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Magnetic and Thermal Design of Litz wire 500 kHz High Power Planar Transformers with Converging Cooling Duct for a dc Transformer Resonant Converter Applications

Current Sharing Waveforms
Fig. 1. Current sharing results with and without "Intra-leaving"
  This work presents the design and analysis of two Litz wire transformers for a 500 kHz, 18 kW partial power processing converter (PPP). Because the two power paths in the PPP operate as “dc transformers” (DCX), both transformers are designed with the goal of leakage inductance minimization in order to reduce gain variation around the resonant frequency. The selected winding topology with the lowest leakage inductance results in an impedance mismatch among parallel secondaries used in the majority power path transformer, resulting in poor current sharing. In order to balance the goals of leakage inductance minimization and even current sharing, a new winding technique called “intra-leaving” is presented which reduces current sharing error from 50% to 5%. Due to increased loss per unit volume in power transformers when operating at higher frequencies, the increase in cooling system size can outweigh the transformer size reduction benefits.

  Conventional air-cooling design simply adopts an over-sized high airflow fan to keep the transformer within a desired thermal limit, which reduces power density and efficiency. This work seeks to address this trade-off by presenting a high density, low profile, forced air cooling duct system for high frequency transformers. Different cooling duct designs are presented and analyzed considering the trade-offs between design complexity and cooling performance. A pair of 500 kHz planar transformers with 18 kW ratings are used to demonstrate the proposed cooling system and achieves a power density of 635 W/in3, peak height of 43 mm, cooling power consumption of 6.8 W, and 8% peak temperature reduction when compared to cooling the transformers with a 120 mm x 120 mm, 25 W fan. Peak winding temperature at 15 kW load was 106° C and peak core temperature was 72° C. The proposed transformer with cooling ducts also achieves four times higher power density than the set up using a 120 mm x 120 mm fan.
Picture of Cooling System
Fig. 2. Proposed Cooling System with Converging Cooling Ducts

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