Exact design of an LLC resonant converter for Wireless Power Transfer

The main goal of many power electronics circuits is to ensure transfer of electrical energy with high efficiency and with a good power density, i.e., the ratio between power and volume occupied. Typical converters, based on PWM, cannot achieve high efficiency at high frequency because they work in hard-switching mode. In fact, during switch’s transitions there is an overlap between the voltage across the power switch and the current through it, which causes the largest part of converter losses. In many times, these converters are replaced by resonant soft-switching converters. As a matter of fact, these converters are capable of a better efficiency at high frequency since they can avoid the overlap between current and voltage. Thanks to the high operating frequency, they can be designed with smaller reactive elements (mostly inductors) providing a higher power density. Currently, the LLC is one of the most employed resonant converters. Its name derives from the main circuit topology, which is composed by two inductors (LL) and one capacitor (C). LLC resonant converter can perform power conversion through frequency modulation (instead of PWM), and it can easily achieve high efficiency. Nevertheless, due to the intrinsic nonlinearity and the presence of the three reactive elements (two inductors and a capacitor), the design of LLC resonant converters is not an easy task to be performed. In the literature the vast majority of the publications which refers to LLC resonant converters, bases its design on the so called First Harmonic Approximation (FHA). This is a simplified approach where current and voltage signals in the converter that are sinusoidal-like, are assumed to be purely single sinusoidal tones. The analysis by means of FHA is strongly simplified and introduces strong approximations which cannot be accepted in practice. Furthermore, FHA is not capable to consider any sources of loss due to non-idealities of the circuital elements. The aim of this thesis regards the design optimization of an LLC resonant converter for Wireless Power Transfer (WPT) by means of a semi-analytical design approach, in which the main sources of losses can be considered. In particular, when the LLC is adopted for WPT, the transformer is replaced with two coupled inductors, which are not well coupled and therefore lead to high leakage inductances. As counteraction, we avoid one inductor of the LLC and exploit the leakage one instead. At the end of this thesis, the exact semi-analytical design is obtained through a switching sequence technique implemented by means of MATLAB scripts. The MATLAB results have been verified to be in agreement with the LTspice simulations. In conclusion, the limits of the LLC converter, when employed for WPT, are highlighted.