In modern power electronics, three-phase DC-DC resonant converters have emerged as a solution for high-power applications, offering reduced component stresses compared to their single-phase counterparts of the same power rating. This technology includes various configurations, with the most conventional being the simple parallel connection, though plagued by current sharing challenges arising from component tolerances. An alternative approach is the Y-connection, ensuring automatic current balance but introducing a persistent DC bias across resonant capacitors, complicating soft start-up procedures.

Fig 1 Δ-connection resonant tank

A three-phase integrated transformer concept has been proposed, using flux cancellation to shrink the core volume. However, this innovation still relies on bulky resonant inductors, limiting overall power density and necessitating complex litz-wire-based designs prone to parasitic issues. WBG devices like SiC and GaN enabled power electronics converters to operate at much higher switching frequencies, reducing the required volt-seconds across transformers and simplifying transformer winding integration through the use of PCBs. Additionally, the CLLC resonant converter, with its symmetrical resonant tank, as shown in Fig. 1, offers bidirectional operation capabilities, making it particularly suitable for high-frequency operations with superior gain boost and reduction capabilities in both directions.

Ⅰ. System Evaluation

The proposed three-phase CLLC resonant converter, depicted in Fig. 2, operates with an 800 V input and produces a 400 V output. For the primary side, high-performance 1.2 kV SiC devices are employed, while 650 V GaN devices are chosen for the secondary side. A half-bridge configuration on the primary side necessitates a 1:1 turn ratio for each transformer phase. Three different SiC devices are evaluated for their turn-off performance under varying conditions. Experimentally, it's observed that conventional TO-247-3L SiC devices exhibit significant gate signal ringing during high-current turn-off, mainly due to common source inductance coupling between power and driving loops. This ringing pushes the gate voltage above the threshold voltage, rendering these devices unsuitable for high-frequency operation. In contrast, Gen. 3 devices with advanced 4-lead packages show improved performance, minimal ringing, and lower turn-off losses. Among the candidates, the Gen. 3 1 kV SiC device is chosen for the primary side.

Fig 2 Three-phase CLLC resonant converter.

On the secondary side, a 650 V GaN device GS66516T from GaN Systems is selected to minimize conduction losses. This comprehensive evaluation and component selection process ensures efficient and reliable operation of the three-phase CLLC resonant converter, meeting the specific voltage and performance requirements for the application.

Ⅱ.  Optimizing Transformer Losses through Integrated Magnetic Design

A. Single phase integrated PCB winding magnetic design: A novel 6-layer PCB winding-based transformer structure is proposed for a 500 kHz 6.6 kW on-board charger, as depicted in Fig. 3. Two of these transformer cells are connected in series on the primary side, resulting in a total transformer turns ratio of 12:6:6. By rearranging the primary and secondary windings on the outer posts and introducing an additional center post, the transformer's leakage inductance can be increased, serving as resonant inductors on both sides. This interleaved winding structure minimizes AC winding losses and allows for precise control of leakage inductance by adjusting the center post's air gap length. Furthermore, all leakage flux is contained within the center post, avoiding interference with nearby components.

Fig 3 Single phase 6-layer PCB winding transformer with an inbuilt resonant inductor.

B. Three-phase integrated PCB winding magnetic design: To achieve a higher power level of 12.5 kW, three single-phase transformers are used as shown in Fig. 4, which forms a core structure with nine posts. By merging the top and bottom plates, these three transformers become a unified three-phase transformer. However, this complex core structure has nine posts, and when examining the core's flux, the 120° phase shift between phases results in a 120° phase shift in the flux within the three center posts, effectively canceling out the total flux. This cancellation is confirmed by a 3D finite element analysis (FEA) simulation, demonstrating negligible flux in the center posts. Consequently, the transformer structure can be simplified by eliminating these three center posts.

Fig 4 6-layer PCB winding transformer with integrated resonant inductors.

C. Three-phase transformer loss model and optimization: To minimize loss in the transformer, a model is constructed, and an optimization process is employed. The transformer's dimensions, as depicted in Fig. 5 involve a circular core post for reduced winding length and fixed 2 mm spacing between windings. Consequently, there are just three design parameters: core post width (a), length (b), and winding width (c).

Fig 5 Proposed three-phase transformer structure with integrated resonant inductors.

Ⅲ.  Results Obtained After the Experiment

A 12.5 kW hardware prototype with a 500 kHz switching frequency was developed using a selected device and a designed three-phase integrated transformer. The power density, including the heatsink, reached an impressive 155 W/in³, significantly surpassing that of a single-phase version, owing to the integration of the three-phase transformer.

Fig 6 Working waveform under 400V and 8A output

Fig 7 Working waveform under 400V and 31A output.

Examining waveforms at 400 V output voltage and 8 A output current (25% load) in Fig. 6 and at 400 V and 31 A (full load) in Fig. 7, we observe excellent current sharing and Zero Voltage Switching (ZVS) due to the primary side-connection.

Ⅳ.  Conclusion

An experiment was conducted to evaluate the performance of a three-phase CLLC bi-directional resonant converter designed for a 12.5 kW off-board charger. The primary-side connection was utilized to take advantage of excellent current sharing and rapid startup capabilities. Additionally, a 6-layer PCB winding transformer, integrating all resonant inductors and three-phase transformers, was proposed using the flux cancellation concept.

The hardware prototype of this converter demonstrated an impressive power density of 155 W/in³, and extensive testing revealed a peak efficiency of approximately 97.3%. This underscores the viability and efficiency of this innovative converter design, highlighting its potential for high-power off-board charging applications.