How DENSO Met the Important Requirements for Flying Cars of Weight Reduction and Increased Efficiency — Mechanism of DENSO’s Proprietary Inverter and SiC

DENSO Tech Links Tokyo #11, an event organized by DENSO Corporation, employees involved in air mobility talked about how they are developing flying cars. In this section, Mitsunori Kimura of the Development Dept., Electrification Components Eng. Div. 1 introduces DENSO’s proprietary inverter and silicon carbide (SiC).

Sep. 13, 2021

【Speaker】

Mitsunori Kimura
Development Dept., Electrification Components Eng. Div. 1, DENSO Corporation

Inverters for Electric Vehicles

Mitsunori Kimura: I’d like to talk about inverters. First, I’ll explain how an inverter works in electric vehicles because many of you may be unfamiliar with this power converter.

An inverter controls the conversion of direct current, or DC power of a high-voltage battery to alternate current, or AC power to drive a motor, and the conversion of regenerated power of the motor to DC power to charge the battery.

This circuit diagram shows the main components. An inverter has six semiconductor switches and a power-smoothing capacitor. The on-off operation of the switches in the semiconductor switching module is controlled to adjust the voltage that is applied to the three-phase winding to drive the motor. This is the main characteristic of an inverter.

These are semiconductor switching devices, but they are not ideal. Power losses are generated during the period when the semiconductor switches are turned on to apply the current and during each switching operation, resulting in heat generation.

When developing inverters, the key point is to reduce heat generation. A water-cooled system is used for automotive inverters. Specifically, heat generated by the semiconductor devices is absorbed by water and radiated into the atmosphere via a radiator.

DENSO’s Inverters

Next, I’d like to explain the characteristics of DENSO’s inverters. DENSO’s conventional inverters had a single-sided cooling structure, so the heat generated by a semiconductor device was removed from one side. The latest inverters have a double-sided cooling structure. Specifically, a cooler is arranged on both sides of a semiconductor module to remove the heat of semiconductor devices from both sides.

We succeeded in almost doubling the cooling performance compared to single-sided cooling. This technology resulted in a compact inverter with high output density.

Based on the idea of removing heat from both sides, this double-sided cooling technology was embodied by combining DENSO’s heat exchanger technology refined through the manufacture of radiators, semiconductor module technology for in-house manufacturing, and the necessary production technology.

Thus far, I have explained automotive inverters. Next, I’d like to explain the characteristics and differences between cars and air mobility.

The total weight of a car is almost the same as that of a flying car. However, the percentage of motors, inverters, and electric drive system in the total weight is significantly different. The weight is an important factor that is closely linked with fuel consumption and range, and so reducing the weight is extremely valuable for air mobility. This is the main difference between cars and air mobility.

We aim to reduce the weight of flying car by thinking at entire system of it. At first we are working on reducing the heat loss of semiconductor devices and developing air-cooled structure by simplifying a water-cooled structure has been used for cars.

I don’t have time here to explain the details of the air-cooling structure, but I’d like to introduce SiC technology, which is the key to reducing the losses. Automotive inverters mainly use silicon devices. DENSO has developed next-generation SiC devices and manufactures them in-house.

Comparison of SiC and Silicon and Their Characteristics

Let me briefly explain the characteristics of SiC and silicon, or Si. This graph compares the physical properties and relevant characteristics of SiC and silicon.

As you can see, SiC is superior to silicon in all the physical properties: dielectric breakdown field strength which is related to low losses and high withstand voltage; thermal conductivity which is related to high-power operation; melting point which is related to withstand capacity; saturation velocity which is related to high-speed operation; and energy gap which is related to high-temperature operation.

This shows that SiC devices are suitable for high output with low losses, and are capable of high-temperature, high-speed drive. As I explained using the first slide, semiconductor switching devices are not ideal, so each switching operation generates losses.

The power losses generated during the period when switches are turned on to apply the current are called “conduction losses,” while those generated during each switching operation are called “switching losses.”

Regarding the conduction losses, the dielectric breakdown field strength of SiC is about 10 times higher than that of silicon. This makes it possible to reduce the thickness of the drift layer, which behaves as a resistor when the current flows, to about one tenth while ensuring the withstand voltage. Thus, the conduction losses can be reduced significantly.

Regarding the switching losses, the devices are not ideal, so losses equivalent to the voltage times the current are generated due to the voltage-current overlap in each switching operation.

SiC can perform high-speed switching, so the switching losses can be reduced significantly by minimizing the switching period.

Characteristics of DENSO’s SiC

Next, I’d like to explain the characteristics of DENSO’s SiC, which is named “REVOSIC®.” This is a collective name for DENSO’s SiC technology to achieve high quality and low losses. It represents our commitment to changing the world with innovative technology.

DENSO has been developing the overall technology, ranging from six-inch wafers, which attained the highest quality in the industry and ultra-low defects, to power modules, which attained high efficiency, to inverters, which drive the power modules.

First, I’d like to introduce low-defect RAF wafer technology, which underpins high quality. RAF stands for Repeated A-Face growth method. This technology minimizes defects by repeating crystal growth in different directions to grow high-quality SiC crystals.

This technology results in both high quality and a high yield for a large area.

The next technology is trench gate MOSFET to achieve low losses. By using the trench gate structure, it is possible to narrow the cell pitch and arrange multiple SiC cells to achieve low on-resistance and low conduction losses. These are the characteristics of DENSO’s SiC.

We have improved various elemental technologies for inverters, which I could not explain here, through the development of automotive inverters. We are using such technologies, as well as SiC technology and air-cooling technology to develop air mobility.