Design Analysis of High-Frequency, High Power 3-Phase DAB Transformer for Electric Vehicle Charger Applications

The power requirements for onboard chargers in Electric Vehicles are increasing due to the need for increased charging speeds. The 3-Phase Dual Active Bridge (DAB) DC-DC converter is an enabling topology for high-power onboard chargers (22kW). However, the design of a 3-phase high-power, high-frequency transformer (3P-HPFT) for the 3-phase DAB poses several challenges. To understand those challenges, two design approaches are considered in this paper: a single-core based design referred to as integrated transformer design, and a three-core based design referred to as 3CT. The results are compared to understand the design challenges. Ansys and PLECS magnetic simulation tools are utilized to validate the design. For a 22kW converter, the integrated 3-phase DAB Transformer has a 19% higher power density (88 mW/mm³), 50% lower core losses (133 W) and is 25% lighter (1.5 kg) in comparison to 3CT which has a power-density of 74 mW/mm³, core losses of 262 W and a weight of 2 kg. Leakage inductance can be utilised as the energy-storing element during the phase shift operation of the DAB thus, the range of achievable leakage inductance in both the designs is explored. The integrated transformer design can achieve a leakage inductance range of 0.5 µH - 9.6 µH. Whereas, the 3CT transformer offers a leakage inductance range of 1.5 µH - 9 µH. To operate at nominal power, apart from the leakage inductance, both designs will require an additional inductance of 5 µH. Soft switching conditions for the nominal power operation are also derived and it was found that both the bridges operate within the soft switching boundaries.