Developments for the high frequency power transformer design and implementation

Abstract

The thesis considers design and manufacturing of ferrite based high frequency power transformers. The primary aim of the work was to study core and winding losses and in particular thermal modeling of high frequency power transformers and to determine appropriate loss and temperature rise modeling methods for power converter applications. The secondary aim of the work was to study improved, mass manufacturable winding methods for toroidal, tube-type planar and disc-type planar high frequency power transformers.

The analytical high frequency power transformer design equations for core and winding losses and transformer temperature rise were reviewed from literature, formulated for spreadsheet type calculations using excitation, material, geometry and winding implementation parameters and validated by in circuit temperature rise comparisons between calculated and measured values using regression analysis. The core and winding loss calculation methods in literature were found to provide appropriate accuracy for the practical design purposes. Thermal test block tests suggested a slight modification for analytical convective heat transfer equation from the literature. The results from in circuit temperature rise comparisons suggest that the transformer total losses can be predicted with the average standard error below 0.2 W with datasheet type information only. Further, if conductive thermal resistance from transformer via printed circuit board substrate to ambient is available the transformer operating temperature could be predicted with appropriate accuracy (5.6 °C) as well.

The new manufacturing methods developed for toroidal, tube and disc-type transformer geometries were proved to be suitable for high frequency operation. With a common mode choke with static shield and windings deposited and etched directly on the toroidal NiZn core a transfer loss resonant frequency above 1.2 GHz was achieved. A multilayer foil winding with interleaved primary and secondary layers resulted resulted leakage inductance of 10 % of the value achieved using a wire-wound winding. The new developments for Z-folded inductive components resulted material cost savings, reduction of winding resistance and adjustability of leakage inductance and winding capacitances.