Design and analysis of an AC-DC LLC resonant converter with new bus voltage stabilization technique

LLC resonant converter is an attractive choice for isolated power conversion stages, particularly because of its high efficiency. However, low power factor is an important issue in AC-DC LLC resonant converters. To improve the power factor, a boost power factor correction stage is usually required at the front end. To reduce the size and cost, both the boost and LLC stages can be controlled by the LLC switches while operating the boost converter in discontinuous conduction mode. However, line voltage variations and transient load changes can cause wide variations in the bus capacitor voltage which can lead to bus capacitor failure and unstable operation of the converter. In this paper, a simplified topology is proposed to avoid these issues. The converter can return the additional energy stored in the bus capacitor back to supply and thereby maintaining the bus capacitor voltage level. Design equations are derived for the converter and the circuit operation is verified using MAT LAB Simulink.