High Voltage Power Modules Research Articles

Electrical Tree Failure at DBC Substrate-Silicone Gel Interface in High-Voltage Power Module Under Positive Square Wave Voltage

Electrical Tree Failure at DBC Substrate-Silicone Gel Interface in High-Voltage Power Module Under Positive Square Wave Voltage

Investigation of Threshold Voltage Instability and Bipolar Degradation in 3.3 kV Conventional Body Diode and Embedded SBD SiC MOSFET

The 3.3 kV SiC MOSFETs are essential for traction applications, so it is important to investigate the reliability of the recently developed high voltage MOSFETs and power modules as they are believed to be more susceptible to the effects of basal plane dislocations (BPDs). This paper presents measurement results and analysis of bipolar degradation and threshold voltage instability in 3.3 kV SiC MOSFETs having two distinct kinds of integrated diode, conventional body diode and embedded Schottky Barrier Diode (SBD). No bipolar degradation was observed both in MOSFET with conventional body diode and with embedded SBD after accumulated test with 100 hours each of 200%, 400% and 600% rated current stress in the 3rd quadrant of operation. However, the output characteristics show 1% (~0.2 mΩ) and 2% (~0.4 mΩ) increase in on resistance (RDS(on)) and 11% (0.23 V) and 5% (0.1 V) increase in threshold voltage (VTH), respectively, after total bipolar degradation test in the case of the MOSFET with conventional body diode and up to 74 hrs of 600% rated current stress in the case of the MOSFET with embedded SBD at 70°C. A rapid large negative VTH shift was observed in the MOSFETs with embedded SBD after ~ 74 hrs of 600% rated current stress. After accumulated Bias Temperature Instability (BTI) test at 150°C, the VTH value at 25°C has increased by 9.7% (0.14 V) and 14.5% (0.2 V) for the MOSFET with conventional body diode and with embedded SBD, respectively, while RDS(on) increased by 1mΩ at 25°C and by 5mΩ at 150°C, for both types of MOSFETs.

Partial discharge characteristics of peek utilized in press-pack IGBT under DC voltage

Modular multilevel converters (MMCs) have been utilized in solid-state transformers (SSTs). With the increasing demand in high-voltage power module in MMC-SST, press-pack IGBTs are very promising. The poly-ether-ether-ketone (PEEK) frame is utilized in the submodules of press-pack IGBT to withstand the high voltage. The electric field simulation in this paper reveals that electric field concentrates on the surface of the PEEK frame. Thus, the surface partial discharge characteristics under direct current (DC) voltage is investigated by the developed test bench. The results show that surface partial discharge model of PEEK sample has polarity effect. Under positive and negative polarity voltage, the average discharge quantity, maximum discharge quantity and discharge repetition rate of the PEEK sample all increase with the voltage amplitude. Moreover, the front time of the surface partial discharge waveform reaches 20 ns, and its frequency spectrum is concentrated in range of 10–20 MHz.

Concepts for realizing High-Voltage Power Modules by Embedding of SiC Semiconductors

Today’s power electronics modules typically consist of a ceramics substrate (DBC – Direct Bond Copper), carrying IGBTs, diodes or MOSFETs. These semiconductors are soldered or sintered to the ceramics and their top sides are interconnected by thick Al wires. An integration of further components or functions on the DBC substrate is difficult or even not possible. Therefore, driver circuits and controllers have to be mounted to a separate substrate, typically an organic Printed Circuit board (PCB). The PCB has to be connected to the DBC by wires or pins. The mechanical integration of the whole system requires a bulky housing. Embedding of power semiconductors like SiC MOSFET allows a significant size reduction, improved electrical performance and a high degree of reliability of such modules. In the last years, the capability of PCB embedding technology for the realization of low and high voltage power modules was demonstrated. Fraunhofer IZM together with partners from the industry demonstrated the feasibility for different applications, from single die SiC packages to high voltage automotive traction inverters. In this paper, a general overview about innovative power module technologies will be given and the embedding of power semiconductors will be introduced in detail. It will address all most relevant point, like the demands on the semiconductor, the thermal and electrical considerations as well as the demands on the used materials. To address the integration of the required driver circuits and controllers, the idea of modularization such electronics systems will also be presented. Here already packaged components will be used and embedded into PCB layers too. As a result, a modular approach to form a complete system will be developed. Different functional layers, e.g. power switches, logic modules, will be formed and finally stacked und connected to form the system. The concept and first realized demonstrators will be discussed.

