Abstract
M.Ing.
This study focuses its attention on conducted common mode EMI. Common mode current is the current that flows from an electrical circuit to a zero reference plane and back to the circuit again. It is known that the manner in which the common mode current flows is through stray capacitances that form between the electrical circuit and the zero reference.
This study was aimed at developing a method to measure the value of the stray capacitance of a switching circuit.
Determining the value of the stray capacitance by taking physical measurements on a circuit board is a challenge for a number of reasons, one of which is that great care should be taken not to add to the stray capacitance by means of the measuring equipment.
By measuring the value of the stray capacitance, it will be possible to model the occurrence of Common Mode EMI better and more accurately. This could, in turn, lead to a reduction in EMI.
Any body of an arbitrary shape, size and material exhibits a self-capacitance with respect to a zero reference frame. This principle, together with the principle of conservation of charge, also applies to electrical components, or circuits as a whole.
The experimental work was performed on a buck DC-DC converter. The circuit was simplified to aid in analysis. By varying the value of an external capacitance and taking basic measurements, it is then possible to uniquely determine the absolute values of the self-capacitances.
For every pair of external capacitance values placed in the circuit (of which one can be =0pF), a value for stray capacitance is calculated. Many data points were recorded with many different external capacitors in the circuit, resulting in a variety of stray capacitance values. In order to obtain a single value, a weighted mean of all the values was calculated.
The values obtained in this proposed method of measuring the stray capacitance compares well with that obtained using the Finite Element Method.
The advantage of the method presented here is that the self-capacitances are determined under the actual operational conditions, no specialised equipment is required and no unique handling of parasitics is needed. The method relies on very simple measurements and no complex data manipulations are required.