As we all know, silicon (Si) semiconductor power electronic devices have long been able to meet the demands of power electronics in terms of high power, fast switching speeds, low on-state resistance, and minimal driving power. However, the performance ceiling of traditional semiconductor power electronic devices, represented by Si, is essentially capped at around 10 (the product of power frequency and the semiconductor material's limitations), largely due to parasitic diode constraints. This has brought us close to the material's physical limits. Currently, power electronic devices based on wide bandgap semiconductor materials like silicon carbide (SiC) are regarded as the next-generation solution to address the requirements of high breakdown voltage and high operating temperatures. These devices could potentially eliminate the need for overcurrent, overvoltage, and overtemperature protection circuits, allowing simpler circuit topologies in high-power applications, thus reducing the failure rates and costs of these devices. Of course, SiC-based devices still face numerous challenges in terms of material and process development, requiring substantial research investment and time to overcome these hurdles. This article explores the advantages and disadvantages of SiC devices and their applications in power systems, ultimately summarizing the future development prospects of SiC devices.
Wide bandgap semiconductors like SiC not only exhibit high breakdown electric field strength and excellent thermal stability but also possess high carrier saturation drift speed and high thermal conductivity. Given the unique properties of SiC materials, SiC power electronic devices offer several benefits. The bandgap of SiC materials is 23 times that of Si and GaAs, enabling SiC devices to operate at higher voltages and temperatures. The high saturation electron drift speed and permittivity of the medium determine the high-frequency and high-speed capabilities of SiC devices. The thermal conductivity of SiC is 33 times that of Si and 10 times that of GaAs, which implies superior heat dissipation, improving circuit integration and reducing the need for extensive cooling systems, thereby significantly shrinking the overall size of the device. Additionally, SiC has an exceptionally high critical electric field strength (4590eV), providing robust resistance to electromagnetic pulse (EMP) impacts and radiation.
However, SiC devices are not without their drawbacks at this stage. For instance, in SiC MOSFETs, the built-in diodes are unreliable at high temperatures. Moreover, the scarcity of natural SiC results in high production costs, making SiC devices more expensive. Despite these limitations, SiC applications are burgeoning, particularly in power systems, offering a promising alternative to Si-based materials.
The application of SiC devices in power systems has become one of the most active research directions in recent years. The advent of new power electronic devices like SiC promises to have a profound impact on power systems. For example, foreign researchers have developed a 10kV SiC MOSFET-based boost converter, an 180kVA three-phase inverter based on SiC turn-off thyristors, and a 4kV DC output, 4kW power boost converter based on SiC-FETs, among other FACTS or HVDC devices.
SiC devices have a significant influence on Flexible AC Transmission Systems (FACTS) and High Voltage Direct Current (HVDC) devices. Firstly, medium and high voltage FACTS and HVDC devices are designed using high-blocking voltage SiC devices, eliminating the need for transformers in grid connection. This results in a more streamlined and efficient main circuit. Secondly, the high-temperature tolerance of SiC devices makes thermal management easier and more reliable. Si devices can typically handle ambient temperatures up to 125°C, whereas SiC devices can withstand temperatures exceeding 500°C. Thirdly, the lower switching losses of SiC devices enhance the efficiency of FACTS and HVDC devices. At high switching frequencies, switching losses constitute the primary component of the total device loss.
Figure (A) illustrates a schematic diagram of a measurement platform built to assess the losses in power electronic devices. Here, V represents the input voltage, L is the inductance, TpA, TnA, SpA, and SnA correspond to the power devices on the upper and lower bridge arms. The output measurement includes AC current, output voltage derived through a differential circuit, C being the filter capacitor, U representing the output inductance, and the output current under load conditions. The system determines the electronic device loss by measuring the differential output voltage and load current.
