Today, with the rapid growth of electronic equipment, Demand for DC power supplies is growing. Rectifiers are widely used in application industries to convert main AC voltage to DC voltage. Since rectifiers include diodes and large capacitor that are nonlinear loads, Therefore, electronic equipment injects harmonics into the power system, which results in increased Total Harmonic Distortion (THD). Rules and standards such as the International Electrotechnical Commission (IEC 61000-3-2), limit the harmonics produced by these devices. At present, Power Factor Correction (PFC) circuits with a wide range of industrial applications is an effective method to meet the aforementioned standards. Single-phase and three-phase ac–dc power converters can be implemented with PFC for low-power and high-power applications, respectively. Among the many options, the Boost converter is widely used as a power factor correction. This converter has several advantages such as simple control and high PF. However, its output voltage should be higher than the peak voltage of the input voltage. Therefore, the researchers introduced the Buck and Buck-Boost converters as PFC for applications that demand low output voltage for electronic equipment. However, for buck types PFC converters, when the output voltage is higher than the input voltage, the dead angle emerges in the input current, which leads to low PF and high current distortion. In this thesis, two bridgeless PFCs with input single phase and a buck-boost PFCs with input three phase are introduced. at first of the thesis, the definition of power factor and its mathematical relation are presented and then advantages and disadvantages of basic switching converters are mentioned. To increase the efficiency, bridgeless PFCs are reviewed. In bridgeless PFCs, rectifier is combined with the PFCs converter to create a new converter. Because the number of semiconductor devices are reduced in bridgeless PFCs, the conduction losses are reduced and efficiency is increased. In the first proposed PFC, converter operates at Discontinuous capacitor voltage mode (DCVM) and discontinuous inductor current mode (DICM); hence inherent PFC is achieved with a simple pulse width modulation(PWM) control. Also, the switch of the proposed converter can operate with Zero Current Switching (ZCS) turn-on and Zero Voltage Switching (ZVS) turn-off without applying any extra element. The second proposed PFC, using lossless snubber, converter can operates under ZVS condition. Laboratory prototype of the proposed bridgeless PFCs are designed and implemented to verify validity of theoretical analysis. Furthermore, the input current of the proposed converters are dead angle free. For design high power converters, three-phase converters to be considered as a better option than single-phase converters. Therefore, in the third proposed PFC, converter with three phase input is simulated. The third proposed PFC structure improves the power density by employing two switches. Due to reduction of the number of semiconductors in the power flow path and providing soft switching condition for semiconductors, efficiency of proposed bridgeless PFCs are high and above %92. Power factor correction - Soft Switching - Dead angle - Total Harmonic Distortion (THD)