SiC power devices improve efficiency, performance, and cost

With the latest introductions of next-generation silicon carbide (SiC) MOSFETs and diodes, SiC power devices are ready to impact mainstream power applications in solar inverters, motor drives, UPS systems, traction, and wind and other industrial power converters. Benefits from their inherent switching efficiency, ruggedness, and high power density can be used to reduce power consumption in power converters, increase the productivity of alternative energy systems, or, in many cases, significantly reduce the size, weight, and cost of systems by enabling higher switching speeds as compared to silicon-based solutions. SiC power devices are now being introduced by many vendors, and, most important, SiC switch technology is maturing, with several vendors introducing or sampling SiC MOSFETs. Cree’s introduction of its second-generation MOSFET technology — including 50-A MOSFETs and Schottky diodes at 650, 1,200, and 1,700 V — is a prime example. The key to realizing the potential benefits of these new SiC devices is targeting the right applications, knowing how to choose the right-size devices for a given system, and optimizing performance with particular design goals in mind. New SiC component costs are coming down significantly and, by taking full advantage of their capabilities, system designers can make higher-performing, higher-efficiency, and lower-cost systems.
In most applications requiring blocking voltages above 600 V, SiC MOSFETs will be competing with silicon IGBTs. The SiC MOSFET has significant switching-efficiency benefits derived from its unipolar architecture. IGBTs depend on bipolar p-n junctions to block and conduct. In contrast, the MOSFET is a field-effect device with no p-n barrier junction to dissipate and no resulting tail currents that waste energy like the IGBT. SiC MOSFETs have only 20% of the switching losses of silicon IGBTs at 1,200 V and less than 15% of the losses of 1,700-V silicon transistors. These benefits grow as switching frequency increases.
Less obvious are the potential advantages in conduction losses from the MOSFET. Figure 1 shows the characteristic IV curves for a 50-A IGBT and a SiC MOSFET chosen to match voltage drop at the maximum 50-A rating. SiC MOSFETs have a linear on-resistance across the entire operating range of the device. In contrast, the IGBT acts as a diode in series with a resistor and exhibits the characteristic knee voltage common with any bipolar device. Most systems do not operate at or even near the maximum operating conditions for the switches. As can be seen in Fig. 1 , at an average operating current of 25 A, the SiC MOSFET shows a significant advantage in conduction losses. The forward drop of the SiC MOSFET is 1 V, whereas the IGBT forward drop is nearly 1.4 V, resulting in 40% higher conduction losses for the IGBT. In many applications, IGBTs are significantly over-rated in order to compensate for this poor switching efficiency, further increasing the MOSFET advantage. As such, smaller SiC MOSFET device ratings can be used to achieve much higher efficiency numbers, which provides benefits regarding the physical size of system implementation, thermal dissipation, and ultimately, overall system cost.

Fig. 1: 50-A IGBT4 and 50-A SiC MOSFET I-V curves.

Figure 2 summarizes the device efficiency advantage. As devices have more amps to address a given system power requirement, the advantage of SiC over silicon IGBTs increases. Additionally, even when the system switching frequency is increased by nearly seven times, the SiC MOSFET remains much more efficient. Higher frequencies enable the reduction of filter inductor and capacitor ratings, resulting in smaller, lighter, and less-costly solutions.

Fig. 2: Conduction plus switching in inverter simulation.

The effect is illustrated in Table 1 , which depicts a two-level, three-phase inverter application that was simulated using different IGBT and SiC MOSFET power module ratings. The simulation was first done at a conservative 3 kHz — the frequency at which IGBTs provide the best advantages. Using IGBT modules rated from 600 to 1,200 A, one can see a respectable efficiency range from 98.9% to 99.1%. However, similar SiC module efficiencies can be achieved with smaller ratings of 250 to 400 A. In all cases, SiC efficiency beats the IGBT solutions ranging from 99.4% to 99.7%. Significant reduction in device rating helps offset the higher cost of SiC and provide smaller, lighter, and cooler-running power modules. Enhanced efficiency also provides lower system operating temperatures, enabling reduced heatsink sizes, fan sizes or, in extreme cases, even eliminating costly liquid cooling. Furthermore, as a SiC module’s frequency is increased to 10 and 25 kHz, SiC efficiency remains as good or better than that achieved by the IGBTs.

Table 1: 250-kW inverter loss analysis MOSFET vs Si IGBT

Figure 3 summarizes switching and conduction losses for a 600-A IGBT solution versus a 300-A SiC MOSFET-based solution. At half the module rating and more than three times the switching frequency, there is still an energy loss reduction of 33%, which drives down operating temperatures and reduces the system’s cooling costs. For grid connected systems — such as solar inverters, UPS units, or industrial power converters — increased frequency will shrink the magnetic and capacitive elements by many times. In addition to the bill-of-material savings, smaller systems are easier to install and maintain, and are much less expensive to ship. In transportation applications, smaller, lighter systems can pay for themselves many times over in energy consumption alone.

Fig. 3: 300-A SiC MOSFET module vs. 600-A IGBT module

These dramatic reductions in required amperage ratings make the new 50-A SiC MOSFET and Schottky diodes equivalent to much larger single silicon devices. As SiC MOSFETS and diodes parallel nicely, the new devices enable modules that can power systems into the hundreds of kilowatts. As the performance of SiC devices improves, larger devices are being introduced, package options are expanding, and the prices of the new devices are drastically decreasing from previous generations, all of which is good news for system designers. Cost reductions of more than half over previous generations are already readily available and, with higher volumes and further device innovations targeted for the future, pricing is expected to continue to come down.