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Magnetic Design and Validation of a 500 kHz, 18 kW Intra-Leaved Litz Wire Transformer for Battery Charging Applications

Planar E Core
Fig 1. Intra-leaved PSPS Planar E Core
  This paper presents the design and fabrication process of a high-density 500 kHz 18 kW litz wire, air-cooled transformer that achieves low leakage inductance and high efficiency for a bi-directional resonant dc-dc converter system. The transformer design procedure analyzes the trade-offs between standard E cores and planar E cores with a focus on low leakage inductance, high efficiency, high power density, and even current sharing among parallel windings. A planar E core with a primary-secondary- primary-secondary (PSPS) configuration is chosen as the final design. Transformer efficiency is 99.61% at full load power (18 kW) and power density is 4277 W/in3. Leakage inductance is 764 nH and 128 nH for the primary and secondary sides, respectively. The transformer winding is potted in thermal epoxy to improve heat spreading and reduce peak internal winding temperature. A simple-to-use potting mold is developed to pot the transformer winding and remove air bubbles from the winding bundle.
  The secondary side is split into parallel secondaries S1 and S2 to handle full-load current (56 ARMS) while still using small diameter litz wire to achieve a high window fill factor. Due to the high operating frequency of 500 kHz, any mismatch in winding leakage inductance will result in unequal current sharing among parallel windings. An "Intra-leaved" winding structure is proposed as shown in Fig. 1. With "Intra-leaving", the winding impedance of S1 and S2 are well matched resulting in improved current sharing over the non-"Intra-leaved" case. Current sharing results are shown in Fig. 2, which validates the impact of the proposed winding strategy. Without "Intra-leaving" S1 takes 53 APeak and S2 takes 27 APeak. With "Intra-leaving" S1 takes 42 APeak and S2 takes 38 APeak, an almost perfect current split between the parallel windings S1 and S2. The proposed winding method, potting method, and analytical insights are applicable to future high-power, high-density planar transformers.

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