Experimental Extraction of Parasitic Capacitances for High-Frequency Transformers

Parasitic capacitances of the high-frequency (HF) transformer used in dc-dc converter impact harmonic analysis of the transformer and interactions with the converter. Resonant frequencies of open- and short-circuit impedances characteristics (ICs) have been widely used to extract parasitic capacitances of HF transformers, but it was reported that some resonant frequencies were unobtainable as they were too high where the ICs fluctuated heavily, and parasitic capacitances remained unevaluated. In this paper, resonance features of ICs and voltage transfer characteristics (TCs) are investigated, and a new frequency of voltage TC is used for extracting parasitic capacitances. By analyzing inner links of known impedance resonant frequencies, why and which resonant frequencies are likely to be unobtainable is revealed. Furthermore, a new frequency corresponding to zero of TC, accounting for parallel resonance between leakage inductance and mutual capacitance of the two windings, is used to extract parasitic capacitances. With this new frequency mutual capacitance of the windings is evaluated, and missing equation due to unobtainable impedance resonant frequency is added. Besides, merits and limitations of extracting parasitic capacitances based on ICs and TCs are analyzed. These techniques are verified with several HF transformer prototypes in the laboratory.