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Advanced Power Electronics Research Center

Power Circuit Integration Team
 Electrical energy is easily transmitted and controlled and efficiency is high; because of this, the end-use form of most energy is as electric power, and this ratio of electrification is projected to further increase. In addition, from the standpoint of reducing greenhouse gases, new forms of power generation such as solar and wind power as well as new applications such as electric vehicles are increasing rapidly. Electrical energy from the power system to consumers is variously converted and supplied according to applications, and to make most efficient use of this power, it is important to develop technology for the next generation of electric power converters (inverters) and introduce them into fields where they are not yet used. The material characteristics of wide-gap semiconductors are different from those of silicon semiconductors, allowing wide-gap semiconductors to operate under conditions of high temperature and high power density and at high speed. These features of wide-gap semiconductors are useful for high-output, low energy-consumption devices.


 From the points of view above, the Power Circuit Integration Team is promoting research on design technology for electric power converters (circuit technology, mounting technology, modularization technology, simulation technology, etc.) for use of wide-gap semiconductors in electrical energy control.

Overview of Research

 Design of electric power converter cannot be achieved with power devices alone, and systematic knowledge (databases) of conventer  systems inclusive of peripheral technology is necessary. In particular, higher-temperature and higher-speed operations are possible with wide-gap semiconductors than with conventional silicon semiconductors; therefore, new design techniques that differ from the conventional are important.

 In addition, electric power converters tend to have an increase in the volume of cooling mechanisms (heatsinks, fans, etc.) with increase in conversion capacity, but with wide-gap semiconductors, higher temperature operation is possible than with silicon semiconductors, so smaller cooling mechanisms can be used. We are introducing power density (= electrical energy/total volume including heatsinks and fans) as an index indicating the performance of electric power converters, and we are promoting research to achieve small, high-efficiency electric power converters .

Three-dimensional Mounting Technology

 Two-dimensional mounting is primarily used in current electric power converters . However, the device switching speed is high in new electric power converters that make use of wide-gap semiconductors; therefore, there are clearly new problems arising, such as parasitic inductance and generation of noise. We are working on developing three-dimensional mounting techniques for solving these problems in circuit integration technology along with improving heat dissipation, size reduction, and reliability.

FIG. 1 Two-dimensional mounting technique
FIG. 2 Three-dimensional mounting technique

High-Temperature Mounting Technology

 High-temperature mounting technology is necessary to make use of the characteristics of wide-gap semiconductors, which can be operated at temperatures of 300°C or higher. Normally, high-temperature solder which can be used at 300°C or higher is used, but since the soldering process temperature and device operating temperature are both higher than the conventional, it has been found that there is a reduction in bonding strength because of diffusion reactions at the solder/circuit board interface. We have been successful in solving these problems by developing surface treatment techniques for circuit boards and controlling the reduction in bonding strength.

FIG. 3 High-temperature junction technology of power device

High-Efficiency, High-Power-Density Electric Power Converter Design Using Wide-gap Power Semiconductors

 We are studying the design and fabrication of high-efficiency converters that make use of the merits of wide-gap semiconductors in order to achieve high-efficiency, high-power-density electric power converters .
  FIG. 4 shows the results of converter performance analysis for SiC power devices. Ploss is the loss, Tj is the junction temperature, and Rth is the thermal resistance of the heat sink of the SiC power device. A large Rth means that a small cooling component can be used; therefore, it was found that in SiC power devices capable of operating at high temperatures with small loss, it was possible to fabricate small electric power converters by using small heatsinks.

FIG. 4 Relationships among thermal resistance (Rth) of heatsink, SiC power device junction temperature (Tj) and loss (Ploss)

Simulation Technology

 In the integration and mounting of devices that operate at high temperatures, it is important to investigate in advance the effects of thermal stress, thermal strain, thermal expansion, etc. at the junction interface for each mounted component with different mechanical and electrical characteristics. By using finite element method, We are carrying research on improving the reliability of structures resistant to high heat along with integration and mounting of devices operating at high temperatures.

FIG. 5 Example of temperature distribution based on the results of a heat transfer analysis of heat generated by high-temperature mounted components incorporating SiC devices.

FIG. 6 Example of equivalent stress distribution based on the results of a coupled analysis (heat transfer analysis and structural analysis) of heat generated by high-temperature mounted components incorporating SiC devices.