Stray capacitances of two winding transformers: equivalent circuit, measurements, calculation and lowering

As long as it works linearly, a two winding transformer is, from an electrostatic point of view, a three port system. Its capacitance matrix includes six independent values. Consequently, whatever its shape, introducing six capacitances in the right places of the equivalent circuit allows the user to account for the whole electrostatic behaviour of this component. With such an equivalent circuit, resonance frequencies can be computed and, reversely, the six capacitances are all deducible from measured resonance frequencies. Thus, using this circuit by hand or by software, interwinding currents of real electronic circuits can be forecasted together with parasitic resonances related to the transformer. After this global approach, a microscopic approach is adopted. A closer look at the winding layer shape and then, at the wire shape itself leads to two simple models which allow the six capacitances of the equivalent circuit to be computed by hand. Values found are approximate but, thanks to analytical expressions, two design rules leading to lower capacitance values are inferred. Measurements are presented to establish the reliability of the equivalent circuit and to evaluate the interest of the proposed designing rules