This paper considers an ideal planar transformer wherein only the electromagnetic parasitics (stray capacitive and leakage inductance) arising out of the transformer geometry are taken into account, assuming lossless conditions. A suitable electrically equivalent circuit model for the planar transformer is used to analyze its frequency and power transfer characteristics; this model was validated by a three dimensional electromagnetic simulation of the planar transformer structure in FEKO electromagnetic simulation software. The effect of dielectric thickness on the bandwidth of the transformer has been analyzed based on the premise that the inherent stray capacitance and leakage inductance elements would affect the power transfer characteristics of the transformer. It has been found that the dielectric thickness of a planar transformer can be optimized so as to maximize the frequency bandwidth. It is also shown that the bandwidth is found to be sensitive to the thickness of the dielectric beyond the optimum thickness threshold topt. Convenient closed form analytic expressions for the optimum dielectric thickness and the resultant maximum bandwidth are derived and presented. It is argued that these results can be readily used to benefit the design of air-core PCB/Planar transformers.
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