This paper proposes an optimized integrated planar transformer structure and a systematic design framework for a three-phase ∆ − ∆ connected LLC converter for 48V light electric vehicle charging systems. The proposed integrated magnetics aims to increase the leakage inductance to reduce the switching frequency range for regulating the voltage with a wide range. The proposed methodology uses newly derived first harmonic approximation current equations in relation with the switching frequency, sweeps the Lm/Lr and Q pairs, and designs integrated magnetics with different asymmetrical winding options, different cores and materials under given constraints. A 1 MHz 3700W / 48V three-phase ∆−∆ connected LLC converter has been optimized using the proposed optimization framework. The resultant design employs an integrated magnetic design with two stacked ER/32/6/25 cores under the constraint of using a 6-layer board and off-the-shelf ferrite cores, with asymmetric winding turns of 5:1 and 3:1 on the central legs. The power stage of the developing prototype has a power density of 387 W/in3.
