June/July 2021

30 APEC 2021 www.apec-conf.org Issue 3 2021 Power Electronics Europe www.power-mag.com Power consumption of data centers is increasing rapidly over recent years. Recently, 48 V power architecture has attracted more interest in data centers as it offers more efficient architecture. This awarded paper (Best Presentation) focuses on the implementation of the DC/DC stage of the 3 kW power supply unit. Full-bridge regulated LLC converter is designed with a matrix of two transformers. Ahmed Nabih, Center for Power Electronics Systems at Virginia Tech (nabih@vt.edu) Resonant converters present an excellent choice for efficient DC/DC converters as they offer soft-switching for both primary and secondary devices. both primary, secondary, shielding, and inductor winding. The transformer of the 48 V resonant converter is excited with four times higher volt.sec compared to a 12 V converter. This makes the core cross-section area (Ae) is almost 4x higher as well. With increased core size at high frequency, the transformer core exhibits dimensional effects, namely dimensional resonance and skin effect. Moreover, the core eddy loss component tends to increase with increased core size. Dimensional resonance Any ferrite material has finite permittivity. This means there’s an induced electric field inside the ferrite material. The interaction between electric and magnetic fields tends to redistribute the magnetic field inside the ferrite core following a decaying sinusoidal distribution. If the core thickness is higher than half the wavelength, the magnetic field starts to flow out-of-phase from the excitation current causing drop in core permeability and increased core loss. Skin effect and eddy loss Any ferrite material has finite conductivity. The source flux induces ac currents flows inside the core. these ac currents are responsible for the eddy loss. These AC currents will also induce eddy fluxes causing the resultant flux to crowd on the surface of the core. The core thickness needs to be smaller than the ferrite skin depth to avoid skin effect. Figure 2 shows a case study on 3 different sample sizes from the same material showing increased core loss density with core size. The ferrite core material ML91 (from Hitachi) was selected for two reasons: ML91 has the least core loss or least hysteresis loss when measured on a small toroid sample and the least AC conductivity value at 1MHz when measured and compared to other materials. Experimental results The converter is implemented on 6-layer PCB, 2 oz. Copper, 35 m Ω /600V GaN devices are used for the primary full-bridge, and 4 m Ω /80 V GaN devices are used for the secondary full-bridge rectifiers. Figure 3 shows the implemented PCB including primary and secondary devices and gate drivers and magnetics. The resonant inductor is designed on UI core with side legs as well and the same thickness as the transformer. The inductor and transformer cores are High Power Density 1 MHz 3 kW 400 V-48 V LLC Converter The LLC Resonant converter, along with GaN devices, breaks new grounds to push the switching frequency to much higher frequencies (>1MHz) than those of the traditional pulse width modulation (PWM) converters. This opens opportunities to reduce the converter size and push for higher power density. When the magnetic size is reduced, planar magnetics (PCB winding) become more relevant. The design of a high-efficiency high power density 1 MHz 3kW full-bridge LLC converter for 48 V front-end power supply a shown in Figure 1 is described in this paper. The converter is designed on 6-layer PCB that includes Figure 1: Full-bridge LLC resonant converter Figure 2: Evaluation of core loss density for different core sizes of the same material (DMR51w) Figure 3: Implemented 1 MHz 3 kW LLC converter