Fig .1. Schematics of Three-Phase Interleaved LLC Resonant Converter.
The LLC converter is the most suitable topology for a dc-dc converter in server and telecom applications. In order to increase the output power and reduce input and output current ripples, the three-phase interleaved LLC converter is becoming more and more popular. By conecting the three phases of the converter in a certain way, the three-phase LLC converter can have the benefit of automatic current sharing. Different topologies of the three-phase LLC converter are surveyed in this paper. And a new topology suit- able for high-frequency operation is proposed, as shown in Fig. 1. The proposed LLC converter employs a delta-connected resonant capacitor network on the primary side to achieve automatic current sharing, a full-bridge secondary side due to high-frequency operation, and a shielding technique to reduce CM noise.
The three-phase LLC converter suffers the drawback of numer- ous bulky magnetics. A novel magnetic structure is proposed to inte- grate three inductors and three transformers into one magnetic core, with 600 V GaN devices from Panasonic and 80 V GaN devices from EPC employed for the primary and secondary sides, respectively. By pushing the switching frequency up to 1 MHz, all the windings for the magnetic structure can be implemented with 4-layer PCB winding. And additional 2-layer shielding can be integrated in the PCB windings to reduce CM noise. A 1 MHz 3 kW 400 V/48 V three-phase LLC converter with the proposed magnetic structure is designed with an estimated peak efficiency > 98%, as shown in Fig. 2, and a power density of > 200 W/in3
Fig. 2. (a) Hardware prototype (b) Estimated efficiency.