This research explores two issues when implementing drain-source voltage sensed synchronous rectification (SR) on LLC DC-Transformers (DCXs). Firstly, a current resonance issue caused by the SR controller, and secondly a early turn-off issue from parasitics present in the drain-source sensing path. Two novel methods are proposed to solve the early turn-off issue, and an FPGA based solution is built to validate and fix the resonance issue. Simulations are run to quantify the amount of rectifier power savings possible with the proposed solutions. / Master of Science / This research explores issues and improvements in synchronous rectifiers used in resonant based power conversion circuits. The two issues explored hurt rectifier efficiency, and thus total power conversion circuit efficiency. Implementation issues are identified, simulated, and new solutions are proposed. Simulations are run to quantify the amount of power savings is possible.

This thesis presents a novel configuration of battery power by the outputs with serial connection of battery power modules (BPMs) for high voltage level loads. As compared to the conventional application of series-connected battery bank, this configuration operates the BPMs individually, and thus has the advantages of flexible control, convenient maintenance, and easily favorable battery management. The associated converter is equipped to a single battery pack, so that has lower component stresses leading to a higher circuit efficiency. The operation and the design of a lead-acid battery power with series boost-typed BPMs are illustrated. The operation and the design of the converter are illustrated. The control of the power converters is accomplished by a complex programmable logic device (CPLD). To improve the converter efficiency, the technique of synchronous rectification is introduced. For the BPMs designed for discontinuous conduction mode (DCM) operation, charge equalization can be automatically achieved under the same duty-ratio, but is adequate only for batteries with a small difference. On the other hand, charge equalization for the BPMs with continuous conduction mode (CCM) can easily be accomplished by regulating the duty-ratios of the converters.

Due to the loose coupling nature and separated primary/secondary side, achieving tight load regulation or implementing closed-loop control of output voltage/current is nontrivial in a wireless power transfer (WPT) system. Previously presented methods for regulating or controlling the output of a WPT system include incorporating either post-regulator stage, wireless communication from secondary to primary side, primary side sensing and modulation scheme, or dual active bridge type of topology. However, all existing methods have limitations and disadvantages in terms of increased size/cost, control complexity, or reliability in electrically noisy environments. This dissertation proposes a single switch control and regulation mechanism based on the secondary side of the WPT system. Specifically, the duty cycle of the secondary side synchronous rectifier (SR) switch is modulated to control the output voltage or current. By modulating the SR duty cycle, output of the WPT system can be controlled without requiring additional regulator stages/power devices, a primary side sensing mechanism, or secondary to primary communication. The proposed control method lowers cost and simplifies the design of WPT systems while improving reliability in noisy environments. The proposed control and modulation mechanism maintains zero voltage switching of all power semiconductor switches so efficiency of the WPT system would not be compromised by implementing the proposed control scheme. The proposed secondary side SR based control method can be applied to dc-dc WPT systems to control output voltage or current, or it could be used in a dc-ac WPT system to generate and regulate ac output if combined with an unfolding stage. When used in dc-ac WPT systems, the bulky output filter stage usually required in conventional dc-ac inverters is eliminated. The proposed control scheme is evaluated with computer simulation as well as hardware implementation and testing. / Doctor of Philosophy / Wireless power transfer (WPT) is an emerging technology that supplies electric power to loads without using wires or electrical contacts. WPT technology has many promising uses in consumer, industrial, transportation, biomedical, and other applications. However, unlike controlling the output voltage of a conventional power supply or power converter, controlling the output of a WPT system is not a simple task due to the physical separation between the transmitting and receiving sides. State-of-the-art methods for controlling the output of a WPT system include adding another power regulator stage to regulate output, incorporating secondary side (power receiver) to primary side (power transmitter) communication so that output information can be passed back to the primary side where that information is used to monitor and regulate output. In some systems, output information may also be estimated indirectly from primary side voltage/current information. However, all these methods have significant disadvantages. Adding another power converter stage increases cost and efficiency loss of the WPT system. Incorporating secondary to primary communication for output control is detrimental to the reliability of the PWT system because communication may be impacted by external noise. The reliability of primary side sensing and regulation is also severely impacted by component parameter variations in the WPT system. This dissertation proposes a new mechanism that controls output of a WPT system at the receiver or secondary side without needing another power conversion stage, communication or any cooperation from primary side. The proposed control mechanism controls the turn on duration of the synchronous rectifier (SR) switch at the receiver side to modulate output voltage or current. Since SR technology is already prevalently used in power electronics systems, including WPT systems, to efficiently convert high frequency ac to dc before delivering power to the load, implementing the proposed control mechanism does not increase complexity or cost of the WPT system. The proposed control mechanism is useful in both dc-dc and dc-ac WPT systems. In a dc-dc WPT system, the proposed mechanism can control or regulate output voltage or current independently from the primary side, while in a dc-ac WPT system the proposed mechanism can generate and regulate ac output. If used in a dc-ac WPT system an unfolding stage needs to be added, but the bulky output filter stage required in conventional pulse width modulation (PWM) dc-ac inverters for suppressing switching ripple is not needed. The proposed mechanism is verified with computer simulation as well as hardware prototyping in this dissertation.

