Stray capacitance of a two-layer air-cored inductor

A new analytic method for predicting the stray capacitance of a double layer coil with conducting shield and air core is presented. The method involves construction of a two-layer turn-to-turn circuit model, which includes the self-inductance and resistance of each turn, the mutual inductance representing the flux linkage between all turns, and the capacitance between adjacent turns and the capacitance between the outer turns and conducting shield. At high, low and zero frequency the model is reduced to an effective capacitance, inductance, and resistance respectively: for frequencies below the first self-resonant frequency the behaviour of the circuit is approximated by an RLC circuit. A new result is a recursive calculation of the stray capacitance of the high frequency circuit model. The method and stray capacitance value are compared to alternative models and stray capacitance predictions, and the limitations of the different approaches discussed. Comparisons with experimental measurements show good agreement at the fundamental self-resonance and confirm that the effect of a conducting shield is likely to be small for a closely-wound coil.