Browsing by Author "Ball, Arthur"
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- Thermal and Electrical Considerations for the Design of Highly-Integrated Point-of-Load ConvertersBall, Arthur (Virginia Tech, 2009-11-18)DC/DC Power converter design has been following a trend of reducing size while also increasing performance for the last several years. This push for higher power output and smaller footprint and profile requires integration and higher switching frequencies in order to continue. Higher frequencies require physical integration to eliminate problems induced by parasitics, which increase losses. GE's Power Overlay and Philip's PCB integration schemes have been clear steps in the quest to reduce size with new system design techniques. However, both have downsides. GE Power Overlay embeds the devices inside a milled AlN ceramic cavity and then layers interconnections on top using polyimide dielectric interlayers. The milling of AlN ceramic is a very costly and time consuming task due to the brittleness of the material, and the interlayers add additional complexity to the fabrication process. Philip's PCB integration was primarily aimed at integrating passives along with the PCB process for reduction of size. Inductor windings and capacitive layers were built up along with FR4 epoxy layers using typical PCB fabrication methods. However, unlike GE's Power Overlay, the substrate material was several times lower in thermal conductivity which invariably has corresponding thermal penalties. The work presented here reconciles the good of both integration techniques. Initially called Embedded Power, alumina ceramic was used as the substrate and rather than milling holes for the devices, holes were laser cut all the way through and interconnections were made by using interlayers and sputtered copper deposition, similar to GE's method. Integration of passives was done using LTCC ferrite to make an inductor of thin profile, rather than embedding cores and windings inside PCB. However, fabrication remained time consuming due to numerous solder masking and sputtering steps and thermal performance was not optimized due to the use of alumina ceramic. A revised design method called Stacked Power is presented in this dissertation that follows on the work of Embedded Power, but improves on it by simplifying fabrication through the elimination of thermally-restrictive interlayers, as well as time consuming sputtering and electroplating of copper interconnections. Instead, AlN Direct Bonded Copper is used as a multifunctional material thanks to its many-times-greater thermal conductivity than PCB or alumina, solderable device dies are implemented in a vertical fashion, and interconnections are simply made using copper straps soldered into place. For applications where moisture contamination and breakdown isolation are potential problems, dip conformal coating can easily be applied, replacing laborious solder masking. The work in this dissertation describes the fabrication methodology for Stacked Power, demonstrates the thermal advantages, and shows examples of high-frequency buck converters that achieve super-high levels of power density in the smallest of volumes and require no more thermal management than modest airflow. The added cost incurred with aluminum nitride is traded for distinct advantages in terms of low-profile, low airflow requirements for the power output, capability of natural convection for use in locations where fans are prohibitive and compact size for ease of implementation.
- Work in progress: Creating a climate of increased motivation and persistence for electrical and computer engineering students: A project-based learning approach to integrated labsBall, Arthur; Baum, Liesl M.; McNair, Lisa D. (2019-06-15)This work in progress studies the impact on students and faculty and their perceived value of integrating project-based labs with lectures on student learning in a sophomore-level electrical and computer engineering course. Historically, engineering courses have been structured with a division between the theoretical lecture and the applied lab, preventing students from making clear connections between the two. Today's students do not find this legacy approach effective [1], [2]. In order to enhance student learning and concept retention in a large electrical and computer engineering program, a faculty team is redesigning the sophomore year experience using a project-based learning approach. This study describes the work of one instructor teaching a freshman-level course as part of the experience of exploring the full integration of labs and lectures that incorporate industry-level, real-world problems. The questions we seek to address are: How does integration of project-based lab and lecture contribute to students' perceptions of value, motivation and success? How does integration of project-based lab and lecture contribute to instructors' perceptions of value and motivation to modernize instruction? In this paper we discuss the historical approach to the design of the course, which we discovered was from the early 1980s, the time of the last major curriculum revision. In addition, using the MUSIC Model of Academic Motivation together with course data, we present baseline data from current students and instructors in regards to overall performance. Finally, using the MUSIC Model and course data from instructors and students in the revised course, we report some insight on perceived value and performance in order to make comparisons between the old and revised curriculum. Additional data sources were pulled from student feedback as well as analytic memos from the instructor. For the purpose of this paper, the combination and cross-analysis of this data resulted in a set of lessons learned and recommendations for faculty looking to adjust the design of their course to be more integrative. For the broader purpose of this grant project, this data will be used to influence the trajectory of the course and refine methods for more thorough integration of the labs. While our original expectation was that the integration of project-based labs would increase student success, as measured by course grade distribution as well as self-reported perceptions through the use of the MUSIC survey, we actually found little to no change in these measures. In addition, we anticipated the results from the research would reveal that having students participate in industry-level, real-world scenarios would contribute to increased authenticity students assign to the course content, also to which we found little to no change. Even though our original presumptions were unfounded in these two specific quantitative measurements, there are several other factors that have arisen that allow us to nevertheless make meaningful recommendations to other electrical and computer engineering instructors, as well as department administration, as we continue to modernize the student experience.