Energy saving and cost reduction are becoming more and more important for today's industry, causing efficiency and power density to become major driving forces in modern power delivery systems. With the development of information technology, computing systems, electric vehicles and other fields, the demand for isolated power supplies with high efficiency and high power density grows significantly. In order to meet the requirements of high efficiency, high power density for data center server power supplies, the switching frequency is increased to 1 MHz. The investigation of traditional LLC converters is carried out to explain the operation principle, as well as the advantages compared with other converters. The GaN MOSFETs, matrix transformer and synchronous rectifier are applied to further optimize the traditional LLC resonant converter. Finally, a 1 kW, 380 V input, 12 V output experimental prototype with power-density of 300 W/in³ is built to verify the theoretical analysis.