Figures (B) and (C) depict the turn-on and turn-off losses of IGBTs, respectively. Pos in the figures indicates power loss. When the DC-side voltage increases, the IGBT switching losses also increase; the turn-on losses at 450V are 240pW and 300pW, respectively. Under the condition of I = 5A, the turn-off losses corresponding to V = 300V and V = 450V are 200pW and 260pW. Since the current decay rate during turn-off is much slower than the turn-on current, the turn-off loss of the device is significantly greater than the turn-on loss under the same DC voltage. Additionally, SiC devices can operate at higher switching frequencies compared to Si devices, enhancing the performance of FACTS and HVDC devices while reducing the size and capacity of passive components. Furthermore, the switching losses of SiC MOSFETs are less influenced by ambient temperature, unlike SiGBTs, which are more susceptible to external environmental conditions.
In wind power generation systems, the use of SiC inverters improves system efficiency, increases output power, and reduces system size and costs. A notable distinction between SiC diodes and Si fast recovery diodes is that SiC diodes exhibit a positive temperature coefficient characteristic in forward voltage, allowing them to operate in parallel, thereby increasing the rated power of DC-AC rectifiers (such as generator rectification).
In New Energy power generation systems, the increased junction temperature due to power electronic device losses necessitates larger and heavier cooling systems. However, SiC devices with their high-temperature and high-frequency characteristics reduce the losses of power electronic converters based on SiC devices to approximately one-third of those based on Si devices. Wind and photovoltaic power generation devices often operate in low-power output states. Utilizing SiC power electronic inverters enhances the efficiency of these power generation devices, especially in smaller power outputs where the efficiency improvement is even more pronounced.
SiC devices also play a crucial role in microgrids, which are considered a key measure to reduce greenhouse gas emissions and address the energy crisis. Traditional microgrid power conversion devices using Si-based power electronic components tend to be bulky and inefficient. SiC material power electronic devices, with their high voltage tolerance, low losses, high temperature resistance, and low reverse recovery current, are particularly suited for medium-voltage microgrid power conversion devices.
Firstly, the high threshold voltage of SiC devices simplifies the main circuit topology of medium-voltage microgrid power conversion devices. Secondly, SiC devices have low on-state losses at high current densities and a positive temperature coefficient, making it easier to expand the capacity of microgrid power conversion devices. Thirdly, SiC devices exhibit shorter turn-off times, lower reverse recovery currents, and reduced turn-off losses, improving the efficiency, stability, and reliability of microgrid power conversion devices.
A key parameter of diodes is the off-time, which depends on the thickness of the i-zone, carrier lifetime, and saturation speed. Increasing the i-zone thickness boosts the device's breakdown voltage but decreases its switching speed. Conversely, a thinner i-zone increases the operating frequency but reduces the breakdown voltage. SiC materials offer significant flexibility in balancing these parameters.
Figure (A) shows the turn-on waveform of a 1.5kV/10A SiC MOSFET at room temperature, with a source-drain voltage VDS of 600V, source-drain current I of 5A, gate-source voltage VGS of 20V, and an external gate resistance of 10Ω. An anti-parallel SiC Tiki diode is also present. It’s evident from the figure that the turn-on and turn-off times of this material are less than 50ns, showcasing its extremely fast switching speed, far surpassing that of SiIGBTs.
Figure (B) illustrates the turn-off waveform of a SiC MOSFET at room temperature. The turn-off time of the SiC device is significantly shorter than that of the Si device, and the maximum reverse recovery current is much smaller, leading to much lower turn-off losses.
Based on the synthesis and analysis of existing research on SiC power electronic devices, the following conclusions can be drawn: The highest voltage level of Si fully-controlled devices is currently 6.6kV, applicable only to power electronic devices below 3kV. These devices suffer from issues such as high prices, low switching frequencies, and large losses. In contrast, SiC devices outperform Si devices in terms of high voltage tolerance, high temperature resistance, high switching frequency, low losses, and excellent dynamic performance. In cases involving higher voltage levels (above 3kV) or higher requirements for power electronic device performance, existing devices fall short, and SiC devices clearly excel. With the commercialization and widespread adoption of SiC devices, power systems involving power electronics—such as FACTS, HVDC, renewable energy power generation, microgrids, and others—are bound to undergo transformative changes.
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