Due to ever-increasing demand for dc–dc converters, using rectifiers is unavoidable. Conventional rectifiers encounter excessive peak input current and total harmonic distortion (THD) which reduces power factor (PF) to about 0.5–0.7. In order to increase PF, PF correction (PFC) converters are employed to decrease these harmonics. A conventional form of a PFC converter, which is usually controlled by average current pulse width-modulation (PWM) method, is a full-bridge rectifier followed by a boost converter . It is desired to reduce the volume and weight of a converter by increasing the switching frequency. Increasing the switching frequency would result in higher switching losses which makes employing soft-switching techniques unavoidable. Consequently, dc–dc power supplies with higher power density and improved efficiency can be obtained. Many ac–dc topologies are introduced to improve the efficiency and PF simultaneously. In order to improve efficiency, switching losses and conduction losses should be reduced. To reduce conduction losses, the rectifier circuit and the PFC are combined to create bridgeless PFC . This combination decreases the conduction losses by reducing the number of semiconductor components in the line current path. To further improve the efficiency by reducing switching losses, various soft-switching techniques are applied to bridgeless PFC converter. In most of these topologies, switch voltage or current stresses are increased. Zero-voltage transition (ZVT) and zero-current transition (ZCT) are soft-switching techniques which provide soft switching while the desirable features of conventional PWM converters remain. ZVT techniques eliminate the turn-on capacitive losses, and thus, MOSFETs are preferred. However, at highpower applications, insulated-gate bipolar transistors (IGBTs) are used because of their lower conduction losses and cost. To eliminate tailing current problem of IGBTs, ZCT techniques are preferred. The ZVT converter provides soft-switching condition for the main switches, but the auxiliary switch is hard switched. In comparison to fully soft switched converters similar to the proposed converter in this paper, the efficiency is usually lower. In addition, in this converter, di/dt and dv/dt are higher than that of the proposed converter, which results in higher EMI. In this thesis, after a survey on isolated and non isolated PFC converters and their problems, some of the applied soft switching methods on non isolated bridgeless PFC converters are presented. The non isolated PFC converters can shap the input current of the rectifiers according to the related standards but because of the variable input power of a sinusoidal voltage, there will be some rippels in the output voltage. Then the proposed PFC converters are presented. The proposed converter is a Zero-Voltage-Switching bridgeless PFC which uses an improved auxiliary circuit to achieve Zero-Voltage-Switching conditions for its main switches and diodes. In this converter all of the resonant current flowes through the auxiliary circuit so there is no extera voltage or current stress on the main switches and diodes, also the auxiliary switch operates in the ZCS conditions so it dose not introduce any switching loss, In addition to the auxiliary circuit the converter uses lower component count compared with the similar ZVS converters. the converter is analysed and a design strategy is presented for each of them and results are verified with PSIM simulation and experimental results. Keywords: Power factor correction (PFC), Bridgeless PFC, Soft switching
