###### Abstract  
Soft switching for both primary and secondary side devices is available by using LLC converters. This resonant converter is an ideal candidate for today's high frequency, high efficiency, and high power density applications like adapters, Uninterrupted Power Supplies (UPS), Solid State Transformers (SST), electric vehicle battery chargers, renewable energy systems, servers, and telecom systems. Using Gallium-Nitride (GaN)-based power switches in this converter merits more and more switching frequency, power density, and efficiency. Therefore, the present paper focused on GaN-based LLC resonant converters. The converter structure, operation regions, design steps, and drive system are described precisely. Then its losses are discussed, and the magnets and inductance characteristics are investigated. After that, various interleaved topologies, as a solution to improve power density and decrease current ripples, have been discussed. Also, some challenges and concerns related to GaN-based LLC converters have been reviewed. Commercally available power transistors based on various technologies, i.e., GaN HEMT, Silicon (Si) MOSFET, and Silicon Carbide (SiC) have been compared. Finally, the LLC resonant converter has been simulated by taking advantage of LTspice and GaN HEMT merits, as compared with Si MOSFETs. LLC; resonant converter; GaN HEMT; review 28 October 2021 25 November 2021 2030 November 2021 2030 November 2021 2030 November 2021 2030 November 2021 2030 November 2021 2030 November 2021 2030 November 2021 2030 November 2021 2030 November 2021 2030 November 2021 2030 November 2021 2030 November 2021 2030 November 2021 2030 November 2021 2030 November 2021 2030 November 2021 20300 November 2021 20300 November LLC converter, first proposed by [PERSON] in 2003, benefits from soft switching for both primary and secondary side devices [5]. LLC resonant converter, with such advantages as high frequency, high efficiency, and high power density, has a wide range of applications, including adapters, UPS, electric vehicle battery chargers, renewable energy systems, servers, and telecom systems [6; 7; 8; 9; 10; 11]. Low EMI and wide voltage range are the other advantages of the LLC resonant converter [12; 13; 14; 15]. ZVS maintained by the LLC converter enables a low turn-off current for primary switches. Therefore, dv/dt at the primary switches is small [16]. Also, the LLC converter has the benefit of ZCS for the Synchronous Rectifiers (SRs) in frequencies lower than the resonant frequency [16; 17; 18; 19; 20; 21; 22; 23]. Consequently, beside galvanic isolation, it achieves higher frequency with better efficiency, more power density, and lower total cost, as compared with the soft-switching PWM converters [11; 19; 21; 24]. For high input voltage applications, the LLC topology is more effective in achieving higher efficiency because the switching losses are more dominant than the conduction ones [25; 26; 27; 28]. In a promising development, GaN-based transistor is introduced as a very hard, mechanically stable wide bandgap semiconductor. With higher breakdown strength, faster switching speed, higher thermal conductivity, and lower on-resistance, GaN based transistors, also called GaN High Electron Mobility Transistors (GaN HEMT), significantly outperform silicon-based devices. Optimized internal structure and smaller chip size, along with lower drain-source resistance in the on-state condition, lead to the shorter deadtime and higher efficiencies, normally exceeding 96% [24]. Less capacitance, resulting in fewer losses while charging and discharging, less power consumption for driving the circuit, and lower price are the other advantages of using GaN-based transistors in the LLC converters [29]. Recent developments in LLC resonant converters with GaN transistors and novel magnetics present more and more switching frequency, power density, and efficiency [11; 30; 31; 32].