermal-magnetic FEM of the multi-chip IGBT module based on electromagnetic theory. Simulation results showed that magnetic flux density had even higher sensitivity than collector current with chip solder degradation. In addition, the magnetic flux density was only related with the current and was not influenced by temperature, which suggested good selectivity. Therefore, the magnetic flux density was selected as the precursor due to its better sensitivity, selectivity, and generality. Finally, a health state assessment model based on backpropagation neural network (BPNN) was established according to the selected precursor. To localize and evaluate chip solder degradation, the health state of the IGBT module was determined by the magnetic flux density for each chip and the corresponding operating conduction current. BPNN featured good self-learning, self-adapting, robustness and generalization ability to deal with the nonlinear relationship between the four inputs and health state. Experimental results showed that the proposed model was accurate and effective. The health status of the IGBT modules was effectively recognized with an overall recognition rate of 99.8%. Therefore, the health state assessment model built in this thesis can accurately evaluate current health state of the IGBT module and support condition-based maintenance of the IGBT module

Measuring Level of Degradation in Power Semiconductor Devices using Emerging Techniques

Title from PDF of title page viewed May 24, 2021Dissertation advisor: Faisal KhanVitaIncludes bibliographical references (page 124-154)Thesis (Ph.D.)--School of Computing and Engineering and Department of Mathematics and Statistics, University of Missouri--Kansas City, 2021High thermal and electrical stress, over a period of time tends to deteriorate the health of power electronic switches. Being a key element in any high-power converter systems, power switches such as insulated-gate bipolar junction transistors (IGBTs) and metal-oxide semiconductor field-effect transistors (MOSFETs) are constantly monitored to predict when and how they might fail. A huge fraction of research efforts involves the study of power electronic device reliability and development of novel techniques with higher accuracy in health estimation of such devices. Until today, no other existing techniques can determine the number of lifted bond wires and their locations in a live IGBT module, although this information is extremely helpful to understand the overall state of health (SOH) of an IGBT power module. Through this research work, two emerging methods for online condition monitoring of power IGBTs and MOSFETs have been proposed. First method is based on reflectometry, more specifically, spread spectrum time domain reflectometry (SSTDR) and second method is based on ultrasound based non-destructive evaluation (NDE). Unlike traditional methods, the proposed methods do not require measuring any electrical parameters (such as voltage or current), therefore, minimizes the measurement error. In addition, both of these methods are independent of the operating points of the converter which makes the application of these methods more feasible for any field application. As part of the research, the RL-equivalent circuit to represent the bond wires of an IGBT module has been developed for the device under test. In addition, an analytical model of ultrasound interaction with the bond wires has been derived in order to efficiently detect the bond wire lift offs within the IGBT power module. Both of these methods are equally applicable to the wide band gap (WBG) power devices and power converters. The successful implementation of these methods creates a provision for condition monitoring (CM) hardware embedded gate driver module which will significantly reduce the overall health monitoring cost.Introduction -- Failure mechanisms of modern power electronic devices -- Existing degradation detection & lifetime prediction techniques -- Accelerated aging methods -- SSTDR based degradation detection -- Ultrasound based degradation -- Degradation detection of wide band gap power devices -- Conclusions and future researc

On integrity assessment of IGBT-based power stacks used in magnet power supplies for particle accelerators

This thesis analyses an electrical testing method for assessing the integrity of an IGBT-based power stack assembly during factory acceptance tests and service stops. The method combines vce measurements with high current in the Zero Temperature Coefficient (ZTC) operating region and with low sensing current within a specific current cycle using a proposed sampling and filtering technique. Two circuits are presented for the vce measurement. The value of this method is the ability to validate the power stack assembly and to detect IGBT aging without the need for power stack modifications for the vce measurement with sensing and load current. Additionally, no dedicated current control of the IGBTs is required. The aging mechanisms that are targeted with this method are the bond-wire lift-off and the solder delamination. As a part of the method, an on-the-stack vce calibration technique at the sensing current level is proposed for the IGBTs avoiding the need to un-mount and characterize them in a thermal chamber. The reference application is a power electronic converter that is used as a magnet power supply in particle accelerators at CERN, the European Organization for Nuclear Research. Based on the specialized application, the levels of ambient air, junction and cooling water temperature change that could have an impact on the method’s precision are defined. Experimental results, which are obtained with the power stack of a power magnet supply, are presented and are compared favorably with results obtained using Finite Element Method (FEM) and Lumped Parameter Network (LPN) simulations to demonstrate the method’s applicability. For the high-current IGBT module of the application, it is shown that the measurement in the ZTC operating region could detect bond-wire lift-offs when more than half of the bond-wires of the chip have been lifted. A measurement in the Positive Temperature Coefficient (PTC) operating region can be used to detect the early stage aging of a bond-wire lift-off, but the measurement precision is, highly, influenced by temperature. Moreover, the detection of manufacturing issues, such as errors in thermal paste application, is proven to be possible with the help of vce measurements with sensing current.As a potential improvement of the future power stack designs for lifetime prolongation, this work investigates the possibility for IGBT module’s thermal stressing mitigation using the specialized application as a reference. This investigation is based on LPN simulations. Prior to the LPN modeling, the extensive operation of the magnet power supply in the Negative Temperature Coefficient (NTC) operation region is, experimentally, examined. It is demonstrated that the current and thermal stressing unbalances among the chips inside the Soft Punch Through (SPT) IGBT module operating in the NTC operating region can be neglected and do not have to be considered for the thermal modeling. Moreover, the impact of the material, of the thickness and of the heat convection of the cooling plate on the junction temperature variation and maximum junction temperature is evaluated. It can be stated that, for long current cycles of the specialized application, a relatively thick aluminum cooling plate (3cm) with a moderate heat convection coefficient (10kW/(\ub0Cm2)) may exhibit almost the same performance as a copper cooling plate of equal or even greater thickness (5cm) with a high heat convection coefficient (10kW/(\ub0Cm2)).Two strategies are proposed with the switching frequency and the gate resistance as parameters for online thermal stressing mitigation. The first strategy reduces the switching frequency in parts of the cycle where a high precision requirement for the output current is not imposed, in order to limit the power losses and the thermal stressing of the IGBT. The second strategy combines the switching frequency reduction in one part of the cycle with the increase of switching frequency and gate resistance in another. By increasing the power losses the junction temperature fluctuation can be limited. Using four typical current profiles from the specialized application, it is shown that both strategies could prolong the IGBTs’ lifetime. It is shown that the contribution of the mitigation strategy to the lifetime prolongation depends on the current profile