Publications & Presentations
R&D Bridging over Device and System Engineers Through the Electric-Mileage Estimation of a Motor System.
The team experimentally showed how different power devices change the efficiency of a motor system. This reserach aims at optimizing the design of the whole system, not focusing only on the device characteristics.
An evaluation of a new type of High Efficiency Hybrid Gate Drive Circuit for SiC- MOSFET suitable for Automotive Power Electronics System Applications
A report about high efficiency hiblid gate driver for SiC MOSFET
High efficiency 3-Phase Power Inverter Using SiC MOSFETs and Trans-Linked Neutral-Point-Clamped Topology
A 50kW three-phase inverter using SiC MOSFETs achieves a maximum conversion efficiency of 99.1%. The inverter utilizes a neutral-point-clamped (NPC) topology combined with a trans-linked circuit. These combined technologies successfully reduce the reactor loss and enable high efficiency operation.
Development of high speed and high accuracy SiC MOSFET model using VHDL-AMS.
Temperature Characterizations of Multi-Unit and Multi-finger Dependencies on AlGaN/GaN
The authors discuss the measurements and simulations of the transconductance characteristics for the compact model of AlGaN/GaN Ridge type HEMTs that we have developed, focusing on the dependence on temperature, number of gate fingers and number of units. In particular, we proposed model equations for the increased drain current due to gate injection and discuss in detail the analysis results of the change in the gate injection current characteristics with device size and temperature.
Comparison in Sintered Silver Die Attach Failure between Thermal Shocked Test and Four-point Bending Test
Sintered silver (s-Ag) has attracted so many researchers due to its high heat conductivity. Applying power module products for die attach part, the degradation has been evaluated by thermal shocked test (TST). Therefore, understanding of the degradation mechanism during TST is an eseential role for designing power modules. The objective of this study is to compare degradation mechanism between TST and four-point bending test to understand s-Ag die attach failure in power module products under operation.
Dominant Model Parameter Extraction for Analyzing Current Imbalance in Parallel Connected SiC MOSFETs
Micro pore structure dependence on pressured-type sintered silver tensile mechanical properties
Experimental Validation of Thermal Couple Impedance Model for Accurate Die Temperature Estimation in Power Modules
SiC wafer cost reduction with remote epitaxy technology through graphene
Expanding SiC power device market requires lowering wafer prices for the majority of device costs. In this talk, the author reports the application of remote epitaxy technology to alternative substrate with epi-membrane. SiC remote epitaxy needs high growth temperature with hot-wall chemical vapor deposition around 1873 K, which is a harsh environment for graphene survival. By replacing the carrier gas from hydrogen to argon, graphene etching was suppressed and peeling was successful. The author believes this approach represents a significant step toward the fabrication of high quality and low cost substrate with epi.
Geometry Independent Hole Injection Current Model of GaN Ridge HEMTs
The Compact model of GaN Ridge HEMTs, especially the drain current increase due to gate injection, has been improved from the one presented at APEC2020, and a scalable model equation has been derived by considering the physical generation mechanism. In addition, we analized the effect of the current gain H21 on the frequency response when the device is operated in AC. With this model, the H21 characteristics in the current-increasing region due to gate injection agreed with the actual measurement with high accuracy.
EV driving pattern simulation with PLECS
We propose a results of a simple motor inverter simulation with WLTC droving mode using PLECS. PLECS is the simulation platform of power electric systems.
TLP Bonding process using In coated Cu sheet for high-temperature dieattach
In coated Cu substrate was used as a low melting temperature bonding. We optimized the joint condition such as a temperature, pressure and process.
Development of the Third-Generation Wireless In-wheel Motor
The third-generation wireless in-wheel motor (W-IWM3) which has the capability of the Dynamic Wireless Power Transfer (D-WPT) on its wheel side has been developed for the Electric Vehicles. Developing concepts of W-IWM3 targeting “all components in wheel” is realized. One of the factors for “all components in wheel” is the small silicon-carbide (SiC) power device authors have developed. This paper showed the test result that the more than 18kW output with 95.2% DC to DC efficiency was achieved.
A High-Speed and High-Accuracy SiC MOSFET Model for Simulating Practical Power Circuits
We propose a SiC MOSFET model for circuit simulation. We validated the accuracy of the proposed model by measured switching waveforms of an inductive load switching evaluation circuit. Simulation speed of the model has been validated by a three-phase inverter simulation. Excellent results have been observed for both accuracy and speed.
