## 1 Introduction
Moreover, the power stage of the phase-shifted full-bridge (PSFB) converter with a current-doubler rectifier (CDR) and synchronous rectification offered the low switching losses in the power switches and reduced the current ripple on the charger output side [10; 11; 12]. Furthermore, this study integrated the proportional (P) control, proportional-integral (PI) control, fuzzy logic control, and multiplexer into the current-loop feedback controller of the charger to mitigate the step current overshoot during the startup phase and prevent false triggering of overcurrent protection (OCP). Only a limited number of studies have effectively addressed and resolved the issue of start-up current overshoot. In [13], the authors have sought to address the current-overshoot-suppression charging method for super-capacitor trans. A cooperative charging protocol was designed to achieve current balancing among chargers, and a set-point modulation method was introduced to smoothly adjust the chargers' set-points, thereby suppressing current overshoot. This study builds upon the overshoot mitigation techniques presented in [14; 15; 16]. The works referenced in [14; 15; 17; 18], as well as the present study, adopt a power stage topology based on a PSFB converter integrated with a current-doubler rectifier. Furthermore, approaches involving the transition from P control to PI control were employed in [14; 15; 16]to suppress the output current overshoot during the start-up phase of the battery charger. In contrast, this study further enhances the control strategy by integrating fuzzy logic control into the current-loop controller, thereby improving the effectiveness of overshoot mitigation. Additionally, the proposed battery charger was specifically designed to charge solid-state lithium battery packs. Technical comparisons of literatures are list in Table 1.  
The charger developed in this study was designed to suppress the current overshoot that occurs during the initial startup phase, specifically when transitioning from constant-voltage to constant-current charging mode. This approach offers the following advantages for solid-state lithium batteries:  
* Excessively high charging current can increase interfacial stress or cause localized heat accumulation, potentially leading to reduced reliability and accelerated battery aging. * Although solid-state lithium batteries have a lower risk of lithium dendrite growth compared to liquid lithium batteries, high voltage and current conditions may still promote dendrite formation. These dendrites can potentially penetrate the solid electrolyte, leading to internal short circuits. * Liquid and solid-state lithium batteries have different state-of-charge characteristics; therefore, it is not recommended to use traditional lithium battery chargers directly. Instead, a charging strategy should be developed based on the specific characteristics of solid-state batteries, followed by the design and development of a dedicated solid-state lithium battery charger. * Current overshoot during charging is an undesirable phenomenon that can accelerate the aging of both the circuit component and the battery. This paper is organized into six sections. Section 2 presents the power stage, focusing on the PSFB converter with a current-doubler rectifier and synchronous rectification. Section 3 describes the control loops for constant-voltage and constant-current charging. Section 4 discusses design considerations for the power stage and the proposed current control loop. Section 5 provides the experimental results. Finally, the conclusions of this study are presented in Section 6.