Design and identification of an equivalent circuit for an LCT component: inventory and representation of losses

A new kind of passive component, the LCT [integrated inductor (L), capacitor (C), and transformer (T)] is becoming very fashionable in the switch mode power supply area. Starting from our knowledge in transformer characterization, we recently elaborated an equivalent circuit for this component. This equivalent circuit is fully deducible from impedance measurements. No dismounting of the component is needed, and no information about its internal design is required. This paper aims mainly to present the new equivalent circuit and the method leading to its identification using a commercial impedance analyzer. The LCT intended to a 300-W power supply working at 200 kHz has been built and characterized between 100 Hz and 40 MHz. Analyzing the frequency dependence of the real part of serial and parallel impedances, several kind of losses rarely taken into account have been identified. Among these losses are those related to the capacitor insulator and those due to the dielectric constant of Mn-Zn ferrite core. Equivalent circuits are supplied to represent every type of loss, and their accuracy is checked.