Ⅰ. Inverter System

The industrial drives used in low-voltage applications (<1000V) are a significant application area for power semiconductors. These drives primarily use two-level inverters that operate at switching frequencies ranging from 4 to 16 kHz and a dc-link voltage of around 600 V, as shown in Figure 1. Two-level inverters are the most commonly used in industrial drives due to their simplicity, cost-effectiveness, and high efficiency.

Fig 1 Schematic figure of a  two-level inverter for low-voltage standard motor drives.

In niche applications, matrix converters, and three-level inverters are used in low-voltage industrial drives. Matrix converters are bidirectional power converters that allow power flow in both directions, making them suitable for regenerative braking applications. They also have the advantage of being able to operate at higher switching frequencies compared to two-level inverters.

The high winding inductance of motors used in industrial drives implies that there is generally no need for higher operating frequencies beyond the range of 4 to 16 kHz, as illustrated in Figure 2(a). The inductance of the motor windings limits the rate at which the current can change, which in turn limits the maximum switching frequency that can be used without causing excessive current ripple or voltage spikes.

Fig 2: a)  Radar chart of low-voltage industrial drives; current status is shown in blue (= 100%), and the desired values are shown in red. b) derived switching device where current status with silicon devices (= 100%) is shown in blue, requirements on the switch level are shown in yellow, and the offered performance of SiC devices is shown in red.

However, in some special high-speed applications where the fundamental frequency is significantly higher than 50 Hz and the inductance values are lower, higher operating frequencies may be required to achieve the desired performance. Cost is a significant driving force for the widespread adoption of inverter drives in industrial applications.

The system requirements for industrial drives are translated to the switch level in Figure 2(b). The current status for silicon devices is shown as the reference level in blue, and the resulting requirements are shown in yellow. The achievable performance with SiC devices is shown in red. It can be seen that the achievable switching speed of SiC and even Si devices is not required in most industrial drive applications. Even Si IGBTs are often slowed down to avoid damaging the motor-winding isolation/bearings with high dv/dt values and to reduce electromagnetic interference.

In addition, the ability of SiC devices to operate at higher junction temperatures can enhance the durability of the converter systems in cases of overload situations. However, when replacing Si bipolar devices with fast SiC unipolar switches, it is crucial to take into account the parasitics in the layout and mechanical design more meticulously because of the fast switching transients that SiC devices generate.

 Ⅱ. DC-DC Converter

The application of dc-dc converters is discussed in two areas i.e low-voltage converters (both non-isolated and isolated) and high-voltage isolated converters. Cost is the foremost important consideration in both the automotive and telecommunication industries due to the high-cost pressure and intense market competition. In the automotive sector, the weight and power density of the converter are crucial factors as they have a direct impact on fuel efficiency, and there is a need for compact designs due to limited available space.  Similarly, the costs of floor space are high in telecommunication systems, making compact designs essential. In the context of the automotive industry, where harsh environmental conditions such as vibrations and extreme temperatures are common, achieving high reliability can be challenging, especially when operating at high temperatures. Reliability is also a critical consideration in telecommunication supplies, as a shutdown of a data center can be incredibly expensive.

a. Low Voltage Converter

1) Nonisolated Automotive DC-DC Converter:

The design considerations for a non-isolated automotive DC-DC converter, which is commonly used in hybrid electric vehicles or fuel-cell vehicles for power management between batteries, supercapacitors, and fuel cells. These converters need to meet the specific requirements of the automotive industry, including low cost, minimal component size and count, and bidirectional power flow to provide energy during acceleration and store energy during braking.

Fig 3 Schematic figure of a Bidirectional buck-boost converter

The schematic of the bidirectional DC-DC converter for automotive applications is shown in Fig. 3. The converter comprises four switches, an inductor, and two capacitors. The design must be compact and lightweight to meet the specific requirements of the automotive industry, including low cost, minimal component size and count, and bidirectional power flow.

2) Isolated DC-DC Converter:

Isolated DC-DC converters in the kilowatt range have a large market, with the telecommunication power supply being a significant application area. Full-bridge converters with soft switching or some form of resonant converters are commonly utilized in this field to meet the high-power density and high-efficiency demands, which have become more critical due to the increase in energy costs. However, initial costs play a crucial role in this market, leading to the use of relatively simple and robust designs. Fig. 4 shows the circuit schematic and a photograph of a full-bridge converter with a current doubler that fulfills these requirements.

Fig 4 (a) Photo of a phase-shift full-bridge converter with a current-doubler rectifier. (b)circuit schematic diagram.

The converter's design utilizes an optimization algorithm that considers not only the electrical model but also the thermal and electromagnetic design of the converter.

b. High Voltage Converter

SiC devices provide certain advantages in low-voltage applications, such as improved efficiency and/or power density, albeit to varying degrees based on the topology and application. Nevertheless, the adoption of SiC devices does not open up significant new application areas, except in high-temperature environments. Furthermore, the higher costs associated with SiC devices and the requirement for new processing technology may limit their usage to low-voltage systems.  By increasing the chip size of the JFETs and MOSFET, the conduction losses in the dc-dc converter can be reduced, and it is possible to achieve efficiencies up to 99% when using SiC JFETs.

In the near future, there are plans to increase the operating voltage level of the Super Cascode by implementing 6.5-kV JFETs. Efficient and compact SiC-based power-electronic converter systems operating above 10 kV face several challenges besides semiconductors. These include addressing transient voltage distribution in magnetic components, developing suitable packaging, and designing low-inductance/capacitance systems that meet high insulation voltage requirements. Nonetheless, resolving these issues will have a significant impact on the distribution and integration of fluctuating renewable energy sources in the future

Ⅲ. Conclusion

For low-voltage inverter drives that operate at a few kilohertz switching frequencies, the performance gain of SiC switching devices over Si IGBTs is limited. Achieving the same system performance as with Si requires a relatively large SiC chip area. However, SiC's high efficiency can be advantageous in converter systems for renewable energy, such as photovoltaics, where the high cost of SiC can be justified by reduced losses and long operating time. SiC's higher junction temperature capability can offer advantages in high-temperature environments, such as those found in the automotive industry near combustion engines.

These devices offer the potential to reduce chip area and achieve desired efficiency in low-voltage DC-DC converters where soft-switching conditions are easily achievable. For isolated DC-DC converters like the phase-shift converter with a current-doubler rectifier, SiC devices can improve power density and efficiency or reduce chip area to approximately 35% while maintaining the same performance.