Accurate prediction of high-frequency power-transformer losses and temperature rise

The high-frequency power-transformer design equations for winding and core losses and temperature rise were reviewed from literature and formulated for spreadsheet calculations using material (Steinmetz loss coefficients, ferrite resistivity, copper resistivity), geometry (core area, core length, winding area, winding length), winding (number of primary turns, copper fill factor, primary to secondary area ratio) and excitation (input voltage, switching frequency, duty cycle, current harmonic components) parameters. The accuracy of each design issue was first validated and quantified separately using regression analysis. Calculated core losses, winding AC-resistance equations and heat transfer capacity calculations were compared with the results from calibrated heat sink measurements, finite-element method (FEM) analysis and measurements using thermal test blocks, respectively. Finally three EFD20 type transformers (solid wire, noninterleaved foil and interleaved foil winding) were fitted into an active clamp forward converter (100-300 kHz switching frequency, 0-78 W throughput power) for comparison between the theory and experiments. Standard error of predicted core losses and heat transfer capacity were determined to be 0.0581 and 0.15 W, respectively. The results of the in circuit tests suggests that the transformer total losses can be predicted with the average standard error below 0.2 W with datasheet type information only. The most significant uncertainty was heat conduction through and losses generated in the interconnecting wires between the test transformer and other converter components.