Yaskawa Electric prototypes new EV driving system using SiC to achieve substantial miniaturization

Yaskawa Electric has produced a prototype electric vehicle (EV) driving system using SiC power components (Figure 1). The system consists of a driving motor and a motor drive unit. By using SiC power components in the drive unit, the size of the drive unit has been reduced to less than half of the original product.

The prototype exhibited this time is based on the electric vehicle driving system "QMET" that Yaskawa Electric has already put into production. Yaskawa Electric is a large supplier of motor and inverter components. Since the company uses SiC power components in the driving system, many visitors came to see the prototype in front of the company's booth at the "2nd EV and HEV Drive System Technology Exhibition".

The motor drive unit of QMET consists of a circuit that switches the coil according to the motor speed and an inverter. The coil switching circuit is Yaskawa Electric's own technology. According to Yaskawa Electric, this technology can achieve efficient driving in all areas from low to high speed ranges.

Figure 1: Using SiC in the motor drive unit
Yaskawa Electric's electric vehicle driving system "QMET" consists of a driving unit including a driving motor, an inverter, and a motor coil switching circuit (a, b). By switching the power element used in the driving unit from the original Si to SiC, the volume of the driving unit has been reduced to less than half of the original size (c).

The existing QMET uses IGBT and Si power elements called power diodes in the main circuit of the inverter and the coil switching circuit of the motor respectively. This time, the IGBT was replaced with SiC MOSFET and the power diode was replaced with SiC Schottky diode (SBD). All of them were developed by ROHM (Table 1). By using SiC power elements, the overall volume of the inverter was reduced to about 1/3 of the original volume, and the volume of the coil switching circuit was reduced to less than half of the original volume. Therefore, the volume of the drive unit composed of the two was reduced to less than half of the original volume.

Shared water cooling mechanism

There are two reasons why the inverter and coil switching circuit can be miniaturized by using SiC power elements. The first reason is that power loss can be greatly reduced. This is because the heat generation will decrease as power loss is reduced, and even if the heat capacity is reduced due to miniaturization, the temperature will not easily rise. The second reason is that SiC power elements can work even at a high temperature of 200℃. In this way, the heat capacity can be further reduced, thereby further miniaturization can be achieved.

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While utilizing these characteristics of SiC, Yaskawa Electric has also made careful improvements to further miniaturize the inverter and coil switching circuits. In terms of the inverter, the large film capacitors used in the existing QMET were switched to small multilayer ceramic capacitors.

The cooling mechanism of the coil switching unit has been simplified. Previously, a dedicated water cooling mechanism was configured, but this time this mechanism was omitted and the water cooling mechanism on the motor side was shared (Figure 2). That is, the SiC power element module is configured on the aluminum (Al) heat dissipation substrate, and the heat released by the module is conducted to the water cooling mechanism on the motor side through the heat dissipation substrate to cool it.

Figure 2: Simplified cooling mechanism
Since SiC power elements can operate at high temperatures, the cooling mechanism can be simplified (a). This time, the water cooling mechanism of the coil switching circuit unit was omitted, and instead, the SiC power element module was configured on an Al heat dissipation substrate (b).

The use of SiC diodes has been basically finalized

According to Yaskawa Electric, the purpose of this trial product is to verify the potential of SiC power devices. It has been confirmed that it can actually work, but reliability tests and long-term driving tests are required for installation in electric vehicles, and these tests will be carried out in the future. In other words, this SiC power device cannot be installed in the drive unit of electric vehicles immediately.

However, it is certain that SiC power devices will gradually replace existing Si power devices in the future. Among the diodes and transistors used in the drive unit, it is estimated that SiC will be used first from diodes. This is because the loss reduction that is constantly required when Si diodes are used to extend the driving distance of electric vehicles is approaching its limit due to technical reasons. "The structure of the diode is relatively simple, so it is difficult to improve performance by reducing losses. This problem cannot be solved without changing the semiconductor material" (multiple technicians who are proficient in inverters).

It has been about 10 years since the first SiC diode was put into production, and its price has dropped so that it can be used in air conditioners. Even if the reliability test time for automotive products is taken into account, it is not surprising that SiC diodes will be installed before 2015.

The application of SiC in transistors is expected to be later than that of diodes. This is because the performance of Si IGBTs and power MOSFETs will continue to improve in the future, and SiC MOSFETs were only put into production at the end of 2010.