“The increasing use of silicon carbide (SiC) transistors in power converters places high demands on size, weight and/or efficiency. In Contrast to bipolar IGBT devices, SiC’s excellent material properties allow it to design fast switching unipolar devices. Therefore, now only in low-pressure environments (
The increasing use of silicon carbide (SiC) transistors in power converters places high demands on size, weight and/or efficiency. In contrast to bipolar IGBT devices, SiC’s excellent material properties allow it to design fast switching unipolar devices.Therefore, now only in low-voltage environments (Introduction to SiC
Power devices based on wide band gaps, such as SiC Diodes and transistors, or GaNHEMT (High Electron Mobility Transistor), have become an important element among today’s power electronics designers. However, compared with silicon, what is the charm of SiC? What are the characteristics that make SiC components so attractive that, despite their higher cost compared to silicon high-voltage devices, they are still frequently used?
In power conversion systems, people have been trying to reduce energy loss during power conversion. Modern systems are based on technology that is used in combination with passive components to turn solid-state transistors on and off. For the losses associated with the transistors used, several aspects are relevant. On the one hand, the losses in the conduction phase must be considered. In MOSFETs, they are defined by classic resistors. In the IGBT, there is a fixed conduction loss determiner in the form of the knee voltage (Vce_sat) plus the differential resistance of the output characteristic. The loss in the blocking phase can usually be ignored.
However, in the switching process, there is always a transition phase between the on state and the off state. The associated loss is mainly determined by the capacitance of the device. For IGBTs, due to the minority carrier dynamic characteristics (conduction peak value, tail current), further contributions are in place. Based on these considerations, people would expect that the selected device is always a MOSFET, but, especially for high voltages, the resistance of silicon MOSFETs becomes so high that the total loss balance is not as good as that of IGBTs that can use charge modulation. Minority carriers are used to reduce the resistance in conduction mode. Figure 1 summarizes this situation graphically.
Figure 1: Comparison of the switching process (left, assuming the same dv/dt) and static IV behavior (right) between MOSFETs (HV means blocking voltage similar to IGBT C 1200 V and higher)
When considering wide band gap semiconductors, the situation changes. Figure 2 summarizes the most important physical properties of SiC and GaN compared to silicon. Importantly, there is a direct correlation between the band gap and the critical electric field of the semiconductor. As far as SiC is concerned, it is about 10 times higher than silicon.
Figure 2: Comparison of important physical properties of power semiconductor materials
With this feature, the design of high-voltage equipment is different. Figure 3 shows the impact with a 5 kV semiconductor device as an example. In the case of silicon, one has to use a relatively thick active area due to a moderate internal breakdown electric field. In addition, only a small amount of dopants can be incorporated in the active region, resulting in high series resistance (as shown in Figure 1).
Figure 3: Dimensions of 5 kV power devices-the difference between silicon and SiC
Since its breakdown field in SiC is 10 times higher, the active area can be made thinner, and more free carriers can be incorporated at the same time, so the conductivity is greatly improved. It can be said that in the case of SiC, the transition between fast switching unipolar devices (such as MOSFETs or Schottky diodes) and slower bipolar structures (such as IGBTs and pn diodes) has now shifted to higher blocking Voltage (see Figure 4). Or, now, for 1200 V devices, SiC can also use silicon in the low-voltage region around 50 V.
Infineon discovered this potential 25 years ago and established a team of experts to develop this technology. The milestone of development along the way was the first global launch of SiC-based Schottky diodes in 2001, the first launch of power modules containing SiC in 2006, and the most recent 2017, the Villach Innovation Factory fully converted 150 mm Wafer technology, which is related to it. The world’s most innovative Trench CoolSiC™ MOSFET debuted.
Figure 4: High-voltage device concept, comparison between silicon and SiC
SiC MOSFET in the field of modern power devices
As mentioned in the previous paragraph, today, SiC MOSFETs are mostly used in areas where IGBTs are the dominant component. Figure 5 summarizes the main advantages of SiC MOSFETs compared to IGBTs. Especially under partial load, due to the linear output characteristics, contrary to the IGBT under the knee voltage, the conduction loss may be greatly reduced. In addition, theoretically, the conduction loss can be reduced to an infinitesimal amount by using a larger device area. For IGBTs, this is excluded.
Regarding the switching loss, the lack of minority carriers in the conduction mode can eliminate the tail current, which may result in a very small turn-off loss. Compared with IGBT, the conduction loss is also reduced, which is mainly due to the smaller peak conduction current. Neither type of loss showed an increase in temperature. However, compared with IGBT, the conduction loss is dominant, while the turn-off loss is very small, which is usually the opposite of IGBT. Finally, because the vertical MOSFET structure itself contains a powerful body Diode, no additional freewheeling diode is required. The body diode is based on a pn diode, and in the case of SiC, its knee voltage is about 3V.
Someone might say that in this case, the conduction loss in diode mode is very high, but it is recommended (for low-voltage silicon MOSFETs, this is the latest technology) to work in diode mode in order to make the diode’s dead time short , Perform hard switching between 200 ns and 500 ns, for
Infineon also recently launched 650 V CoolSiC™ MOSFET derivatives, which will be deployed in the complete 650 V product portfolio. This technology can not only complement the IGBT of this blocking voltage level, but also the successful CoolMOS™ technology. Both devices have fast switching and common linear IV characteristics. However, SiC MOSFETs can make body diodes work at hard switching and switching frequencies higher than 10 kHz. Compared with super junction devices, their charge in the output capacitor is much lower (Qoss) and smoother capacitance vs. drain voltage characteristics. These features allow SiC MOSFETs to be used in high-efficiency bridge topologies such as half-bridges and CCM totem poles, while CoolMOS™ devices have advantages in applications where hard commutation on conductor diodes is not present or cannot be prevented.
This laid the foundation for the successful coexistence of SiC and super-junction MOSFETs between 600 V and 900 V voltage levels. Application requirements will provide designers with the most suitable technology options.
Figure 5: Overview of the advantages of SiC MOSFETs compared to IGBTs: dynamic losses on the left, conduction behavior on the right, integrated diode on the top left
Infineon’s equipment design has always been oriented towards useful cost-effective evaluations, with special emphasis on outstanding reliability, which customers are accustomed to obtaining from Infineon. Infineon’s SiC trench MOSFET concept follows the same philosophy. It combines low on-resistance and optimized design to prevent excessive gate oxide field stress and provide gate oxide reliability similar to IGBT.
Dr. Peter Friedrichs of Infineon Technologies
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