Biography

Dr. Liu (Fellow of IEEE, 2013, Fellow of CAE, 2018) received his Bachelor and Master degree from the Department of Electrical Engineering from Zhejiang University, China, in 1984 and 1987, and Ph.D. degree from the Department of Electrical and Computer Engineering, Queen’s University, Kingston, ON, Canada, in 1994.

He was a Technical Advisor with the Advanced Power System Division, Nortel Networks, in Ottawa, Canada from 1994 to 1999. Since 1999, he has been with Queen’s University, where he is currently a Professor with the Department of Electrical and Computer Engineering. His current research interests are listed as following:

• How to best utilize the benefits of GaN and SiC devices to explore the full advantage of these devices so that small size and high-efficiency power converters can be designed.
• Extremely high efficiency (>99%)  and extremely high power density (>2000W / inch³) DC-DC Bus converter for data center application
• High power density LLC resonant converter for different power levels (from 65W to 4000W) for different applications (such as PD adapter, data center power, EV on-board DC-DC converter)
• Digital Control technologies for accurate current sharing of multi-phase interleaved LLC resonant converter to achieve high efficiency and high power density simultaneously.
• Digital control technology for fast dynamic response of DC-DC converters.
• High efficiency, high power density AC to DC power converter for USB C Power Delivery (PD) application using GaN switches.
• On-board EV DC-DC converter with high efficiency and high power density using GaN switches.
• High power, high efficiency, and high power density single-phase AC–DC rectifier suitable for On-Board Chargers using GaN switches.

He has authored around 300 technical papers in the IEEE Transactions and conferences (View Dr. Liu’s Publications), and holds 68 U.S. patents (View Dr. Liu’s Patents). He has written a book on “High Frequency MOSFET Gate Drivers: Technologies and Applications”, published by IET. He is also a Principal Contributor for two IEEE standards. He received “Modeling and Control Achievement Award” from IEEE Power Electronics Society in 2017. He received Premier’s Research Excellence Award in 2000 in Ontario, Canada. He also received the Award of Excellence in Technology in Nortel in 1997.

Dr. Liu was the Vice President of Technical Operations of the IEEE Power Electronics Society (PELS, from 2017 to 2020). He was the general chair of ECCE 2019 held in Baltimore, USA in 2019, http://www.ieee-ecce.org/2019/). Dr. Liu serves as an Editor of the IEEE Journal of Emerging and Selected Topics of Power Electronics (IEEE JESTPE) since 2013. His other major service to IEEE is listed below: a Guest Editor-in-Chief for the special issue of Power Supply on Chip of IEEE Transactions on Power Electronics from 2011 to 2013; a Guest Editor for special issues of JESTPE: Miniaturization of Power Electronics Systems in 2014 and Green Power Supplies in 2016; as Co-General Chair of ECCE 2015 held in Montreal, Canada, in September 2015; the chair of PELS Technical Committee (TC1) on Control and Modeling Core Technologies from 2013 to 2016; chair of PELS Technical Committee (TC2) on Power Conversion Systems and Components from 2009 to 2012.

Ultra-High Efficiency Multilevel, Inductor-Less Converters

With the demand for cloud computing and Internet-of-Things connected devices increasing exponentially there is a huge demand for extremely high efficiency DC/DC converters (>99% efficiency) to be utilized in next-generation data center and server power supply designs.

Multilevel converters work by reducing the voltage stress of the semiconductor devices, allowing for substantial performance improvements to be achieved over traditional topologies. While the performance of semiconductor devices continuously improves year after year, magnetic components have not seen the same increase in performance. Inductor-less topologies, such as Switched Capacitor converters and similar derivative topologies that reduce the reliance on magnetic components have thus seen a resurgence of research interest due to the extremely high efficiency and power density that can be achieved by these converters.

A series of new DC – DC converters with Zero-Inductor-Voltage (ZIV) have been proposed, analyzed and tested. Two key benefits of these topologies are (1) the voltage across the inductor does not depend on the input and output voltage, instead, it depends only on the ripple voltage of the capacitors; (2) the operation is not sensitive to the power train component values The tolerance of the capacitors does not impact the circuit operation. Therefore, with moderate switching frequency of 60kHz, we have achieved 99%+ efficiency for 48V to 12V application with inductance value of only 220nH (0.22uH) and power densities in excess of 2kW/in³. This makes them a very attractive solution for size-critical applications where extremely high efficiency is needed.