Characterization of Mechanical Properties of Pb-2Sn-2.5Ag Solder Using Instrumented Indentation Microscopy with Optically Transparent Indenter
Pb-2Sn-2.5Ag alloy solder is one of the most popular materials for die attach in electronic devices, however its mechanical properties have not been reported. In this study, we measured the visco-elasto-plastic properties of Pb-2Sn-2.5Ag solder using Instrumented Indentation microscopy with an optically transparent indenter.
Study of the Microstructure and the Mechanical Properties of Pb-2.0Sn-2.5Ag Solder Joint
Since high lead solder has been used in industry for many years, few basic studies have been reported that discuss the microstructure and mechanical properties. In this study, the relationship between the microstructure and mechanical properties of high lead solder joints was investigated.
Influences of Device Parameters Variability on Current Sharing of Parallel-Connected SiC MOSFETs
Influence of device parameter variation on current sharing between SiC MOSFETs connected in parallel has been studied. From the simulation results, we reveal the device parameters which mainly affetct the static and dynamic current sharing behavior of the devices. We also evaluate the effect of the current sharing mismatch on energy loss for each device.
Pore Size and Shape Dependences on Quasi-Static Tensile Characteristics of Sintered Silver Films
The presentation reveals the pore size and shape dependences
on sintered silver mechanical durabilities.
From stress-strain results, uniform crystal structure make sphere and small pore shape. In terms of thermal cycle reliability using sintered silver materials, uniform structure with small and sphere pore state is essential.
Anisotropic temperature distribution causing the incremental saturated drain-current observed in the I-V characteristic of the SiC MOSFET
The Id-Vd charactaristic in high-Id and high-Vd range of SiC-MOSFET shows incremental trend instead of saturated Id behavior. By TCAD simulation, the self-heating at measurement reproduce well this incremental trend.
Zn-doped GaN Mesa Structure As a Gate for Normally-Off AlGaN/GaN-HFET
In the evaluation of the impurity level depth in Zn-doped GaN, it was found that Zn behaves as an acceptor impurity in GaN by forming the state at 0.3 eV from the top of the valence band. Normaly-off operation was confirmed in an AlGaN/GaN HFET with a 0.1 μm mesa shape using Zn-doped GaN as the gate layer.
Current-and-Voltage Hybrid Source Gate Driver for Maximizing the Switching Capability of SiC-MOSFETs
To maximise the switching capability of SiC-MOSFET, we introduce a current and voltage hybrid source gate driver and design method.
Miniaturized 48 V‒12 V insulation-type DC/DC converter miniaturized by using GaN transistors operating at 2-MHz switching frequency
The adoption of small Qoss and small Qg GaN transistors was found to be a useful option to solve both the miniaturization and high power conversion efficiency of switching power supplies. Isolated 48 V-12 V LLC converter with a switching frequency of 2 MHz and 100 W class was fabricated, and the maximum power conversion efficiency reached 95.3%.
Research Activities to Maximize the Capability of New Power Devices
The basic characteristics of the latest generation of ROHM's SiC-MOSFETs are presented and compared with the current generation of devices in production. On the other hand, it is often assumed that better power device characteristics improve power-circuit performace, but it is not that simple. For better system-level performance, our researches of power applications and the related simulations are also presented.
Study on Degredation Mechanism of Sintered Silver Mechanical Joint
Quasi-Static Tensile and Fatigue Tests of Sintered Silver Film
Temperature Dependency on Thermal Reliability Test Result by Sintered Silver Interconnection
The Influence of Mechanical Property on the Heat-Cycle Reliability of Sintered Silver Die Attach
This research investigates how the mechanical properties of sintered silver (s-Ag) as a die attach material influence its heat-cycle reliability. s-Ag mechanical fatigue life was 500 times under elastic stress. To investigate the correlation between fatigue mechanical test and thermal shocked test (TST), we performed TST with different temperature patterns. From TST results, higher TST gives faster degradation of s-Ag.
Flatbands in millimeter-scale twisted bilayer graphene
Twisted bilyaer graphene can be mocified the energy band structure by changing the rotation angle. Especially, the 1.1degree called as a magic angle can be shown the superconductivity. However, the next challenge is to obtain big scale (millimeter-scale) TBG samples. This research was shown to obtain it with using transfer graphene after annealing.
