Analysis, Design and Construction of an LLC Resonant Converter

Abstract

The trend in DC/DC converters development is toward higher frequency, power density and efficiency. The traditional hard-switched converter is limited in switching frequency and power density. The phase-shift full-bridge PWM Zero-voltage-switching (ZVS) converter has been used widely because of its ZVS working condition. But it has a problem due to the reverse recovery of the diodes, which reduces its efficiency. Nowadays, the LLC resonant converter is a popular research field to consider for increasing converter efficiency. In this thesis, the conditions to achieve the ZVS mode of the LLC resonant converter are studied using the method of fundamental element simplification. Six different conducting stages have been introduced individually and analyzed. Also, the operating region of the LLC resonant converter has been studied, along with the relationship between the input and output voltage as affected by the load and the switching frequency. After analyzing the DC characteristics of the LLC resonant converter, its small signal model is presented and is developed using the method of extended describing functions. Using this model, a voltage control was designed and implemented using an MC34066 analog control chip. A prototype LLC resonant converter was constructed and tested in the laboratory.