Power loss estimation in LLC MOSFETs : a time interval analysis

LLC resonant converters have become a mainstream topology in DC/DC power conversion applications such as electric vehicle charging, renewable energy generation and low power energy conversion. This converter presents advantages such as the soft-switching of active devices, which reduces power losses in the energy conversion process, allowing for a more efficient operation when compared to hard-switched topologies. Losses in the MOSFETs of the inverting and rectifying stages of this converter should be accurately determined so to allow for proper heat management design and thermal dissipation. However, determination of losses through simulation can be challenging due to the significant difference between time constants of electrical and thermal phenomena. Moreover, the information presented by the datasheet of power electronic devices is often limited to select operating points, which may compromise the accuracy of power loss estimation. In order to overcome the limitations imposed by the datasheet, a detailed characterization of the main loss mechanisms in the operation of LLC MOSFETs is presented. To avoid the time-consuming and computationally-intensive process of simulation, steady-state time-domain expressions for the converter are developed, based on the electrical behavior of the topology. These equations, based on the Time Interval Analysis, are able to predict key electrical and thermal behaviours based on circuit design considerations and operating conditions, being easily implementable in software such as MATLAB or MS Excel. As verified by simulation and experimental results, estimation of losses using the proposed method is considerably more precise than using the well-established yet oversimplified First-Harmonic Approximation (FHA). In the inverting stage, the observed error in loss determination is reduced from an average of 19%, using FHA, to 2.8% using the proposed method. When it comes to the rectification portion of the circuit, the reduction in error observed is from 12% to 2%. Such improvement in power loss estimation before the converter is built is fundamental for the design of an agile and cost-effective thermal management approach which guarantees the integrity and reliability of the power electronics device.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofGraduat