Overcoming the Effect of Test Fixtures on the Measurement of Parasitics of Capacitors and Inductors

This paper analyzes and compares available measurement techniques that can be used to assess the effect of parasitics on the impedance of capacitors and inductors within the frequency range, where most electromagnetic compatibility regulations impose limits to conducted emissions of power converters. Direct measurement of impedance is compared with measurement of the transmission coefficient of the component. For impedance measurement, a criterion is identified to determine whether cables and test fixtures used to connect the component to the measuring device will have an impact on the measurements. It is shown that this criterion is more restrictive than the well known requirement of electrically short cable. We also show that, by contrast with direct impedance measurement, the measurement of the magnitude of the transmission coefficient is not affected by the cables in the frequency range of interest. Therefore, to carry out compensation previous measurements are not required. This permits us to quickly evaluate the effect of parasitics on the impedance of inductors and capacitors and also allows us to readily construct a circuit model for the component, which can be used to improve accuracy in the prediction of conducted emissions of power converters by simulation. Experimental validation is presented for practical capacitors and inductors.