Improvement of electrical insulating properties for defective metal/epoxy resin interface in power modules by micro-plasma jet

High-voltage power modules inevitably face the critical problem of electric field distortion at triple-junction point (Cu/insulation substrate/potting material) and accidental introduction of unexpected micro-metal defects on insulation substrate, which damages electrical insulating properties and induces breakdown. In this study, a novel precise surface treatment method based on atmospheric pressure micro-plasma jet (μ-APPJ) is proposed for rejuvenating and improving electrical insulating properties of defective Cu/epoxy resin (EP) substrate in power modules. Results show that a typical μ-APPJ with a diameter of about 200 μm is developed to precisely treat the micro-level metal defect in Cu/EP interface, and a spherical-clustered Si-containing functional film (∼Si-O4) can be fabricated on EP substrate by optimized Ar/Hexamethyldisiloxane (HMDSO)/μ-APPJ, which significantly enhances the hydrophobicity of EP surface. Moreover, the plasma-assisted film deposition around μm-defect improves interface electrical properties of defective Cu/EP samples, evidenced by higher flashover voltages and suppressed partial discharge. Further analysis based on simulations reveals that the deposited typical film not only promotes charge mobility and creepage distance on EP, but also introduces additional trap levels that capture electrons and inhibit electric field around metal defects, thereby suppressing partial discharge failure and improving electrical insulating properties.

Two-Stage Electric Field Optimization Method for High-Voltage Power Module Using Dielectrically Graded Insulation Micro-Region

Dielectrically graded insulation (DGI) with spatially inhomogeneous dielectric parameter distribution can provide extraordinary insulation performance for high-voltage power equipment. The most crucial objective in the application of DGI is to identify the optimal distribution of dielectric parameters. However, the calculation efficiency and electric field optimization results of the existing methods are poor under complex insulation structures. Also, the dielectric loss of DGI calculated by these methods is relatively large. In this work, an efficient DGI parameter optimization method was developed. Firstly, a variable density topology optimization model was proposed to improve the electric field uniformity of the power module and suppress the electric field stress at the triple point. Secondly, the influence of different model parameters on the mitigation degree of the electric field was studied to obtain the optimal parameters. Finally, to reduce the dielectric loss of the insulation, the DGI micro-region was constructed by narrowing the optimized region. Numerical results indicate that the model only needs 23 iterations to obtain the optimal result. After optimization, the electric field stress at the triple point is reduced by 71.7%. DGI micro-regions reduce dielectric loss by 85.1% compared to global DGI. This study shows that the proposed method is anticipated to provide a new route for the preparation and application of DGI.

High Temperature Characteristics of Composite Materials Composed of Silicone Gel and Barium Titanate in High Voltage Power Modules

The blocking voltage of silicon carbide (SiC) device is greater than 10 kV. High voltage can cause high electric field inside SiC power module. Improving the insulation properties of the encapsulation material of power module becomes an urgent requirement. The electrical field-dependent permittivity (FDP) material composed of silicone gel and barium titanate (BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) can be used to reduce the high electric field. However, the related research on FDP materials is limited to the room temperature conditions. It is not enough since the operating temperature of power module is higher than the room temperature because of the power loss, and the temperature has a great influence on the performance of FDP material. This paper focuses on the high temperature performance of this composite material in reducing electric field. Firstly, the simulation results show that the triple point between metallization, ceramic and silicone gel is both stressed by high electric field and high temperature. Increasing the dielectric permittivity of the encapsulation material can reduce electric field stress. Secondly, the breakdown voltage and aging properties of silicone gel and the composite were analyzed. The Curie temperature of the FDP material was measured experimentally, which proved that the Curie point of the FDP material would not be changed by the silicone gel. Thirdly, the permittivity of FDP material at different temperatures and the mechanism of reducing the electric field stress are investigated.

