Advances in Resonant Power Conversion for Offline Converter Applications

Abstract

Switch-mode power supplies (SMPS) have long served as the driving force that boosted technological innovation and human advancement. As the technologies behind different industrial electronics keep evolving, with the aim for improved performance, reliability and portability, additional requirements are continuously placed on power supply technologies to process more power in less volume. Hard-switched pulse-width-modulated (PWM) converters have been the primary candidate for the different power supply conversion stages. They can provide high power quality and efficiency with simple control. However, they typically operate at low frequencies, in the range of few hundred kHz, in order to limit the switching losses, which results in large sizes for the passive components needed to store and process the energy transferred to the load every switching cycle. On the other hand, high-frequency designs (1 MHz and above) have less efficiency and may incorporate a heatsink for thermal management, which counteracts the gain in power density. Accordingly, soft-switching resonant converters have been receiving increased attention in recent years. Thanks to their zero-voltage-switching (ZVS) and/or zero-current-switching (ZCS) characteristics, they incur substantially lower switching losses compared to their PWM counterparts. That makes them a primary candidate for achieving high efficiencies at high frequencies, resulting in reduced sizes for the passive components and, therefore, higher power densities. In addition, high-frequency operation offers higher loop-gain bandwidths and faster transient responses. This has led to the investigation of their adoption into different applications conventionally dominated by PWM converters, one of which is the grid-powered offline converters. This thesis presents advances in the design and implementation of resonant converters for offline converter applications, and introduces several topologies and techniques that allow for the design of high-frequency designs with high efficiency. Among the main research contributions of this thesis are the following: With respect to the converter power stage, the thesis investigates resonant front-end AC-DC converters with inherent power factor correction (PFC) capability to eliminate the current regulation loop and simplify control. A 1-MHz 50-W charge-pump-based class-DE seriesresonant converter is presented, achieving an inherent power factor of 0.99, a total harmonic distortion (THD) of 8.6 % and an efficiency of up to 88 %. Several solutions are presented for the front-end DC-DC stage. A 1-MHz 65-W LLC converter is designed and implemented, achieving up to 96% efficiency with inherent load regulation capability. The power stage architecture is studied, and a converter structure integrating the two stages in a 1.5-stage structure is proposed, where a prototype design for LED driver applications achieves an output power of 42 W with power density of 2 W/cm3 , power factor of 0.99, THD of 6 %, and a peak efficiency of 90 %. With respect to high-frequency modeling and control challenges, this thesis introduces an improved linear model for high-frequency resonant converters. Compared to prior art, the model incorporates the converter parasitics and reduces the DC-gain error by more than 7 dB, with more accurate dynamics. An analysis of the different switching loss modes in class-DE resonant converters with frequency control and fixed dead time is conducted, and a feasible operating region for a given upper-loss bound is defined. Finally, a study of the figures of merit of the best-in-class switching devices and magnetic materials for high-frequency offline converters is presented, with an investigation of GaN devices reverse conduction characteristics and their corresponding losses in the different implementations. Through the application of the research described in this work, high-power density resonant converters with frequencies in the range of 1-2 MHz can be implemented with robust control and high power factor and efficiency.