A WBG-Based Three Phase 12.5 kW 500 kHz CLLC Resonant Converter with Integrated PCB Winding Transformer
The three phase dc-dc resonant converter offers an attractive solution to high-power applications, due to its reduction in component stresses compared with a single phase resonant converter with the same power rating. A three phase CLLC bidirectional resonant converter is adopted (as shown in Fig. 1) for a 12.5 kW off-board charge. The Δ-connection on the primary side is used to take advantage of the strong current sharing and fast start up. In addition, wide bandgap devices, including both silicon-carbide and gallium-nitride devices, are adopted to push the switching frequency over 500 kHz. With the help of high frequency, a 6-layer only printed circuit board (PCB) winding transformer is proposed. In addition to all of the resonant inductors being integrated into the transformer, the three-phase transformers are also integrated together to further reduce the size. As a result, only one magnetic component is needed for the whole three-phase CLLC resonant converter.
By redistributing the primary and secondary windings on the outer posts and adding a center post, the leakage inductance of the transformer can be increased, and serves as the primary and secondary side resonant inductors. Compared to other integration methods, the interleaved winding structure is maintained, and thus a smaller ac winding loss is achieved. Also, with the additional center post serving as the leakage flux path, the leakage inductance (resonant inductors) can be controlled by adjusting the center post air gap length. In addition, all the leakage flux is confined in the center post, having no impact on the surrounding components. If we investigate the flux in the core, due to the 120° phase shift between each phase, the flux in the three center posts also have 120° phase shift. As a result, the total flux in the center posts is cancelled out. This can be verified through 3D finite element analysis simulation. We can see that the flux in the three center posts is almost zero. Therefore, we can simplify the transformer structure by removing the three center posts.
Using the designed three-phase integrated transformer, a 12.5 kW hardware prototype with 500 kHz switching frequency is developed, as shown in Fig. 2. The resulting power density, including the heatsink, is 155 W/in3. This power density is much higher than the single phase version, due to the three-phase transformer integration. The efficiency of the three phase CLLC resonant converter is also tested, and we can see that the peak efficiency of 97.3 percent is achieved.