IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 7, JULY 2014 3473 A Bidirectional-Switch-Based Wide-Input Range High-Efficiency Isolated Resonant Converter for Photovoltaic Applications Thomas LaBella, Student Member, IEEE, Wensong Yu, Member, IEEE, Jih-Sheng (Jason) Lai, Fellow, IEEE, Matthew Senesky, Member, IEEE, and David Anderson, Member, IEEE Abstract—Modular photovoltaic (PV) power conditioning sys- tems (PCSs) require a high-efficiency dc–dc converter stage capable of regulation over a wide input voltage range for maximum power point tracking. In order to mitigate ground leakage currents and to be able to use a high-efficiency, nonisolated grid-tied inverter, it is also desirable for this microconverter to provide galvanic isolation between the PV module and the inverter. This paper presents a novel, isolated topology that is able to meet the high efficiency over a wide input voltage range requirement. This topology yields high efficiency through low circulating currents, zero-voltage switching (ZVS) and low-current switching of the primary side devices, ZCS of the output diodes, and direct power transfer to the load for the majority of switching cycle. This topology is also able to provide voltage regulation through basic fixed-frequency pulsewidth mod- ulated (PWM) control. These features are able to be achieved with the simple addition of a secondary-side bidirectional ac switch to the isolated series resonant converter. Detailed analysis of the op- eration of this converter is discussed along with a detailed design procedure. Experimental results of a 300-W prototype are given. The prototype reached a peak power stage efficiency of 98.3% and a California Energy Commission (CEC) weighted power stage efficiency of 98.0% at the nominal input voltage. Index Terms—High-frequency ac switch, hybrid resonant PWM converter, isolated dc–dc microconverter, LLC converter, photo- voltaic (PV), resonant converter. I. INTRODUCTION P HOTOVOLTAIC (PV) energy has been the fastest growing renewal energy source in recent years and is expected to continue this trend throughout the near future [1]–[3]. In or- der for the solar energy market to continue this growth and to become more competitive with traditional energy sources and other forms of renewables, both PV panels and PV power con- ditioning systems (PCSs) need to be designed to perform more efficiently while becoming lower in cost. The subject matter of Manuscript received May 27, 2013; revised July 29, 2013; accepted Septem- ber 2, 2013. Date of current version February 18, 2014. Recommended for publication by Associate Editor D. Xu. T. LaBella, W. Yu, and J.-S. (Jason) Lai are with the Future Energy Electron- ics Center, Virginia Tech, Blacksburg, VA 24061 USA (e-mail: tlabella@vt.edu; wensong@vt.edu; laijs@vt.edu). M. Senesky is with Amber Kinetics, Fremont, CA 94538 USA (e-mail: senesky@gmail.com). D. Anderson is with Texas Instruments Kilby Labs, Santa Clara, CA 95051 USA (e-mail: David.I.Anderson@ti.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPEL.2013.2282258 this paper focuses on improvement of the performance of the electronics involved in the PCS. There are three main types of PV PCS configurations: central- ized, string, and modular [4]–[7]. The first two configurations are comprised of several series- or parallel-connected PV modules that are fed into a large centralized inverter or several smaller string inverters. These configurations suffer from the inability to have each individual PV module operating at its own max- imum power point, but are relatively cheap to implement and therefore are more common in todays’ PV installations [6]. PV modules exhibit nonlinear power output characteristics that may differ for each module depending on the PV cell material and environmental conditions such as temperature, solar irradiance, dirt and debris collected on the panels, etc. [8]–[11]. Because the optimal operating point may differ for each module at any given time, it is desirable for each panel to have its own power optimizing electronics implementing a maximum power point tracking (MPPT) algorithm. The third type of PV PCS config- uration is comprised of several PV modules, each with its own power converter, or module integrated converter (MIC), which tracks the MPP of each module individually and either outputs ac directly to the grid (microinverter) or outputs dc (microcon- verter), which can be connected in series or parallel with other microconverters to feed a centralized or string inverter. These modular configurations are very attractive because of more ef- ficient energy harvesting and simple scalability [7]. Both microconverter and microinverter modular PCS require a front end dc–dc converter that is responsible for tracking the MPP of each panel. These converters require the ability to op- erate over a wide input voltage range in order to accurately track the MPP of different PV panels under different operating conditions. It is also beneficial for galvanic isolation to be imple- mented in the dc–dc stage rather than the inverter stage to reduce the size of the isolation transformer and to increase the overall system efficiency [12], [13]. Additionally, in the case of front end dc–dc converters for microinverters and parallel-connected microconverters, it is necessary to provide a high boost ratio to boost the low voltage dc PV panel output to a high voltage dc bus capable of feeding a grid-tied or standalone inverter. Be- cause of the wide input range, isolation, and high boost ratio requirements, it is difficult to develop a simple converter topol- ogy that is capable of reaching high efficiency over the entire input voltage and output power range. There have been many approaches to meeting the require- ments of isolated dc–dc microconverters in literature. The 0885-8993 © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. 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