Fast chargers play an important role in supporting smart city development by enabling access to mobile applications, location-based services, and real-time data systems. As mobile internet technology advances, smartphones are becoming increasingly powerful, but their energy consumption continues to rise. However, the lack of compatibility between fast-charging protocols from different manufacturers presents a challenge for universal charging solutions. To address this, a multi-protocol fast charging charger has been designed to support a range of fast charging technologies. The charger adopts a quasi-resonant (QR) flyback architecture with a GaN power stage and a dedicated controller. This design allows for higher efficiency and compact size, making it well-suited for modern mobile device demands within the broader context of smart city infrastructure. In this work, an experimental prototype was successfully built. The experimental results show that the fast-charging charger can achieve a fast-charging function with a maximum output power of 30 W and has Type-C and Type-A interfaces. It is convenient for fast charging of various data lines. And support 90Vac ~ 264 Vac wide voltage input in line with the AC input of various countries. The Type-C interface supports up to 20 V/1.5 A output. It meets the limits of U.S. DOE Level VI and EU Regulation 2019/1782 in our measurements. In addition, the compatibility of the charging and discharging fast charging protocol of the system prototype has been tested, and it can be quickly charged with a variety of mobile phones that support the fast-charging protocol. It has advantages of high-power density, high conversion efficiency and strong compatibility.