Power electronics is evolving day by day, and wide band gap semiconductor technologies are becoming more and more popular. Thanks to their high operating temperatures and higher switching voltages and frequencies, they are practically replacing all older silicon solutions. Wide Bandgap (WBG) semiconductors undoubtedly constitute a true revolution for the electronics of the future. The most important of which, such as silicon carbide (SiC) and gallium nitride (GaN), are revolutionizing the world of electronics thanks to their exceptional properties compared to traditional silicon-based semiconductors.
Introduction
WBG semiconductors are covering the entire electronics market, until now occupied by silicon. Silicon carbide (SiC) and gallium nitride (GaN) fall into the category of Wide Bandgap (WBG) semiconductors and offer several advantages over traditional silicon semiconductors, which have now reached their performance limits. They are characterized by a particularly wide difference between the valence and conduction bands. The gap of the band is approximately three times higher than that of silicon and this allows higher working temperatures to be reached, together with higher operating voltages.
One of the great advantages of SiC and GaN MOSFETs is represented by the low value of the resistance between the source and drain channels when the component is conducting. This parameter is defined as Rds(ON) and is one of the first values that designers must observe in the datasheets. This value, especially for SiC devices, can be even a thousand times lower than a silicon component. The new WBG semiconductor materials therefore represent a clear improvement compared to existing technologies and the advantages concern lower power losses, greater robustness at high temperatures, and the possibility of working at significantly higher switching frequencies and operating voltages.
Both these semiconductors find application, especially in power electronics. Generally, GaN is faster than SiC, allowing for faster switching speeds. SiC offers higher breakdown voltages and prices are slightly lower. Furthermore, they are distributed in very common packages such as the TO-220, allowing simple and quick replacement in existing projects. As can be seen from Figure 1, semiconductors, in a general sense, are materials that have an intermediate electrical conductivity between that of conductors and that of insulators.
This conductivity can be adjusted by applying an electric field to a certain contact of the component. Silicon carbide is one of the most promising WBG semiconductors and offers higher energy efficiency, lower heat generation, longer life, and resistance to shock and vibration. GaN’s greater electron mobility makes it much more suitable for high-frequency applications because its gate has a very low electrical capacitance. On the other hand, SiC has higher thermal conductivity and is ideal for low-frequency circuits and high-power applications (e.g. automotive and solar) where high-frequency switching is not needed. Still, the system operates primarily in high voltage.
Applications
WBG semiconductor electronic components are ideal for power applications, but their use is highly diversified in many sectors. Furthermore, SiC and GaN are quite effective for creating highly efficient and compact devices. One of the major fields of use today is represented by the electric mobility sector, dedicated to electric cars and vehicle charging infrastructures, sometimes capable of delivering hundreds of kilowatts. Sectors related to clean energy production, such as wind and solar, are also a focus of SiC and GaN components, for which the systems often work with enormous powers, even in the megawatt range.
Naturally, the sector of railway transport is not excluded, with the management of extremely powerful engines and industrial automation with the control of robotic devices and automatic machines. Going into more detail, WBG components find wide applications in energy converters, in the driving of electric motors, inverters, LED lighting systems, power supplies, and also in communication devices. Silicon carbide is particularly suitable for high-power applications because it can withstand high voltages, much more than those of silicon. They are characterized by increased thermal conductivity with low power losses. SiC components are also particularly suitable for applications in electricity transmission systems, electric vehicles, photovoltaic systems, and charging stations. With them it is possible to have vehicles with greater autonomy, also allowing faster charging with a lower environmental impact.
An important field of application is represented by renewable energy. SiC converters, for example, are used in solar and wind energy systems to improve the efficiency of electricity conversion, increasing the production of energy from renewable sources. With the high voltage used, the sections and costs of the cables can be reduced. With the higher switching frequencies of WBG devices, much higher than Si and Ge, it is possible to create faster and more precise devices but, above all, less bulky ones, as the inductive components (inductances and transformers) can have smaller dimensions.
The Rds(ON) parameter
One of the most important parameters of SiC or GaN MOSFETs is the resistance of the drain-source channel during its conduction, defined as Rds(on). In silicon devices, it is significantly linked to the temperature and it increases much more than proportionally. This problem is also present in SiC devices but to a much lesser extent. GaN devices are also superior to silicon in this respect. For this semiconductor as well, the Rds(on) value is decidedly reduced. In switching applications, but also in static ones, WBG MOSFETs offer lower power losses and can operate at very high voltages and frequencies.
