**1. Introduction**

Nowadays, with climate change around the world being evident, electrification is considered as a viable solution for the energy transition [1]. Therefore, power electronic converters face numerous new applications [2]. Among di fferent emerging applications, there is a demand for high-performance DC–DC converters suitable for the integration of low-voltage energy sources and battery energy storage in DC microgrids [3]. In some cases, like photovoltaic (PV) module-level power electronic applications, both high-voltage step-up and the wide input voltage range regulation capability are required to interface individual PV modules that can supply their maximum power at very di fferent voltages due to shading e ffects [4]. Therefore, the associated DC–DC interface converter has to regulate the input voltage in a wide range while providing high e fficiency to draw the maximum available power from a PV module.

Usually, high-voltage step-up applications require galvanic isolation as a high-frequency transformer to step up voltage e fficiently. Several DC–DC converters have been proposed to solve the voltage variation [5–7]. These topologies vary in their structure, complexity, and other aspects. The isolated buck-boost converters were justified as a suitable solution for high step-up wide-range applications. Usually, they have active switches at both sides of the converter to implement voltage buck and boost functionalities on di fferent converter sides. Among these topologies, the series resonant converters (SRCs) have demonstrated high performance in target PV applications. They provide soft-switching of semiconductor components and good utilization of the isolation transformer [8,9]. The SRC topology is similar to the LLC converter topology that is investigated in many industrial applications [10,11]. However, the ratio between the magnetizing and the resonant inductances is several times higher in the SRC compared to the LLC converter. In general, the resonant converter applies the frequency modulation to control the DC voltage gain. However, this study targets low-power compact SRCs that use a small (low-cost) resonant inductor in the resonant tank. Even though such implementation results in low values of the quality factor, their DC voltage gain can be controlled using the pulse width modulation (PWM), which simplifies the converter design.

In the galvanically isolated buck-boost SRCs, the input voltage buck functionality is usually implemented by PWM [12] or phase-shifted modulation (PSM) [13] of the front-end inverter. These modulation methods have been already verified in numerous studies that date back as far as 1988 [12]. From the recent reports, it could be concluded that the highest e fficiency is achieved for the input voltage buck operation by using PSM and hybrid PSM methods [14]. Currently, much attention is given to the input voltage boost implementation in the SRC [8]. The implementation of a boost rectifier usually achieves this [15]. As this study targets high-voltage step-up applications, the boost rectifiers are based on the voltage-doubler rectifier (VDR) to minimize the transformer turns ratio. Typical boost VDR is based on replacing diodes with the metal oxide semiconductor field-e ffect transistors (MOSFETs) and their control with short pulses [16] or double-pulse modulation [17]. Power losses in the boost VDR could be reduced if only one diode is replaced with a MOSFET, which, however, results in higher peak current of the resonant inductor and thus can compromise its size [9]. On the other hand, the implementation of a four-quadrant bidirectional switch in parallel to the transformer secondary winding allows for a significant reduction of the switch voltage stress and thus switching losses. At the same time, it keeps the positive and negative magnitudes of the resonant current balanced. Due to these advantages, topology in [18] is considered in this study. Comprehensive analysis shows that this topology cannot operate in a wide range of voltages and power. Therefore, a new converter is proposed to extend the input voltage regulation range by rearranging positions of the resonant tank elements. The proposed converter topology is based on the topology in [18], where the position of the resonance capacitor is moved to be placed between the bidirectional switch and the transformer secondary winding. The proposed converter is feasible in a wide range of the resonant inductance values without affecting the input voltage regulation range, which is not feasible for the baseline topology from [18]. This paper proposes a new SRC topology with a modified boost VDR and verifies it in the voltage range suitable for the module-level PV applications. The main hypothesis is that it is possible to extend regulation voltage and power range of the baseline topology by rearranging the resonant capacitor position. There are three main contributions: identification and experimental verification of the limits of the input voltage and power regulation range in the converter [18], synthesis of the converter with improved input voltage and power regulation range, and derivation of its steady-state mathematical model that is verified experimentally.

The rest of the paper starts with Section 2 that describes the proposed topology and provides its comprehensive analysis. Section 3 presents a comparison between the proposed and the baseline SRC topology. The results of experimental verification are discussed in Section 4. Finally, Section 5 draws the conclusion.

## **2. Topology Description and Modulation**
