In high efficiency high power density applications, series resonant converter (SRC) has been used widely for its soft-switching characteristics, which can operate at high frequency with a negligible switching loss. To further improve the efficiency at high frequency, an input-series output-parallel (ISOP) cellular SRC using low-voltage and low-NFoM devices has been proposed to reduce the conduction loss of the MOSFETs and a resonant drive transformer has been illustrated to minimize the driving loss. However, the space occupied by the power transformers is still very large even at 1 MHz switching frequency, which restricted the further improvement of the power density. In this paper, a multi-window EI core is used and designed to replace the conventional UI core, and the airgaps of the core are analyzed and optimized for even magnetic flux distribution. In this way, the power density of the transformer and the prototype can be improved significantly. A 380 V-12 V 1 MHz DCX prototype using the proposed multi-window EI core is built up for verification, where 98.4% peak efficiency is achieved and the power density is improved to 1.8 kW/in³.