## 10 GaN-Based LLC Converter  
The use of 600 V E-mode GaN HEMTs has been proposed in [2] to expand the LLC resonant converter input voltage range from 340-400 V to 280-400 V. This LLC resonant converter with GaN HEMT has 140 W/inch3 power density, which is 2-3 times better than its silicon MOSFET counterpart. By the substitution of silicon FET with GaN HEMT in a 300 W AC-DC converter operating at \(<\)100 kHz with a power density of 8-10 W/inch3, operation in 500 kHz switching frequency with 29 W/inch3 power density can happen [123]. The researchers are moving toward applications with higher voltage levels. Multi-level converter topologies have been studied widely to adapt high input voltage with low voltage devices [124; 125]. [PERSON] et al. presented the superior performance of 900 V GaN HEMT in a 300 kHz 2 kW LLC resonant converter bridging high voltage and low voltage systems [126]. All GaN power supply units including the LLC converter with 3.2 kW power rating and 73 W/inch3 power density are also reported in the literature [127]. Further, using GaN HEMT instead of Silicon (Si) MOSFET resulted in 100% power density improvement in a 150 W commercial adapter [128]. Silicon-based conventional resonant converters in the power range of 1-3 kW normally operate with a frequency up to 350 kHz [1; 129]. A resonant converter with the frequency of 1 MHz and peak efficiency of 94.1% is enabled using GaN HEMT in [130]. By using monolithic high voltage GaN power ICs that integrate GaN HEMT with the gate drive circuit in a small package, higher efficiency up to 98.3 % has been reported for the LLC resonant converter operating in 1 MHz switching frequency in a 3.2 kW power supply [131]. It should be noted that designing the drive circuit for E-mode GaN devices needs a tightly regulated power supply and a voltage clamping mechanism to prevent the excessive gate voltage [132]. As an example, a 2.5 kW LLC converter with 48 V output and 270-330 V input is considered. The design of the converter has been carried out according to the test procedure \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Properties** & **GaN** & **Si** & **SiC** \\ \hline E\({}_{\text{G}}\)(eV) & 3.4 & 1.12 & 3.2 \\ E\({}_{\text{BR}}\)(MV/cm) & 3.3 & 0.3 & 2.2 \\ n\({}_{\text{i}}\)(cm\({}^{-3}\)) & 2\(\times\)10\({}^{-10}\) & 1\(\times\)10\({}^{10}\) & 8\(\times\)10\({}^{-9}\) \\ V\({}_{\text{S}}\)(\(\times\)10\({}^{7}\)cm/s) & 2.5 & 1.0 & 2.0 \\ \(\mu_{\text{n}}\)(cm\({}^{-2}\)/Vs) & 990-2000 & 1500 & 650 \\ \(\lambda\)(W/cmK) & 1.3 & 1.5 & 3.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Material properties in Gallium-Nitride (GaN), Silicon (Si), and Silicon Carbide (SiC)-based switches. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Technology** & **GaN** & **Si** & **SiC** \\ /**Parameter** & **GaN** & **GaN** & **Si** & **SiC** \\ \hline Part number & GS66508B-TR & FCB09N65S3 & SCT3080 AW7 TL \\ Manufacturer & GaN Systems & ON Semiconductor &
\begin{tabular}{c} ROHM \\ Semiconductor \\ \end{tabular} \\ Drain-source voltage (V) & 650 & 650 & 650 \\ Drain current–continuous (A) & 30 & 30 & 29 \\ Rds–on (\(m\Omega\)) & 63 & 99 & 104 \\ Vgs (V) & \(-\)10, +7 & 4.5 & 5.6 \\ Qg–gate charge (nC) & 6.1 & 61 & 48 \\ Price \$ & 15.75 & 3.67 & 12.67 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of 650 V Field Effect Transistor (FET) switches available in the market from GaN, Si, and SiC technology. As shown in the figures, the use of GaN technology permits the reduction of power losses, hence improving the efficiency in the whole regulation range. At the same time, it would be possible to shift the switching frequency range at higher frequencies to reduce the size of the magnetics and increase the power density. Higher switching frequencies up to 27 MHz are reported in the literature for an output power lower than 300 W without sacrificing efficiency or voltage gain [133; 134; 135; 136].