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High-efficiency, High-power-density 3 kW LLC Converter with Integrated Printed-circuit-board-winding Matrix Transformer

LLC converter prototype
Fig. 1. The 500 kHz 3 kW LLC converter prototype with integrated PCB winding matrix transformer
The 3 kW 48 V output voltage power supply is promoted to replace the 12 V power supply in order to mitigate the heavy bus bar loss and reduce energy conversion stages. The conventional Si-based 48 V front-end converter has fixed DC-link voltage, thus the output voltage is regulated with an LLC DC/DC (D2D) converter through modulating the switching frequency. With the output voltage varied in 40 - 60 V, the switching frequency range of the LLC D2D converter is wide, from 0.7fr to > 3.0fr. Therefore, the switching loss is increased especially in light load and the efficiency of the LLC D2D converter is comparatively low. In order to increase the efficiency of the LLC converter, a variable DC-link voltage solution is proposed in this paper. By running the LLC converter as a DC transformer (DCX), the LLC converter can always work at resonant frequency to achieve high efficiency. Accordingly, So the output voltage is actually regulated by a power factor converter (PFC). With SiC and GaN devices in the primary and secondary side respectively, the switching frequency of the LLC DCX con-verter is pushed to 500 kHz. The main challenge for high power density and high efficiency is the transformer winding and core design. The total transformer ratio for the LLC converter is 12:1. With four elemental transformers, the turns ratio for each elemental transformer is 3:1. A four layer printed circuit board (PCB) winding structure, with two layers for secondary side winding and another two layers for primary side winding, is the general practice with PCB winding matrix transformer design. In the case with an even number of turns in the primary side, half of the turns are arranged in each layer. However, with an odd number of turns in the primary side, it is physically impossible to accomplish half of one turn. This paper proposes a novel winding structure that allows an odd number of turns for the primary side winding. The proposed structure has a simple winding structure and low winding loss. Four elemental transformers are integrated into a single planar magnetic core by flux can-cellation. The winding width and core radius are optimized to achieve lower winding loss and a smaller footprint. Moreover, a shielding technique is applied by simply inserting two layers into the PCB winding to reduce the common mode electromagnetic interference (EMI) noise. A 3 kW PCB winding matrix transformer is demonstrated based on a 500 kHz LLC resonant converter with a wide-bandgap device. A turns ratio of 3:1 is achieved in each elemental trans-former. The prototype achieves a peak efficiency of almost 98.5% and a power density of 400 W/in3.
Tested efficiency result
Fig. 2. The tested efficiency versus output power for the 3 kW LLC converter
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