Rapid simulation of multi-resonant LLCC converters with capacitive output filter based on an extended first harmonic approximation

In case of resonant converters, the corresponding steady-state analysis is in general more cumbersome than in case of PWM converters, as often a multitude of modes has to be regarded and resonant oscillations with durations impossible to calculate directly are encountered. Therefore, approximations are required in order to facilitate the design process. An extension to the well-known first harmonic approximation is investigated for the multi-resonant LLCC converter with capacitive output filter in this paper, revealing a new degree of simplification regarding the resulting closed-form solution. By means of this approximation, the design of the fourth-order resonant LLCC converter is significantly simplified, providing a tool for rapid simulation combined with a very high accuracy. The accuracy of the investigated approach was double-checked by means of exact calculations. The loss-breakdown on several prototypes shows a very high correspondence between prediction and measurement.