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The paper deals with the analysis of the characteristics of the resonant converters that make them suitable to be modelled and studied using the envelope approach, which is based on the hypothesis that the converters process in a liner way the envelope of the input quantities. The Modulated Variable Laplace Transform method is reviewed and applied to find the envelope model of a wireless power transfer battery charger, considered as study case converter; then it is shown that in certain conditions the envelope model fails in reproducing the system behavior because the latter one performs a non linear processing of the input envelopes. Different types of not linearity are recognized, and the causes of each of them are investigated, detailing the characteristics of the transfer function of the converter that make it more or less suitable for an envelope modelling. The qualitative evaluation of the results coming from the simulation of the study case system is carried out in parallel to the development of a general mathematical analysis of the behavior of a system supplied by a modulated quantity so that each step of the latter one is validated before going ahead to the next and the obtained results can be applied to any kind of system.