High density, high efficiency power conversion, conditioning, and control is critical to achieving the SWaP (Size, Weight and Power) metrics necessary in current and future military systems. Power needs can differ depending on the application. They range from hundreds of watts for application-specific integrated circuits (ASICs) to hundreds of kilowatts and even megawatts for next generation radars and directed energy weapons, such as high energy lasers and high-power microwave systems.
Over the past decade, power electronics have experienced a revolutionary advancement in performance. The rapid maturation and adoption of power switching devices based on wide bandgap (WBG) semiconductors has resulted in significant improvements in areas such as efficiency, determined by the ratio of power out to power in; power density, or the amount of power per unit volume; and transient response. WBG semiconductors enable the synthesis of power devices with improved Figures of Merit (FoMs), capable of operating at higher voltage levels, switching frequencies, and temperatures as compared to their conventional silicon counterparts.1
Figure 1 provides a comparison of semiconductor materials and FoMs as described by Jensen2. The associated FoM definitions and their application from Wang3 are shown in Figure 2.
As Figure 1 indicates, Silicon Carbide (SiC) and Gallium Nitride (GaN) are the most mature WBG semiconductor devices to date, emerging as front-runners to replace Silicon Insulated Gate Bipolar Transistors (IGBTs) and Metal Oxide Semiconductor Field Effect Transistors (MOSFETs). SiC MOSFETs are proving to be excellent candidates for replacing Si IGBTs in industrial motor drives and electric vehicle powertrains, and in replacing Si MOSFETs in AC/DC and DC/DC power supplies for telecom and datacom applications. The higher critical electric field of SiC and GaN enables power switching devices with lower on-state resistance, lower gate charge, and lower chip area, resulting in lower power dissipation, higher operating frequencies and power density. The higher thermal conductivity results in lower temperature rise, enabling higher operating temperatures; hence lighter and smaller cooling systems. SiC devices maximize the system level enhancements at voltages greater than 600V over their Si counterparts. GaN FET power switching devices are replacing Si MOSFETs in lower power, lower voltage (typically < 600V) applications, enabling switching frequencies that approach the MHz range.
Through both internal research and development and industry collaboration, Raytheon has taken full advantage of the significant advancements in WBG semiconductors in their use in power conversion systems across many product lines. High-power active electronically scanned arrays (AESA) and radar systems benefit from a power conversion design utilizing SiC MOSFETs, while lower voltage systems, including DC/DC converters driving ASICs, FPGAs and other digital systems, are achieving increased performance through the implementation of GaN FET-based power converters.
With the introduction of SiC MOSFETs and GaN FETs to its power converters, Raytheon has experienced appreciable improvements in SWaP metrics. One example of this is point of load GaN-based isolated regulators in an AESA system, which are distributed across the power system to provide galvanic isolation between tightly integrated power buses and high density, kW-level DC/DC converters. This configuration significantly reduces weight by eliminating high current cable harnesses. Another example can be seen in the improved efficiency of the SiC-based DC/DC converter illustrated in Figure 3. Here, the efficiency of an isolated DC/DC converter operating from 600V DC input to provide regulated 30V to a radar system is shown for both the SiC- and Si-based implementations. As seen in the figure, the measured efficiency with a SiC MOSFET is between 2% and 7% greater than a comparable Si-based DC/DC converter.
Figure 4 shows the efficiency of a GaN-based, hard-switching DC/DC converter operating at multiple input voltages, providing regulated 28V output. An efficiency of more than 95% in a hard-switching configuration at the specified switching frequencies is difficult, if not impossible, to achieve with a Si-based solution. In addition to the above applications, SiC MOSFET modules with integrated gate drives are used in motor drives for both control and actuation systems.
Raytheon’s power team encourages and works with suppliers to accelerate the insertion of SiC and GaN devices wherever the advantages of WBG technology can be realized. In addition, as members of a companywide WBG working group, Raytheon power subject matter experts (SMEs) meet regularly to identify, capture, and disseminate the latest technology information from industry and academia. SMEs produce manuals that capture critical design, manufacturing, process, and qualification guidelines. More focused efforts are undertaken to develop high-fidelity simulation models of WBG devices that include device parasitic elements and temperature effects. Collaboration with leading circuit simulation tool vendors helps to create models appropriate for the stringent application environments of deployed systems.
Raytheon actively participates in university and industry consortia such as Power America at North Carolina State University, and Center for Power Electronics Systems (CPES) at Virginia Tech, to form working relationships within the WBG power electronics technical community. These relationships provide the opportunity to learn about and leverage the latest developments in WBG devices and applications, influencing advanced research, and helping to attract high quality talent to the field.
WBG semiconductor power electronics delivers the performance and system benefits predicted for many years and is a key technology in the future of Raytheon’s power products. With WBG applications ranging from device development to converter topology and interface circuit optimization, Raytheon continues its focus on providing technologies that contribute to the standards and quality that bring reliable military and aerospace applications to customers’ missions.
— Sriram Chandrasekaran
— Jon Rawstron
1 Wide Bandgap Semiconductors: Pursuing the Promise, Department of Energy, Office of Energy Efficiency and Renewable Energy, https://www.energy.gov/sites/prod/files/2013/12/f5/wide_bandgap_semiconductors_factsheet.pdf.
2 Jensen G., Chabak K., Green A., Moser N., McCandless J., Leedy K., Crespo A., Tetlak S. (2017). “Gallium oxide technologies and applications,” Proceedings of the 2017 IEEE Compound Semiconductor Integrated Circuit Symposium (CSICS), pp 1-4.
3 Wang H., Wang F., Zhang J. (2008). “Power Semiconductor Device Figure of Merit for High-Power-Density Converter Design Applications,” IEEE Transaction on Electron Devices, vol.55, No.1, January 2008, pp.466-470.