Power Electronics Europe Feb/March 2023

22 POWER TRANSISTORS www.epc.co.com Issue 1 2023 Power Electronics Europe www.power-mag.com 4:1, desired output power of 1 kW at 1 MHz resonant frequency, and maximum size of 17.5 mm x 23 mm. Building on the experience from previous work [1,2] and aided by Finite Element Simulations a core shape with a single 6 mm diameter center post and four satellite flux return legs was designed and shown in Figure 2 (right). The 6 mm diameter for the center post was found to be the optimal dimension considering conduction losses in the copper windings and core losses, as analyzed in [1]. The final dimensions of the top and bottom side caps of the core shown in Figure 2 (left) are a compromise between flux density and magnetic core utilization, to open areas for placing components without increasing core losses. Note that the PCB real state close to the transformer windings is of utmost interest to minimizing parasitic inductance. As reported in the literature, this parasitic inductance in the secondary is detrimental to the performance by as much as 30% [4]. ML91S [9], the same soft-ferrite material used in prior work [2], was used for the transformer core. It provides good stability over temperature and frequency, even beyond 1 MHz, as well as less than 200 kW/m3 of flux density volumetric power loss. The airgap between the two core halves was tuned to realize a magnetizing inductance of approximately 1.8 µH. PCB Design With the transformer core dimensions defined, the primary and secondary windings were distributed over 16 layers routing the current around the center post of the transformer core. A single 3oz per layer was dedicated to each primary turn and three 3oz layers and one 2oz layer were paralleled for each branch of the secondary. The inner twelve layers are fabricated with standard PCB technology, whereas HDI technology was utilized for the outer layers. This way the primary and secondary components can be placed on the top and bottom sides of the board and the current efficiently routed down into the transformer windings. Testing Results and Next Steps To test the converter described in the previous sections, a motherboard was developed to provide input/output connections for the module, additional bus capacitance, housekeeping power supplies, sense connections for accurate efficiency measurements, and a connector for the controller board. A photo of the setup is provided in Fig. 3 (left), along with waveforms at full load (center) and the efficiency curve (right). Peak efficiency of 96.3% could be measured at 25 A and 93.8% at 84 A (1 kW). In the next iteration of the converter, the controller and housekeeping power supplies will be integrated in the module while maintaining the same overall size. Moreover, a small resonant inductor will be added in series with the transformer to increase the Q factor while maintaining the same resonant frequency. The PCB will also experience changes as the 16-layer board will be replaced with a two-PCB solution to reduce copper losses and improve manufacturability of the overall system. Conclusions The module presented in this article demonstrates that GaN FETs can enable unprecedented levels of power density (>4 kW/in3) in 48 V to 12 V power converters, such as those needed in datacenters with a 48 V architecture. In particular, the combination of GaN technology featuring chip-scale packaging like those of eGaN transistors, and carefully designed magnetics, allow 1 kW load capability at 1 MHz frequency with peak efficiencies and full load efficiency of 96.3% and 93.8% respectively. References [1] M. de Rooij and A. Negahdari, “Beyond 4 kW/in3 Power-Density for 48 V to 12 V Conversion using eGaN FETs in an LLC DC-DC Bus Converter,” PCIM Europe 2022; International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management, 2022, pp. 1- 9, doi: 10.30420/565822013. [2] M. de Rooij, J. Wang, A. Negahdari and Y. Zhang, “A 1 kW eGaN FET-based LLC Resonant Converter in them 1/8th Power Brick Size for 48 V Server Applications,” PCIM Europe digital days 2021; International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management, 2021, pp. 1-8. [3] M. H. Ahmed, F. C. Lee and Q. Li, “Two-Stage 48-V VRM With Intermediate Bus Voltage Optimization for Data Centers,” in IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 9, no. 1, pp. 702-715, Feb. 2021, doi: 10.1109/JESTPE.2020.2976107. [4] D. Huang, S. Ji and F. C. Lee, “LLC Resonant Converter With Matrix Transformer,” in IEEE Transactions on Power Electronics, vol. 29, no. 8, pp. 4339-4347, Aug. 2014, doi: 10.1109/TPEL.2013.2292676. [5] EPC. (2021). “EPC2218 datasheet,” [online]. Available: https://epc- co.com/epc/Portals/0/epc/documents/ datasheets/epc2218_datasheet.pdf. [6] uPI SEMI. (2021). “uP1966E datasheet,” [online]. Available: https://www.upi - semi.com/files/2279/19724ef5-d308- 11eb-8d5f-d30ecdd7d08d.pdf. [7] EPC. (2021). “EPC2067 datasheet,” [online]. Available: https://epc- co.com/epc/Portals/0/epc/documents/ datasheets/epc2067_datasheet.pdf [8] TI. (2018). “LMG1020 datasheet,” [online]. Available: https://www.ti.com/lit/ds/symlink/lmg1 020.pdf?ts=1664290417482.pdf. [9] Hitachi, Material characteristics of ML91S. http://www.hitachi - metals.co.jp/ To receive your own copy of Power Electronics Europe subscribe today at: www.power-mag.com