10 kW High Efficiency Compact Gallium Nitride-based dc-dc Converter Design
The LLC resonant converter has been widely adopted in telecom and transportation applications for its high efficiency performance, as well as its ability to achieve zero-voltage switching (ZVS) for all switches under different load conditions. In the LLC resonant converter design, the adoption of wide-bandgap (WBG) devices, especially gallium nitride (GaN) devices, contributes to a significant improvement of efficiency and power density, due to high switching speed, low switching loss, no reverse recovery loss, and small package. This paper presents a 10 kW LLC converter design using 650 V 60 A e-mode GaN HEMTs with wide regulation, achieving high efficiency and high power density simultaneously. To conduct high current and further reduce the on-resistance, the switching cell of a half bridge with paralleled GaN HEMTs is introduced. Fig. 1 shows the PCB design with four devices in the half bridge located on the top layer of the PCB, and the bus decoupling ceramic capacitor at the back side, which achieves 0.814nH power loop inductance based on Q3D simulation. A universal connector employed for the switching cell, enables easy replacement and debug capa-bilities. The heat sink dedicated to the GaN-based switching cell, is designed according to the requirements of device loss, room temperature, and device operating temperature. For verifica-tion, a complete procedure in which conducting reverse current with negative gate voltage to measure the thermal resistance is developed, and can be generalized to other converter thermal management applications. In order to achieve high efficiency and high power density, the transformer and the resonant inductor are integrated together on an E I core with litz-wire. A large resonant inductor, which can be controlled by a center leg width, helps to achieve wide regulation range. The transformer dimensions and inductances are optimized based on the Pareto front between converter volume and frequency. Using the selective design above, a compact structure is achieved, as shown in Fig. 2. Vertical space within the converter can be fully utilized with the same height of the switching cell, the fan, the transformer, and the input and output capacitor. Furthermore, the air flow from the fan can be effectively used for the heat dissipation of the transformer and the devices. This paper presents a complete and detailed design procedure for a 10 kW GaN-based LLC resonant converter with a paralleled 650V 60A e-mode GaN. The work includes both layout con-sideration and thermal management. The integrated transformer design with a Pereto front is introduced. The final LLC resonant converter features a dimension of 278 x100 x 45 mm, achiev-ing a 97.9 % peak efficiency, and a 131 W/in3 (8 kW/l) power density.