Magnetic and Thermal Design of Litz­wire 500 kHz High­power Planar Transformers with Converging Cooling Duct for “dc Transformer” Resonant Converter Applications


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Virginia Tech


This work presents the design and analysis of two Litz wire transformers for a 500 kHz, 18 kW input­parallel output­series partial power processing converter (IPOS PPP). Because the two power paths in the IPOS 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 “intra­leaving” is presented which reduces current sharing error from 50%, to 5%. A design rule for “intra­leaving” is also established which extends the winding method to different winding configurations and higher numbers of parallel winding. A novel cooling duct designed with computational fluid dynamics is used for transformer thermal management. The cooling duct uses two 30 mm 7.7 CFM fans to cool the transformer winding and achieves a small height of 43 mm and only 6.8 W power consumption. Using the cooling duct, 106 °C peak winding temperature and 76 °C peak core temperature is achieved at 15 kW load, an ∼ 8% reduction compared to using a conventional 120 mm fan 41 CFM fan. The two transformers with the cooling system achieve 635 W/in3 power density, 1U height compliance, and 99.4% peak efficiency.



High Frequency Transformer, Thermal Design, Cooling Duct, Planar Transformer, Resonant Converter, Litz Wire