The reduction of this parameter therefore contributes to drastically reducing conduction losses. The electrical diagram in Figure 2 is an application example in which a very high current passes through a power load (R1). The SiC device, in this example used in static mode, has the following characteristics:
- Model: UF3C065030K3S
- Drain-source voltage: 650 V
- Typical drain-source on-resistance: 27 mΩ
- Continuous drain current: 85 A
- Pulsed drain current: 230 A
- Power dissipation: 441 W
- Maximum junction temperature: 175°C
In the diagram, the voltage on the gate allows the MOSFET to conduct a very high current, about 32 A, into the 3Ω load. During the circuit’s steady state operation, the following powers can be detected:
- Power generated by generator V1: 3045.47 W
- Power dissipated by load R1: 3019.17 W
- Power dissipated by MOSFET M1: 26.30 W
The power related to the MOSFET gate are irrelevant as they are in the order of uW. Knowing the electrical voltage on the drain of the device and the current flowing through the load, it is very simple to calculate the Rds(on) value as follows:
The newly calculated value of Rds(on) confirms the data published on the official datasheet of the component and is quite stable and constant for a wide range of operating conditions.
Now, it is fascinating to analyze and observe the variations of the Rds(on) based on changes in various parameters. The graphs in Figure 3 highlight the trend of this parameter with the change in other values and in particular:
- The first graph at the top “Rds(on) vs. temperature” shows the resistance values of the device in conduction, varying the junction temperature between -20°C and +180°C, for the same load. As can be seen, the values of this resistance are extremely low
- The second graph in the center “Rds(on) vs. drain current” shows the resistance values of the device in conduction, varying the current flowing on the drain of the MOSFET at temperatures of 0°C, 70°C, and 140°C. In this case, the value in question is very stable
- The third graph below “Rds(on) vs. gate-source voltage” shows the resistance values of the conducting device, varying the driving voltage of the MOSFET on the gate. The analysis of the graph starts from a voltage of 7 V since lower values would not allow the device to conduct
Lower switching losses
Since WBG semiconductors have lower conduction resistance than silicon and germanium, they dissipate less heat when conducting current. Furthermore, as a result of their higher breakdown voltage, they can withstand higher voltages without being damaged. High voltages and low currents allow the creation of smaller and more efficient devices, which require less cooling and consume less energy. Due to the high frequencies involved, inductive components can also be much smaller.
Today, with the revolutionary characteristics of WBG components, drive systems of motors or, in general, power devices control pulse width modulation (PWM) to vary the power on the load. Such methods allow to obtain high torques in motors and are highly efficient at any speed regime. However, they are affected by inevitable high-frequency switching losses, as electronic switches are not ideal components, they are not characterized by infinite speed and have an albeit small input capacitance which prevents perfect and clean signal switching. Some studies are focused on understanding the origin of these losses and the switching frequencies of the system.
The amount of energy lost depends on the type of electrical diagram and is directly related to the switching frequency. It is quite natural that higher switching frequencies increase switching losses due to the greater number of logic change events that occur. The electronic components, in fact, from certain points are no longer able to accurately “follow” the switching. Therefore, the selection of an appropriate switching frequency is very important to optimize the overall efficiency of the power system. In Figure 4 it is possible to observe the moments in which the switching losses occur, i.e. precisely in correspondence with the rising and falling edges of the gate signal, which is the one used to open or close the DS channel of the MOSFET. In the instants following the switching of the logic level, the voltage, and current transients are not immediate and sudden, so there are some moments in which these values, which are not zero, cause an increase in the power dissipated by the MOSFET. Furthermore, as mentioned above, the value of the losses increases as the working frequency increases, and already after 30 kHz, depending on the model used, these losses could be unacceptable.
Conclusion
As can be inferred from the previous paragraphs, semiconductors with WBG technology offer important advantages in terms of reliability, energy efficiency, power density, and cost reduction. SiC and GaN components are ideal for power applications, such as automotive, transportation, energy transformation, and the renewable energy sector. With WBG semiconductors there are undoubtedly fewer losses during device operation, allowing for more efficient performance and better energy conversion, combined with less heat dissipation. With their massive use, a great reduction in environmental impact is achieved for energy sustainability, as they also contribute to reducing greenhouse gas emissions.
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