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Design and Optimization of High Frequency High Current LLC converter with Planar Transformers

Fig.1: DC power distribution system with one stage DCX for data center
Presently, most multi-core processors for server applications are powered from a 12-volt bus. The i2R related copper loss for 12 V bus is excessive. To reduce the loss in the power distribution, the power conversion stages is required to be in vicinity with the processor. The DC power distribution system was proven to be a high efficiency structure, compared with AC power distribution system. For to further improve the system efficiency, a 380V-12V unregulated converter (DCX) is used to replace the multiple-stage DC/DC converters in the DC distribution systems, as shown in Fig.1.

The LLC resonant converter was chosen as a candidate for next generation high-power-density high efficiency isolated converter for datacenter applications. Regarding the magnetic design, matrix transformer is a perfect fit for this application because of high step down ratio and high output current. The matrix transformer can simplify the winding structure, by reducing the turn ratio of each individual transformer. Such simplification is important for the high current application and PCB winding.

In this work a matrix transformer with optimized termination and non-perfect interleaving was chosen as it shows better performance compared to perfect interleaving with non-perfect termination. The LLC converter with 2 core structure power architecture and is shown in Fig.2. In this paper a detailed modelling and optimization method is presented to achieve high power density and efficiency design. A comparison between two design approaches of 2 core and 4 core structures was discussed showing total transformer loss comparison as in Fig.3 where the 4 core structure is much better in comparison with 2 core structure. Finally, the termination of the output capacitors was studied in details which is very important for conduction loss. Im-proper design of output loop and capacitors can induce significant amount of loss. Experiments verified the design and analysis. Finally, a cost effective 1kW 1MHz 380-12V LLC converter de-signed achieving efficiency of 97% and 700W/in3 power density as shown in Fig.4.

Fig.2: converter power architecture and individual transformer footprint
Fig.3: Comparison of two-core matrix trans-former and four core matrix transformer

Fig.4: Designed converter and measured efficiency