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.

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.