3D-Matrix Inductor-Transformer with Fractional-Turn Interleaving in a CLLC Resonant Converter for Bidirectional Onboard Chargers

Planar magnetics are widely recognised as compact solutions for high-frequency isolated dc-dc converters such as electric vehicle chargers. Meanwhile, PCB windings can drastically reduce costs and improve manufacturability. This paper presents a 3D-matrix inductor-transformer (3D-MIT) that reconciles two design approaches for planar magnetics that, thus far, have been considered incompatible. First, a novel 3-dimensional core and winding structure is proposed to integrate the series resonant inductors and transformer of a CLLC resonant converter. Second, a fractional-turn interleaving structure is presented, which halves the eddy current-inducing field strength surrounding the windings compared to fully interleaved windings. In doing so, the copper losses are drastically reduced in high-frequency operation. The combination of both presents the first integrated inductor-transformer concept capable of achieving completely interleaved windings. Modelling and design methods are presented, and software implementations thereof accompany this paper. A prototype 3D-MIT is designed and optimised for a 4-kW 500-kHz GaN CLLC resonant converter in a liquid-cooled bidirectional onboard charger. The converter achieves a power density of 4.0 kW/L including the cooling system and 5.6 kW/L excluding it, with a peak efficiency of 97.5%.