Simulation analysis of a specification package for high density and high voltage power module

A novel packaging designing for high density and high voltage power module, namely TPak was proposed. It uses flip-chip welding technology and through-hole technology to avoid the parasitic parameters introduced by wire bonding in traditional electrical interconnection, so that the proposed efficiency of the entire high-voltage module is close to 100%, parasitics are reduced by an order of magnitude, and module manufacturing is simplified and the module mechanical stress capability is improved. The finite element simulation anakysis of heat and force are carried out by means of simulation software, and the problems existing in the current Tpak packaging are analyzed from the perspective of materials, and improvement methods are proposed to optimize performance and improve its reliability.

Enhanced field-dependent conductivity and material properties of nano-AlN/micro-SiC/silicone elastomer hybrid composites for electric stress mitigation in high-voltage power modules

This paper reports an enhancement of the nonlinear conductivity, thermal and mechanical properties of micro-silicon carbide/silicone elastomer (m-SiC/SE) composites by adding nano-aluminum nitride (n-AlN) for power module encapsulation applications. The electrical properties (such as nonlinear conductivity characteristics and transient permittivity obtained from polarization current, and trap distributions obtained from thermally stimulated depolarization current) and material properties (including thermo-gravimetric analysis, coefficient of thermal expansion (CTE), and thermal conductivity, tensile strength, strain at break and Young’s modulus) of the pure SE, m-SiC/SE microcomposites, m-SiC/n-AlN/SE hybrid composites are investigated. The effect of the m-SiC fillers and n-AlN fillers on physicochemical properties of the SE matrix is analyzed by FT-IR spectroscopy and crosslinking degree. The measured nonlinear conductivity and transient permittivity are used for electric field simulation under DC stationary and square voltages. It is found that the addition of n-AlN fillers in the SE hybrid composite improves the nonlinear conductivity characteristics and mitigates the electric field under DC stationary and square voltages, compared to the SE microcomposite. Furthermore, the m-SiC/n-AlN/SE hybrid composite has a higher thermal degradation temperature, thermal conductivity, tensile strength, Young’s modulus, and crosslinking degree than the SE microcomposite, whereas their CTE and strain at break are lower. It is elucidated that the m-SiC/n-AlN/SE hybrid composite with enhanced nonlinear conductivity and material properties is a promising packaging material for high-voltage power modules.

Space Charge and Trap in High-Voltage Power Module Packaging Insulation: Simultaneous Measurement and 2-D Simulation

Space charge problems have been widely acknowledged for solid insulation such as polyethylene for high-voltage dc cable insulation. However, the effect of space charge on the reliability of high-voltage power module packaging insulation has not been systematically investigated. In this article, a simultaneous measurement of space-charge and isothermal relaxation current (IRC) is performed on silicone packaging insulation. An iterative algorithm based on the nonnegative least-square method is introduced to estimate multilevel trap energy distribution in silicone insulation. It is found that homocharges are accumulated around the electrodes, and more charges are accumulated with the increase in applied electric field and temperature. The local electrical field is distorted, and the threshold of the external applied electric field for charge accumulation decreases with increasing temperature. The trapped charges tend to be trapped in deep traps as the temperature increases. Furthermore, to investigate the charge dynamics in the complex power module package structure, a 2-D space-charge simulation model based on the combination of the bipolar charge transport model and finite-element method (FEM) is proposed with the parameters derived from the simultaneous measurement. According to the simulation results, the maximum electric field is located at the triple-junction position of the high-voltage power module package. With the accumulation of homocharges and the increase in temperature, the electric field decreases at the triple-point position but increases between the injected positive and negative charges. Space-charge behavior affects the local electric field of power module packaging insulation and should be thoroughly investigated in the future.

