Four grad students win IEEE APEC Best Presentation Awards

Enver Candan, Christopher Brandon Barth, , and Thomas Peter Foulkes, all ECE ILLINOIS graduate students advised by Assistant Professor Robert Pilawa-Podgurski, won best presentation awards at the 2017 IEEE Applied Power Electronics Conference (APEC). Both Foulkes and Prof. Pilawa-Podgurski are CSL affiliates. Pilawa-Podgruski is also a Helm Fellow in Electrical and Computer Engineering.

IEEE APEC is the leading conference in the field of power electronics. “The program includes excellent technical talks and workshops from the leading experts, enough free time to explore the industry representatives in the expo area, multiple networking events, and a poster session,” said Candan. 

According to Barth, the trade show at the conference allows for interactions with leading edge suppliers. This means that the personal connection not only enhances knowledge of the products, but can also lead to the acquisition of engineering samples before their release to the public.

“I’m incredibly excited that the hard work of my graduate students was recognized by these awards,” said Prof. Pilawa-Podgurski. In addition to producing high-quality research, his students have worked hard to present their groundbreaking research results effectively, both in written and oral form. “Receiving a best presentation award at APEC is a significant achievement for any power electronics engineer. The fact that our group received four such recognitions this year was certainly exciting, and encourages us to continue exploring innovative solutions to problems in the area of power and energy, with large impact on society.”

Candan’s paper, “Unregulated Bus Operation of Server-to-Virtual Bus Differential Power Processing for Data Centers,” contributes to the development of series-stacked server power delivery architectures for high-efficiency data centers. “I had a unique opportunity to explain the impact of my work in one-on-one dialogues with the attendees and receive instant feedback,” said Candan.

His approach involves servers connected in a similar fashion to LED arrays, a method which minimizes power conversion. His control algorithm and hardware allow for voltage balancing even when the source voltage is not constant. The unregulated bus operation enables possible integration of batteries to the proposed approach.

This work resulted in 99.4 percent power delivery efficiency, which is a remarkable improvement, as state-of-the art power converters for data centers are around 96-97 percent efficient.

“This work is the first successful hardware demonstration of the series-stacked servers that are connected to a DC bus voltage with decaying voltage, proving the real-life implementation of this innovative approach is possible without sacrificing ultra-high energy efficiency,” said Candan.

Barth’s paper, “Experimental Evaluation of a 1 kW, Single Phase, 3-Level Gallium Nitride Inverter in Extreme Cold Environment,” characterized the impact of low-temperature operation on a single-phase inverter.

Liquefied natural gas (LNG) is being considered as a fuel for small aircraft due to its high energy density, availability, and clean burn. Its storage temperature of -161°C allows it to be used as a heat sink during flight that can cool propulsion electronics to low temperatures and reduce conduction losses. Additionally, using waste heat to warm fuel, prevents the system from being forced to use additional energy.

Barth’s work investigated a 1 kW, GaN-based, 3-level power converter down to -140°C. “It is clear that there is an opportunity for additional gains in efficiency by combining high-performing GaN field-effect transistors (FETs) with passive components optimized for low temperature operation,” said Barth.

Stillwell’s paper, “A 5-level Flying Capacitor Multi-Level Converter with Integrated Auxiliary Power Supply and Start-up,” encompassed the 5-level Flying Capacitor Multi-level (FCML) converter with power supplied to the controller and gate drive circuitry by an integrated auxiliary power converter. His research contribution could lead to smaller and more efficient power converters for electric vehicles, ranging from personal automobiles to off-highway and airborne vehicles. His paper describes a method for extracting auxiliary power from within an existing power converter, thereby reducing the power losses and minimizing size. This results in a smaller auxiliary converter along with a 75 percent decrease in voltage rating requirement.

However, the size reduction does not come without some additional challenges. For example, internal components (flying capacitors) witness an imbalance due to the auxiliary load current. Stillwell’s remedy involved an active balancing method, adjusting the phase and duty cycle of traditional Phase-Shifted Pulse-Width Modulated (PSPWM) control signals. 

“The active balancing requires a single capacitor voltage measurement, no additional components and proportional control. A start-up circuit is proposed to power the auxiliary power supply and soft-start the FCML converter,” said Stillwell. The demonstration of a practical solution to these challenges was well received by both academic researchers and industry participants, making possible industry adaption of this new technology. 

Foulkes, who is also an NSF Graduate Research Fellow, has had the opportunity to explore a steerable cooling mechanism for power electronics involving jumping droplet condensation. His collaborators include Junho Oh and Patrick Birbarah in the Mechanical Science and Engineering (MechSE) department and his co-advisors Pilawa-Podgurski and Prof. Miljkovic (MechSE).

His paper, “Active Hot Spot Cooling of GaN Transistors with Electric Field Enhanced Jumping Droplet Condensation,” addressed the increasing demand for enhanced cooling of power electronics within mobile platforms. The technology impacted has ranged from automobiles to aircrafts. “Conventional, passive thermal management strategies are not sufficient to remove heat from the internal hot spots of compact converters,” said Foulkes. This places limits on how much  the volume of the converter can be reduced. However, his paper on jumping droplet condensation presents a disruptive solution. 

“This cooling approach is fundamentally different from any prior thermal management strategy for power electronics since the jumping droplets can be actively steered using electric fields toward hot spots inside of the converter, which enables further reduction in total converter volume,” said Foulkes.  Foulkes identified  UIUC’s collaborative, interdisciplinary culture as the cornerstone of the project’s  success. “Our  team has benefited a great deal from collaborating with researchers at Sandia National Laboratories, such as Dr. Jason Neely, and the POETS NSF ERC community,” said Foulkes.