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Low-Profile and High-Efficiency 3 kW 400 V-48 V LLC Converter with a Matrix of Four Transformers and Inductors for 48V Power Architecture for Data Centers

Data center structure
Fig. 1. Narrow range 48 V bus – OCP V3
  The use of cloud computing services is surging at a rapid rate. These trends are motivation to improve the cost efficiencies of building data centers, either by reducing initial construction cost or increasing Power Usage Effectiveness (PUE) to reduce running costs. In 2020, OCP V3 was released to modify the 48 V bus to a more efficient structure. A narrow range 48 V bus is proposed where the bus voltage is fixed to 50 V during normal power delivery conditions. In case of an energy shortage, the power flow is switched naturally to the BBU when the bus voltage drops to 48 V. This solution offers multiple advantages such as enabling efficient 4:1 fixed ratio converters to downstream conventional 12 V PoL converters, and eliminating oversized current/voltage designs.

  In this paper, high efficiency and low-profile design are proposed for the 48 V LLC converter for the data center. The converter utilizes PCB magnetics for better thermal management, easy assembly, and lower cost. The matrix transformer is optimized through multiple aspects. The number of elemental transformers is selected based on the required number of parallel SRs. The turns number of 2:2 per elemental transformer is found to achieve better light load efficiency compared to the 1:1 turns ratio as the core loss reduces. The switching frequency is optimized and 250kHz is found to have the least total converter loss given the selected devices and ferrite material. The matrix inductor concept is presented to reduce PCB inductor winding loss and reduce thermal stress on PCB. The matrix inductor concept helps to avoid the extreme increase in ac resistance due to the MMF build-up. The ac resistance is found to be reduced by a factor of 1/7 for the case of 4 turn inductor. The converter achieves 98.8% efficiency and 600 W/in3 power density. The converter has an extremely low profile of <11mm. This reduces the PCB-to-air thermal resistance and facilitates thermal management.
Hardware prototye
Fig. 2. Hardware prototype of 400 V- 48 V 300 kHz LLC with integrated PCB magnetics.

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