Investigation of Electrical and Thermal Properties of Epoxy Resin/Silicon Carbide Whisker Composites for Electronic Packaging Materials

This article explores the potential of epoxy resin/silicon carbide whisker (EP/SiCw) composites (1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%) as the field-dependent conductivity (FDC) packaging materials for the high-voltage power modules by investigating their thermal and electrical properties at different temperatures. The cross-sectional morphology of the prepared materials is characterized by field emission-scanning electron microscopy (FE-SEM). The thermal analysis consists of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The electrical properties, such as dc conductivity, dielectric spectroscopy, thermally stimulated discharge current (TSDC), space charge distribution, and dc breakdown strength, are measured. The molecular orbital energy levels of the EP and SiCw monomers are calculated by quantum chemical calculation (QCC) to analyze the effect of EP interfacial trap sites on the electrical properties. The effect of the EP/SiCw interfacial charges and electrical field distortion on the electrical properties of the EP/SiCw composites with different filler content is investigated by the finite-element method (FEM) simulation. It is found that the 2 wt% EP/SiCw composite has a better thermal performance. The 1 wt% has lower real permittivity, lower trap density, lower space charge accumulation, and higher breakdown strength. The EP/SiCw composites have a higher trap level than the pure EP. The nonlinearity percolation threshold of the EP/SiCw composites is 3 wt%. As the filler content increases, the nonlinear conductivity coefficient of the EP/SiCw composites becomes higher, whereas the breakdown strength decreases due to the electric field distortion near the electrodes caused by the apparent hetero charge accumulation at high filler content.

A New Structure of Ceramic Substrate to Reduce the Critical Electric Field in High Voltage Power Modules

Abstract The blocking voltage level of silicon carbide (SiC) can reach 10–25 kV, which will significantly increase the power density and capacity of power modules. However, high voltage can induce a high electric field, increase the risk of partial discharge (PD), and threaten the insulation reliability. This paper focuses on the triple points between the metal electrode, silicone gel, and ceramic in power modules. The influencing factors of the electric field at different triple points are fully analyzed. PD experiments are performed and the results show that the interface between silicone gel and ceramic is a weak area of insulation. Therefore, this paper demonstrates that area of weak insulation and high electric field meet at the triple point. To solve this problem, a new structure of the ceramic substrate is proposed, which isolates the interface area from the high electric field. At the same time, the new structure can significantly reduce the high electric field reinforcement.

Tailoring Electric Field Distortion in High-Voltage Power Modules Utilizing Epoxy Resin/Silicon Carbide Whisker Composites with Field-Dependent Conductivity

This paper reports the performance of an epoxy resin/silicon carbide whisker (EP/SiCw) composite (1–5 wt %) as the field-dependent conductivity (FDC) layer for electric field reduction in the high-voltage power module. The experiments consist of a field emission scanning electron microscope (FESEM), thermal conductivity, Fourier transform infrared (FT-IR) spectroscopy, thermally stimulated discharge current (TSDC), space charge, DC conductivity, and dielectric spectroscopy. The DC conductivity and dielectric spectroscopy are used for DC and AC stationary electric field simulations, respectively. The electric field reduction of EP/SiCw composites in the power module is analyzed, and the void defect in the FDC layer is also identified. The observed percolation threshold of the EP/SiCw composites is 3 wt %, and the DC electric field near the triple point decreases significantly by 74.8% under 10 kV when a 5 wt % EP/SiCw composite is applied for the FDC layer. It was found that the efficient threshold operating frequency of the FDC layer is around 10 kHz. The FDC layer can significantly reduce the electric field under AC voltage below 10 kHz. Although the power loss with the FDC layer increases obviously without the FDC layer, it is still lower than 1 W at 1 MHz, which is negligible for industrial applications. Notably, the void in the FDC layer is identified by the slowly increased dielectric loss with the increase of frequency through dielectric spectroscopy simulation.