Rotation angle control of multilayer graphene on SiC (000-1)
Semi-Theoretical Prediction of Turn-off Surge Voltage in a SiC MOSFET Power Module with an Embedded DC-link Decoupling Capacitor
This paper presents a semi-theoretical method for predicting the turn-off surge voltage (Vsurge) in double-pulse test (DPT) using a power module (PM) with SiC MOSFETs and an embedded DC-link decoupling capacitor. In this scheme, multiple Kirchhoff’s laws-based equations are simultaneously solved, and drain-source voltage and drain current are mathematically expressed. This calculation process is semi-theoretical, and sufficiently predicts the Vsurge observed in DPT performed under various conditions.
Modeling of Switching Device and Simulation of Power Converter, and its Trends
The Influence of Sintered Silver Mechanical Property on the Heat Cycle Reliability Life Time
Effect of the sintered silver die attach thickness in hert-cycle reliability
Modeling of SiC UMOS chip and its application to Power Module
The modeling method developed in SiC DMOS was applied to the SiC UMOS and its versatility was verified. The model was also evaluated by comparing the switching waveforms fo a power module with SiC UMOS chip. As a result, the SiC UMOS model reproduced the measurement data with high accuracy as well as the SiC DMOS.
Dynamic On-State Resistance Measurement of GaN-HEMT by Double Pulse Test
Demonstrate the dynamic on-state resistance of GaN-HEMT and estimate inpact of loss of devices.
Mechanical Property of Nano Porous Sintered Silver: Toward Reliability Estimation
The fatigue mechanical property of sintered silver plays a critical role to estimate the reliability of systems. The tensile fatigue test is performed for the sintered silver with p=5% and the bulk silver. The fatigue lifetime of the silver films is shorter than that of the bulk silver one.
A gate Driver Maximizing the Switching Capability of SiC MOSFETs
This study report that high speed switching gate drive circuit for SiC MOSFETs. The provided gate drive circuit adapted subsidery capacitor in order to add gate voltage, enables increase gate current. This allows fast charge and discharge and reduced switching losses. This study confirmed that the provided gate drive circuit reduce switching loss compared to conventional one.
Dynamic Measurement Method to Extract High Voltage and High Current I-V Characteristics of SiC MOSFET with Reduced Self Heating
A novel measurement method to extract high voltage and high current I-V characteristics (HVHC I-V)of SiC MOSFET is proposed in this paper. The method could extract the HVHC I-V with extremely reduced self heating of the device. By using the method, we can measure higher voltage and current I-V characteristics than those extracted by the conventional method. It is possible for us to measure the HVHC I-V of the next generation SiC MOSFET, whose rated volatge and current is more than those of the conventional device.
Switching behavior based method to estimate the intrinsic gate resistance of a transistor by using gate plateau voltage
The internal gate resistance of SiC MOSFET effects its switching behavior larger than Si devices because it can switch at high speed with low external gate resistance. However, the convention mesurement method of internal gate resistance is not accurate. Then, we devised the new measurement method of it at switching operation by the switching waveforms.
Tensile Mechanical Fatigue Properties of Sintered Porous Silver Films
A Study of Adhesion Interface about Die Bonding Structure Using Conductive Adhesives
Research for maximizing the potentials of wide bandgap semiconductor power devices
Measurement scheme to model an SiC MOSFET for simulating its switching behaviors
The precise modeling of SiC MOSFETs are difficult by means of conventional modeling method of Si devices because of the device characteristics. Then, we invented the new modeling method for SiC MOSFETs which can reproduce its switching behavior precisely.
Thermal Warpage Behavior Analysis of Semiconductor Package of Semiconductor Packaging Structure
A High Efficiency 3-Phase 400V 15kW Power Inverter Using SiC MOSFETs and Trans-Linked Topology
A 3-phase15kW using SiC MOSFETs and trans-linked topology achieved maximum efficiency of 99.2%, and verified the benefit of combining SiC MOSFET with trans-linked technology.
High Speed Gate Circuit for SiC-MOSFET Utilizing CapacitorT
IoT Application from the Viewpoint of Electronic Devices and Potential of Energy Harvesting
A Study of Adhesion Interface about Die Bonding Structure with Conductive Silver Paste
Influence of porosity on Tensile Mechanical Properties of Sintered Porous Silver Films
This paper investigates the porosity-dependent tensile mechanical properties of porous 8-10 μm thick silver films. The silver films are fabricated by pressure press, the variety of which changes the porosity (p) ranging 5% to 25%. p is determined by use of scanning electron microscopy cross-section images of the films. Stress-strain (S-S) curves are obtained by tensile tests performed for the porous and bulk silver films.Breaking strain and Ultimate tensile strengh decrease almost linearly with increase of p.
