LLC resonant converters possess excellent soft-switching characteristics and are extensively applied in the dc–dc converter field. To achieve closed-loop control of the LLC resonant converter, it is crucial to establish an accurate small-signal model. Traditional modeling methods suffer from the problems of decreasing accuracy away from the resonant frequency or high model complexity in practical applications. This article proposes a small-signal modeling method for an LLC resonant converter based on time-domain correction, which provides practical analytical small-signal models below, at, and above the resonant frequency. The proposed method strikes a balance between accuracy and complexity. Equivalent models of components and the average equivalent circuit of the LLC converter are established using simplified time-domain analysis to describe the average characteristics of the LLC converter. Small-signal perturbations are applied to the average equivalent circuit to derive an analytical second-order average small-signal model that can guide closed-loop regulator design and stability analysis of the LLC converter. Finally, the correctness of the proposed small-signal modeling method is verified through PLECS ac sweep simulations and experiments.

The traction converter is the core component of an electric multiple unit (EMU), with the Insulated-Gate Bipolar Transistor (IGBT) serving as the key element that directly determines the operational status of the entire traction system. Predicting IGBT service life is therefore essential for fault prediction and health management in trains, providing critical support for ensuring operational safety, guiding condition-based maintenance decisions, and reducing life-cycle costs. In this study, we summarize the basic structure and failure mechanisms of IGBTs and propose a theoretical relationship between their performance parameters and degrees of aging, subsequently developing a method for predicting IGBT service life based on void ratio and service time. Additionally, a DC power cycle aging test is designed and carried out on the IGBT module according to the actual working conditions and the range of acceptable junction temperatures. The life prediction method proposed in this paper is used to analyze the test data, providing the predicted service life of the traction converter’s IGBT module. The results show that a new IGBT module can be used for 29.66 years or 17.8 million km, which can be used to support future maintenance work.