A Quasi-Two-Level Medium-Voltage SiC MOSFET Power Module With Low Loss and Voltage Self-Balance

High-voltage and fast-switching silicon carbide (SiC) power modules are needed to construct high-voltage and high power-density converters. Stacking multiple low-voltage SiC devices to increase the equivalent blocking voltage shows advantages in <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> -state resistance, current capacity, and cost over one single high-voltage device. An indirect series-connected SiC MOSFET power module using quasi-two-level hybrid-clamp topology is proposed in this article. The voltages across the devices are automatically balanced with an open-loop modulation strategy to avoid sensors and control algorithm. Moreover, only ceramic capacitors are needed besides the MOSFETs and diodes in the topology. Thanks to this, the topology was highly integrated into a power module using SiC dies, which is equivalent to a general-purpose two-level medium-voltage SiC MOSFET power module. The switching losses of the power module show advantages over other stacking methods and even a single high-voltage switch and it can be evaluated with a two-level half-bridge (HB). To achieve this, the automatic voltage-balancing performance is analyzed in detail and the parasitic output capacitance in the topology is investigated, based on which the design considerations are presented. The simulation results from LTspice confirm the voltage-balancing and a 3600V/20A HB power module is built in the lab and the experimental results verify the design.

Space-Charge Accumulation and Its Impact on High-Voltage Power Module Partial Discharge Under DC and PWM Waves: Testing and Modeling

Emerging applications of compact high-voltage SiC modules pose strong challenges in the module package insulation design. Such SiC module insulations are subjected to both high-voltage dc and pulsewidth modulation (PWM) excitations between different terminals during different switching intervals. High <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dV/dt</i> strongly interferes with partial discharge (PD) testing as it is hard to distinguish PD pulses and PWM excitation-induced interferences. This article covers both the testing and modeling of PD phenomena in the high-voltage power modules. A high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dV/dt</i> PD testing platform is proposed, which involves a super-high-frequency (SHF) (SHF >3 GHz) down-mixing PD detection receiver and a high-voltage scalable square wave generator. The proposed method captures SHF PD signatures and determines PD inception voltage (PDIV) for packaging insulation. Using this platform, this article provides a group of PDIV comparisons of packaging insulation under dc and PWM waveforms and discloses discrepancies in these PDIV results with respect to their excitations. Based on these PD testing results, the article further provides a model using the space-charge accumulation to explain the PD difference under dc and PWM waveforms. Both simulation and sample testing results are included in this article to support this hypothesis. With this new model, the article includes an updated insulation design procedure for the high-voltage power modules.

Thermal and Electrical Performance in High-Voltage Power Modules With Nonmetallic Additively Manufactured Impingement Coolers

As high voltage, wide-band-gap power modules increase in power density with high switching frequency, they create localized hot spots and harmful electromagnetic interference (EMI). If not treated properly, these issues can drastically reduce the reliability of the devices. In this article, we propose a solution to address both of these challenges created by power module building blocks which are designed for use in a hybrid-electric aircraft traction inverter. Additive manufacturing is used to fabricate a nonmetallic jet impingement device to decrease power module temperatures. Moreover, the nonmetallic structure limits the accentuation of EMI created by the module operating at high switching frequencies. Conjugate heat transfer simulations are used to predict the thermal performance of the cooler. In this article, a reduction of maximum die temperatures is observed, along with an increase in temperature uniformity throughout the base plate. Experimental results show a minimum thermal resistance of 0.1 K/W and an average heat transfer coefficient up to 3600 W/m2.K at peak operation. Finally, EMI tests are performed to show a two orders of magnitude reduction of parasitic capacitance using the nonmetallic cooler compared with a metallic heat sink. This directly leads to a 25 dB decrease in the common mode noise.

