Modelling approaches for audio frequency power transformers

Abstract: The development of HVDC converters, local generation andtractions systems increases the demand for power converters.While a lot of studies have been conducted on the powerelectronics, there are still open questions concerning thecontained power transformer that operates at a few kilohertz.As power and loss increase with frequency, a smallertransformer can be constructed supposed cooling is increased orlosses are reduced. Losses can be reduced by use of a low losscore material. A DC-DC converter is likely to be of resonanttype, which puts constraints on the leakage inductance of thetransformer and other parasitic elements. These factors can beinfluenced by changes in the topology of the transformer, i.e.the geometry.Novel modelling approaches have been studied and comparedwith existing models. This concerns the electrical, magneticand thermal behavior of the transformer. The overall model hasbeen run for a realistic geometry. A new field homogenizationprocedure for a magnetic lamination is suggested. Simulationsprove that this method allows reducing the required number oflumped elements. A novel 2D measurement set-up forcharacterization of magnetic sheets has been designed andconstructed to operate up to a few kilohertz. The experimentalverification of the set-up proves that the magnetic fieldamplitude and especially homogeneity increased considerably.The latter allows larger pick-up coils. The set-up is muchsmaller than the previous and allows small samples, which isimportant in the audio frequency range, where core materialoften is manufactured on small rolls. In addition, thenecessary electric excitation decreased substantially.Measurements are required to characterize and evaluatetentative transformer core materials such that they can be usedas a material database for a simulator program. The first stepwas the development of a measurement program includingautomatic demagnetization. The hysteresis model developed atthe department was tested successfully. An idea is formulatedto optimize the model numerically.Keywords:Power transformer, Audio frequency, Loss,Hysteresis, Eddy currents, Skin effect, Leakage inductance,Parasitic elements, Thermal effects, Modelling approaches,Magnetic measurement set-up, Magnetic materials.