F.C.Y. Lee

• Virginia Tech

A solid-state transformer (SST) offers direct medium voltage to low voltage conversion, resulting in higher system efficiency and a smaller footprint compared to traditional transformers. Among the popular SST architectures, a two-stage system comprising a cascade H-bridge (CHB) as the first stage and a resonant converter as the second stage has ga...

The LLC converter is the most efficient topology in server and telecom applications. Furthermore, three interleaved LLC converters have been shown to yield efficiency advantages at power levels of several kilowatts. The magnetic components of a multiphase LLC , on the other hand, are complex, laborious, and difficult to build cost-effectively...

To date, $LLC$ resonant converters have been deployed in many applications for improved efficiency, density, and reliability. With the introduction of wide-bandgap devices coupled with the soft switching feature, the switching frequency can be extended beyond megahertz. With the significant increase in operating frequency, complicated magnetic co...

Recently, resonant DC-DC converters have been deployed in many fields such as information and communications technology (ICT) and with electric vehicles (EVs) due to their improved efficiency, density, and reliability. With the introduction of wide-bandgap (WBG) devices coupled with the soft-switching feature, switching frequencies can be extended...

Power consumption of data centers is increasing rapidly over recent years. Recently, 48V power architecture has attracted more interest in data centers as it offers more efficient architecture. This paper focuses on the implementation of the DC-DC stage of the 3 kW power supply unit. Full-bridge regulated LLC converter is designed with a matrix of...

To date, LLC Resonant converters, are deployed in many applications for improved efficiency, density, and reliability. With the introduction of WBG devices coupled with the soft switching feature, the switching frequency can be extended beyond MHz. With the significant increase in operating frequency, complicated magnetic components can be broken d...

In this paper, balance technique is applied into two-channel interleaved three-phase acdc converters with critical conduction mode based soft switching modulation. By adding additional inductors and coupling them with the original ones, the realization of the balance becomes possible with proper design; thus reducing the common-mode (CM) EMI noise....

Abstract—Increased attention is being paid to the solid-statetransformer (SST) based charging station which requires a high efficiency high power density DC/DC stage. This framework would provide universal charging for all electric vehicles (EVs) no matter their battery voltage is 400 V or 800 V. In this paper, a two stage DC/DC converter consistin...

Abstract— The bi-directional five-element CLLC resonant converter is attracting great attention for battery charger applications. The three-phase interleaved CLLC converter can deliver more power with lower conduction losses as compared to the single-phase CLLC. Resonant converters, including the threephase interleaved CLLC, suffer from high levels...

Recently, omnidirectional wireless power transfer (WPT) systems have been studied intensely, due to their improved flexibility as compared to their planar counterparts. The LCCL-LC resonant converter topology is selected due to its current source characteristics in this paper. The system frequency is pushed to megahertz (MHz) to increase the spatia...

Resonant converters, especially LLC converters, are deployed in many applications thanks to their reliability and high efficiency. With the introduction of WBG devices and the soft switching feature of the LLC converter, the switching frequency of the LLC can be pushed to the order of MHz, to shrink the converter size and increase the power density...

In this paper, a high-efficiency and power-density unregulated LLC converter (DCX) is proposed for a first-stage converter in a two-stage 48V voltage regulator module (VRM) for data center applications. The intermediate bus voltage in the two-stage architecture is evaluated by designing two LLC-DCX converters with fixed transformation ratios of (4:...

48 V bus converters are widely used in telecommunications and network power architectures. A single-stage regulated LLC bus converter can achieve high efficiency and high power density. This paper addresses the control challenges of a regulated LLC converter for 48 V bus converter applications. Simplified Optimal Trajectory Control (SOTC) is used t...

At the present time, current-mode constant on-time (CMCOT) control is widely used in voltage regulators (VRs) for higher light-load efficiency, higher bandwidth, and a simpler compensation network. In today’s VRs, a high load step with a very high slew rate is a very common requirement in microprocessor or memory applications. One issue of this rip...

