Design considerations of 1 MHz LLC resonant converter with GaN E-HEMT

LLC resonant converters with high switching frequency can show high power density by reducing the size of passive components, such as the output capacitor and transformer. However, it is difficult to operate the PWM generator and at a high switching frequency. Moreover, soft start operation requires much higher switching frequency than the nominal one. Therefore, this paper proposes a new soft start algorithm to suppress high inrush current with limited switching frequency. In addition, stable operation of the LLC converter at the high switching frequency is considered. GaN E-HEMTs are selected to achieve the high switching frequency operation due to its small drain-source resistance and small parasitic capacitance. However, GaN E-HEMTs also have different switching operation characteristics t. In this paper, the design and implementation of a 1 MHz LLC resonant converter are proposed to verify the improvement of power density reducing the passive component size. The soft start algorithm for high switching frequency is analyzed for small inrush currents at the cold start condition. Simulation and implementation are used to verify the validity of the soft start algorithm. The side effects of high switching frequency operation are analyzed to design the power components and PCB. The high speed switching characteristics of the GaN E-HEMT are also analyzed to obtain proper operation for a half-bridge type LLC resonant converter using a boostrap circuit. Simulation and experimental results are presented to show the validity of the proposed analysis and design methods with a 1 MHz prototype converter using GaN E-HEMTs.