A Three-Level Half-Bridge Flying Capacitor Topology for Single-Stage AC-DC LLC Resonant Converter

LLC resonant converters are one of the best choices in AC-DC converters. But, low power factor is a serious problem in AC-DC LLC resonant converters. Adding a boost circuit in front of the converter improves the power factor albeit the increase in circuit complexity. To reduce the size and cost, both the boost and LLC stage can be controlled by the same switches. However, these single stage topologies are less efficient in regulating DC bus capacitor voltage pertaining to transients in the line voltage and load conditions. The paper proposes a new single stage topology for AC-DC LLC resonant converters. The circuit consists of three-level half-bridge flying capacitor topology. Discontinuous conduction mode ensures inherent unity power factor operation. Switching frequency control and PWM duty ratio control are used to regulate the output voltage and bus capacitor voltages of the converter respectively. This dual control scheme makes the converter DC bus voltage nearly constant over a wide range of load and line voltage variations. Furthermore, the three level topology guarantees reduced switching stress and losses. The operation of the topology has been validated using MATLAB simulation.