USB Type C PD (Power Delivery) Adapter

USB - PD is a battery fast-charging protocol that is proposed by over 10 leading companies and organizations in the consumer electronics field. The protocol works with USB-C ports and cable, and compatible source (power supplies) and sink (cellphone, laptop, gaming etc.) devices. The source device and sink device will handshake and negotiate the charging profile from 5V to 20V voltage and up to 3.25A of charging current. It is an all-in-one charging solution to consumer electronics.

Our research focuses on the improving the performance and miniaturizing the footprint of the source device, i.e. the PD adapter. We take advantage of the soft-switching feature of resonant converters and push the switching frequency beyond Megahertz with Gallium Nitride devices. With the new and patent pending digital LLC technology, our 65W PD adapter achieves best-in-class efficiency and roughly half the size as compared with the smallest GaN switch based PD adapter in the market. With digital LLC technology, very wide output voltage variation range (such as 5V to 20V) can be achieved while maintaining all the benefits of the LLC converter.

Interleaved LLC DC-DC Converter for EV Battery Charger

Since increasing demand of environmentally friendly energy, the research and development of Electric Vehicles (EVs) technologies are becoming more significant. For an EV power system, a DC-DC converter is needed to convert the power from high voltage battery (250V to 430V) to low voltage battery (9V to 16V) for the lighting, audio, air conditioning and other functions. The required power can be as high as several kilowatts with the output current at hundreds of amperes, which are critical obstacles for efficiency improvement and size reduction.

The LLC converter due to its soft-switching characteristics is widely used in datacenter and server applications at kilowatts level and has been proved to be a compact and efficient approach. It is well-known that multi-phase interleaving technology can increase the current capacity and reduce the voltage ripple on the output capacitor filter. However, component tolerance among resonant tanks causes severe load sharing problem, leading to performance degrading. A patented Switch-Controlled Capacitor (SCC) technology can be added into resonant tank to compensate component tolerances and achieve accurate load sharing while maintaining all the benefits of LLC converter. With the mentioned technology and using GaN HEMETs, a power density of 3kW/Litre and peak efficiency of around 97% can be achieved for a DC-DC converter in EV application.

Single-Stage LLC AC-DC Converter with PFC

Power Factor Correction (PFC) standards have been proposed to limit the current harmonics of AC-DC converters that is usually implemented by a two-stage structure. Various PFC circuits have been proposed to improve the performance; however, the topologies still need to be further modified.

LLC converter is a well-known resonant converter widely used in DC-DC power conversion and it is rarely used in AC-DC application in previous research studies. High voltage gain at light-load feature enables LLC converter to operate with fluctuating AC input voltage and high-speed digital control provides proper control scheme to modulate the switches. Using wide band gap semiconductors such as GaN and SiC devices enable high switching frequency with improved efficiency performance. By using a single-stage LLC as a PFC, the number of switches and magnetic components is reduced, so high efficiency and high-power density can be achieved.

SCC-LCLC Resonant Converter for Wide DC Input Voltage

A wide DC input voltage operation is required in some applications such as in datacenter to provide power during hold-up time. Resonant converters are widely used in datacenters as a front-end converter to convert 400 V dc bus to 12 V for server motherboard low voltage bus due to their outstanding features such as Zero Voltage Switching (ZVS) and Zero Current Switching (ZCS) for power switches. The 12 V dc bus then is connected to point of load converters to supply CPU, memory and other chips. LCLC resonant converter demonstrates an improved hold-up performance compared with an equivalent LLC converter. In order to go beyond the power limit of resonant converters in datacenter application, which is caused by high current level at low voltage side bus (i.e. 12 V), multi-phasing and interleaving techniques should be implemented to distribute the current stress and provide phase-shedding to improve efficiency profile.

In out research, a patented Switch-Controlled Capacitor (SCC) with either half-wave or full-wave operation can be implemented in the resonant tank to compensate the resonant capacitance value and tune the voltage gain to ignore any imbalance caused by component tolerances. Moreover, we take advantage of GaN HEMETs with TO-220 package that ease the thermal management and enable high power processing. The SCC-LCLC resonant converter can operate with 250 V to 400 V input voltage and a fixed 12 V output voltage with accurate current sharing performance among all phases and a flat efficiency curve with more than 96% over a wide load range (from 25% to 100%).

For more information on Dr. Liu's research, visit the Power Group (PG) web page.

Professor Yan-Fei Liu publications

Teaching

• ELEC 431
• ELEC 490/ELEC 498
• ELEC 832