As information technology advances, so does the demand for power management of telecom and computing equipment. High efficiency and high power density are still the key technology drivers for power management for these applications. In order to save energy, in 2005, the U.S. Environmental Protection Agency (EPA) announced the first draft of its proposed revision to its ENERGY STAR specification for computers. The draft specification separately addresses efficiency requirements for laptop, desktop, workstation and server computers. The draft specification also proposes a minimum power supply efficiency of 80% for PCs and 75% to 83% for desktop derived servers, depending on loading condition and server type. Furthermore, recently some industry companies came out with a much higher efficiency target for the whole AC/DC front-end converter over a wide load range. Distributed power systems are widely adopted in the telecom and computing applications for the reason of high performance and high reliability. As one of the key building blocks in distributed power systems, DC/DC converters in the front-end converter are also under the pressure of increasing efficiency and power density. Due to the hold-up time requirement, PWM DC/DC converters cannot achieve high efficiency for well known reasons when they are designed for wide input voltage range. As a promising topology for this application, LLC resonant converters can achieve both high efficiency and wide input voltage range capability because of its voltage gain characteristics and small switching loss. However, the efficiency of LLC resonant converter with diode rectifier still cannot meet the recent efficiency target from industry. In order to further improve efficiency of LLC resonant converters, synchronous rectification must be used. The complete solution of synchronous rectification of LLC resonant converters is discussed in this thesis. The driving of the synchronous rectifier can be realized by sensing the voltage Vds of the SR. The turn-on of the SR can be triggered by the body-diode conduction of the SR. With the Vds compensation network, the precise voltage drop on Rds_on can be achieved, thus the SR can be turned off at the right time. Moreover, efficiency optimization at normal operation over wide load range is discussed. It is revealed that power loss at normal operation is solely determined by the magnetizing inductance while the magnetizing inductor is designed according to dead-time td selection. The mathematic equations for the relationship between power loss and dead-time are developed. For the first time, the relationship between power loss and dead-time is used as a tool for efficiency optimization. With this tool, the efficiency optimization of the LLC resonant converter can be made according to efficiency requirement over a wide load range. With the expectation to achieve high efficiency at ultra-light load, the green mode operation of LLC resonant converters is addressed. The rationale of the issue with the conventional control algorithm is revealed and a preliminary solution is proposed. / Master of Science

The advent of wide band-gap semiconductors in power electronics has led to the scope of efficient power conversion being pushed further than ever before. This development has allowed for systems to operate at higher and higher voltages than previously achieved. One area of consideration during this high voltage transition is the synchronous rectifier, which is traditionally designed as an afterthought. Prior research in synchronous rectifiers have been limited to low voltage, high current converters. There is practically no research in high voltage synchronous rectification. Therefore, this dissertation focuses on discovering the unknown nuances behind high voltage synchronous rectifier design, and ultimately developing a practical, scalable solution. There are three main issues that must be addressed when designing a high voltage synchronous rectifier: (1) high voltage sensing; (2) light load effects; (3) accuracy. The first hurdle to designing a high voltage SR system is the high voltage itself. Traditional methods of synchronous rectification (SR) attempt to directly sense voltage or current, which is not possible with high voltage. Therefore, a solution must be designed to limit the voltage seen by the sensing mechanism without sacrificing accuracy. In this dissertation, a novel blocking solution is proposed, analyzed, and tested to over 1-kV. The solution is practical enough to be implemented on practically any commercial drain-source SR controller. The second hurdle is the light load effect of the SR system on the converter. A large amount of high voltage systems utilize a LLC-based DC transformers (DCX) to provide an efficient means of energy conversion. The LLC-DCX's attractive attributes of soft-switching and high efficiency allure many architects to combine it with an SR system. However, direct implementation of SR on a LLC-DCX will result in a variety of light load oscillation issues, since the rectifier circuitry can excite the resonant tank through a false load transient phenomena. A universal limiting solution is proposed and analyzed, and is validated with a commercial SR controller. The final hurdle is in optimizing the SR system itself. There is an inherent flaw with drain-source sensing, namely parasitic inductance in the drain-source sense loop. This parasitic inductance causes an error in the sensed voltage, resulting in early SR turn-off and increased losses through the parallel diode. The parasitic will always be present in the circuit, and current solutions are too complex to be implemented. Two solutions are proposed depending on the rectifier architecture: (1) multilevel gate driving for single switch rectifiers; (2) sequential parallel switching for parallel switch rectifiers. In summary, this dissertation focuses on developing a practical and reliable high voltage SR solution for LLC-DCX converters. Three main issues are addressed: (1) high voltage sensing; (2) light load effects; (3) accuracy. Novel solutions are proposed for all three issues, and validated with commercial controllers. / Doctor of Philosophy / High voltage power electronics are becoming increasing popular in the electronics industry with the help of wide band-gap semiconductors. While high voltage power electronics research is prevalent, a key component of high voltage power converters, the synchronous rectifier, remains unexplored. Conventional synchronous rectifiers are implemented on high current circuits where diode losses are high. However, high voltage power electronics operate at much lower current levels, necessitating changes in current synchronous rectifier methods. This research aims to identify and tackle issues that will be faced by both systems and IC designers when attempting to implement high voltage synchronous rectifiers on LLC-DCXs. While development takes planes on a LLC-DCX, the research is applicable to most resonant converters and applications utilizing drain-source synchronous rectifier technology. This dissertation focuses primarily on three areas of synchronous rectifier developments: (1) high voltage compatibility; (2) light load effects; (3) accuracy. The first issue opens the gate to high voltage synchronous rectifier research, by allowing high voltage sensing. The second issue explores issues that high voltage synchronous rectifiers can inadvertently influence on the LLC-DCX itself - a light load oscillation issue. The third issue explores novel methods of improving the sensing accuracy to further reduce losses for a single and parallel switch rectifier. In each of these areas, the underlying problem is root-caused, analyzed, and a solution proposed. The overarching goal of this dissertation is to develop a practical, low-cost, universal synchronous rectifier system that can be scaled for commercial use.