Power application research to utilize the full potential of wide band-gap power semiconductor devices
Tensile Mechanical Properties of Sintered Porous Silver Films
The mechanical properties of die-attach materials are a decisive factor in determing the reliability of the system. Sintered porous silver is attracting much attention as a next-generation die-attach material,however still do not clalify the mechanical properties. Authors prepared thin sintered silver and bulk silver films.Sintered silver films were fabricated under various pressure. From tensile tests, Sintered porous silver films exhibit brittle characteristic in comparison with silver films and show high strength under high process pressure.
A Small Signal AC Model Using Scalable Drain Current Equations of AlGaN/GaN MIS Enhancement HEMT
A Scalable drain current model and a small-signal AC model have been developed for AlGaN/GaN MIS HEMTs with embedded source field plate. The model was found to reproduce the device characteristics well by comparing the Id-Vds, Id-Vgs, S11and H21 characteristics of the device.
A Trans-Linked 5-kW Inverter Using SiC MOSFETs to Achieve Fan-less Operation
A trans-linked 5kW interleaved inverter using SiC MOSFETs drastically reduced both tansistor loss and reactor losses, resulting in high efficiency over 99% and fanless operation.
Electro-Thermal Simulation for Predicting the Temperature of SiC Dies in the Power Module of a High Frequency Operating Power Converter
The accurate electro-thermal simulation is presented by using a new SiC die model to predict the power loss and temperature of SiC MOSFET dies in a power module. The new model incorporates a temperature dependent I-V model and body diode model which is dependent on the gate-voltage. The simulation using the model yields an accurate power loss and temperature estimate of the SiC module in a buck converter.
S-parameter Based Simulation Modeling a Power Module Independent of Measurement Data
S-parameter based simulation model for power module (PM) except for the semiconductor chips was created with electromagnetic simulation. This PM model was combined with the chip model in order to build the model of the whole structure. As a result of verifying the model by the double pulse test, the simulation waveforms which reproduce the measurement results well were obtained.
Circuit Simulation of a Silicon-Carbide MOSFET Considering the Effect of the Parasitic Elements on Circuit Boards by Using S-parameters
The high speed switching of SiC MOSFET is affected by the parasitic elements of circuit board. Therefore, we analyzed the impact of stray capacitance in circuit board on the switching behavior and tried circuit simulation of the effect by use of precise model and electromagnetic simulation results of circuit board.
IMC formation and suppression at the interface of high Pb containing solder / Cu frame
800 V Three-Phase LLC Series Resonant DC/DC Converter Using SiC MOSFETs
This study report that a three-phase, 5-kW LLC series resonant dc/dc converter utilizing SiC MOSFETs. The high-break down voltage of SiC MOSFETs, enables increasing the input voltage up to 800 V. Around 160kHz switchig frequency successfully reduces the volume of isolation transformers. Current-balancing transformers among each phases effectively suppress a peak current from arising in the circuit and contributed that miniaturizes the input and output capacitances. The proposed power supply weighs 1.55 kg with dimensions including a width of 18 cm, a length of 12 cm, and a height of 12.5cm. The conversion efficiency of the converter reaches 98.1% at 5-kW operation.
Thermoelectric enhancement in the two-dimensional electron gas of AlGaN/GaN heterostructures
Influences of Magnetic Core Materials to Power Inductors Fabricated by Silicon Technologies
SiC-FET Chip Model to Simulate its Switching Behaviors in High Voltage and Current Region
Until now, the transientcharacteristics of circuit simulations using the SiC device model have not been consistent. Since EMC must be considered when using SiC Devices, it is important to reproduce their transient characteristics. Therefore, we have developed a novel SiC devie model by measureing the Id-Vds characteristics with the high-voltage and high-current region and the on-state capacitance of the SiC MOSFET to reproduce the transient characteristics. As a result the novel SiC device model successfully reproduce the switching taransient waveform with high accuracy.
Self-Sustained Oscillation in Half Bridge Circuit of Silicon Carbide Devices with Inductive Load
The self-sustained oscillations are sometimes obserbed when SiC MOSFETs switch at high speed in the half bridge configuration. The oscillation is regard as nonlinear self-excited oscillation as a result of its device characteristics. We tried to analyze it from the aspects of device characteristics of MOSFET and trajectory of the oscillation in the I-V plane.
Design and Integration of Power Inductor using Silicon Technology