Deployment of a Bidirectional MW-Level Electric-Vehicle Extreme Fast Charging Station Enabled by High-Voltage SiC and Intelligent Control

Considering the fact that electric vehicle battery charging based on the current charging station is time-consuming, the charging technology needs to improve in order to increase charging speed, which could reduce range anxiety and benefit the user experience of electric vehicle (EV). For this reason, a 1 MW battery charging station is presented in this paper to eliminate the drawbacks of utilizing the normal 480 VAC as the system input to supply the 1 MW power, such as the low power density caused by the large volume of the 60 Hz transformer and the low efficiency caused by the high current. The proposed system utilizes the grid input of single-phase 8 kVAC and is capable of charging two electric vehicles with 500 kW each, at the same time. Therefore, this paper details how high-voltage SiC power modules are the key enabler technology, as well as the selection of a resonant-type input-series, output-parallel circuitry candidate to secure high power density and efficiency, while intelligently dealing with the transient processes, e.g., pre-charging process and power balancing among modules, and considering the impact on the grid, are both of importance.

Trap characteristics and its temperature-dependence of silicone gel for encapsulation in IGBT power modules

Silicone gel is a prevailing material for encapsulation in insulated gate bipolar transistor (IGBT) power modules. The space charge transport behavior in silicone gel is significant to evaluate the electrical insulation characteristics. This paper focuses on the trap characteristics and electrical properties of the silicone gel, which were rarely studied before. The experiments are performed on the surface potential decay of silicone gel after the charge injection. Then, the energy distributions of electron or hole traps are determined by a double-trap energy level model, which can be fitted by the Gaussian distribution. In addition, the mobilities of positive and negative charges are determined, which are 1.38 × 10 −12 m 2 ·V −1· s −1 and 1.74 × 10 −12 m 2 ·V −1 ·s −1 , respectively. Furthermore, considering the heat as a byproduct resulting in thermal issues, the temperature-dependence of surface potential decay characteristics are also studied in this paper. When temperature rises, the decay rate of surface potential increases, especially when the temperature is higher than 80 °C. Finally, the contrastive analysis illustrates that the trap characteristics of silicone gel are between the trap characteristics in liquid-state material and solid-state material, which supports the phenomenon that silicone gel is more resistive to the sharp edges in power modules. This work can provide a useful reference for the design of encapsulation in high-voltage IGBT power modules.

Characterization of Nonlinear Field-Dependent Conductivity Layer Coupled With Protruding Substrate to Address High Electric Field Issue Within High-Voltage High-Density Wide Bandgap Power Modules

In addition to higher blocking voltages of wide bandgap (WBG) power modules, their volume has been targeted to be several times smaller than that of Si-based modules. This translates into higher electric stress within the module and, in turn, a higher risk for unacceptable partial discharge (PD) activities, leading to aging and degradation of both the ceramic substrate and the silicone gel. Due to the small dimensions of power module geometry, in the mm- or $\mu \text{m}$ (for protrusions)-range, and due to its extremely non-uniform electric field geometry, conventional high-voltage testing electrode geometries cannot simulate real conditions. On the other hand, university-based laboratories often cannot provide manufacturing/factory conditions for testing samples and for high-quality materials. Thus, it is difficult to determine the efficacy of electric field control methods through experiments. In these situations, numerical electric field calculation is the only feasible way to evaluate different electrical insulation designs. To this end, the finite-element method (FEM) models of the electrical insulation system used in WBG power modules are developed in COMSOL Multiphysics. It is shown that the current geometrical techniques alone cannot address the high-electric field issue within high-density WBG modules. To address this issue, for the first time, nonlinear field-dependent conductivity (FDC) materials applied to high-electric stress regions in combination with a recently introduced geometrical technique known as the protruding substrate is proposed. In this regard, the nonlinear FDC layer is characterized and various designs to reduce the electric field are evaluated. Moreover, the effect of the operating frequency on the performance of the solution mentioned above will be studied.

Humidity robustness for high voltage power modules: Limiting mechanisms and improvement of lifetime

During the last years applications for power modules with harsh environmental conditions have gained increasing importance. Static humidity tests with T = 85 °C and RH = 85% with high voltage have established as basic testing method for the humidity robustness of semiconductor power modules. The present work is showing the improvement of humidity robustness with respect to the established test methods. Electrical performance monitoring and analytic approaches to prove the results with respect to established work on the field of humidity testing.