In this paper, an LLC converter utilizing gallium nitride (GaN) transistors is proposed for a 48V regulated and isolated bus converter. Compared to pulse width modulation (PWM) based topologies, the soft switching capability of an LLC allows operation at very high frequencies. In addition, the magnetic components size is reduced without sacrificing...

Intermediate bus architecture employing 48-V bus converters is widely used in power supply applications. With the rapid increase of demanded power by these loads, higher efficiency and power density are driving better performance power management solutions. In this article, a Gallium Nitride (GaN) based design of a two-stage solution is proposed. T...

Recently, omnidirectional wireless power transfer systems have been studied intensely due to their improved flexibility when compared with their planar counterparts. In this paper, a novel wireless charging bowl with multiple transmitter coils is proposed to power portable devices. The bowl-shaped transmitter coil is optimized to provide sufficient...

The two-stage 48 V voltage regulator module (VRM) with an unregulated first stage DC/DC converter providing intermediate bus voltage, followed by a multi-phase buck converter, has proven to be a scalable and efficient approach for 48 V rack architecture in data centers. In this work, eGaN® based first stage unregulated LLC converters with integrate...

48 V bus converter is widely used in telecom and networks power architecture. This paper presents LLC converter as single stage solution for isolated and regulated bus converter, and addresses the control challenges for this converter. In this paper, Simplified Optimal Trajectory Control (SOTC) is used to control 1 MHz LLC converter with input volt...

The LLC converter is deemed the most widely used topology as DC/DC converter in server and telecom applications. To increase the output power and reduce the input and output current ripples, three-phase interleaved LLC converter is becoming more and more popular. It has been demonstrated that three interleaved LLC converter can achieve further effi...

This paper develops a 200-W wide-input-range (64-160-to-24-V) rail grade dc-dc converter based on gallium nitride devices. A two-stage configuration is proposed. The first regulated stage is a two-phase interleaved buck converter (>400 kHz), and the second unregulated stage is an LLC (2-MHz) dc transformer. In order to achieve high frequency and hi...

This paper proposes a novel two-stage topology for a 6.6-kW on-board charger. The first stage, employing an interleaved bridgeless totem-pole ac/dc in critical conduction mode to realize zero-voltage switching, is operated at over 300 kHz. A bidirectional CLLC resonant converter operating at 500 kHz is chosen for the second stage. A variable dc-lin...

The light-load efficiency has drawn more and more attention for the LLC resonant converters in information technology industry. Burst mode is an effective and popular method to improve the light-load efficiency. The conventional burst mode control has limited efficiency improvement on LLC converter due to the dynamics of the resonant tank. The simp...

The conventional slope compensations for current mode controls do not take all the parameters' changes into account, so the variations on small signal transfer functions limit the voltage loop bandwidth. In order to optimize the dynamic performance of current-mode control, this paper proposes a universal auto-tuning technique for external ramp slop...

Isolated high output current DC/DC converters are critical for power supplies in information technology industries. LLC converters with a matrix transformer are suitable for these applications due to its high efficiency and high power density. By pushing switching frequency up to MHz with GaN devices, planar matrix transformers with PCB windings ar...

Synchronous rectifiers are important for high-output-current LLC converters in order to reduce the secondary conduction loss. Conventional SR driving schemes are not suitable for high-frequency LLC converters: the analog driving schemes will lose a large portion of duty cycle due to SR parasitic inductance; the digital driving schemes will require...

A wide-band-gap based bidirectional on-board charger (OBC) has been proposed with a variable DC-link voltage, which tracks the battery voltage fluctuation. Although this approach is deemed most efficient for the resonant DC/DC stage, it posts significant challenges for the rectifier/inverter stage which operates in the critical mode (CRM) to realiz...

Efficient power delivery architecture is gaining more attention in the design of future generations of data centers in order to minimize the ever increasing trend of power consumption. Silicon based power delivery architectures and design have already reached a level of maturity in terms of efficiency and density. Recent studies indicate that 48V v...

These days, constant on-time current mode (COTCM) control scheme is widely used in the voltage regulator (VR) controllers because it has a higher light-load efficiency and a higher BW design capability while maintaining a simpler compensation requirement. One issue plaguing the COTCM control is its slow transient response which is caused by its fix...