Design and Optimization of HF Transformers for High Power DC-DC Applications

Sammanfattning: Increasing the operational frequency is the most common solution to achieve higherpower densities, since the weight and volume of the magnetic part, the bulkiestelement in power electronics converters, are then decreased. This solution is wellestablished in low power high frequency applications, while in the recent decade,the possibility of utilizing high frequency at higher power and voltage levels hasgenerated wide interest as well.This work proposes a design and optimization methodology of a high power highfrequency transformer accounting for the tuned leakage inductance of the transformer,as well as high isolation requirements, particularly in DC offshore applicationwhere a converter module should withstand the MVDC or HVDC link voltage. Toachieve this goal, several models were proposed and developed in order to accuratelycharacterize such a transformer. One of these models is a so called pseudo-empiricalexpression derived from a rigourous regression algorithm based on an extensive 2Dfinite element simulation scenario, resulting in an accurate analytical expressionwith an average unsigned deviation of 0.51% and the extreme deviations not higherthan 9%. Moreover, using the energy method, an analytical expression to preciselycalculate the leakage inductance of high power density magnetic components is proposed.In addition, using the proposed modification of the Steinmetz equation forcore loss calculations, general expressions are derived and presented for a rectangularwaveform with its associated duty cycle and rise time.Applying the proposed design methodology, in which all the aforementionedmodels are implemented on a 1 MW case study transformer, indicates that sucha transformer can achieve a power density of about 22 kW/L and the efficienciesas high as 99.74%. Moreover, with respect to the isolation requirements, desiredleakage inductance and the magnetic material used, a critical operating frequencycan be found above which the transformer does not benefit from volume reductionanymore.