Nowadays, by ever-increasing number of electronic devices, the number of connected rectifiers to the networks is increasing constantly. Because these rectifiers are non-linear loads, they produce current harmonics and the input current shape is distorted compared to its ideal sinusoidal form.This undesired harmonics cause problems such as: heating the motors and transformers, increasing current flow in parallel capacitors and in the neutral wire in three-phase four-wire systems, deforming the voltages shape and so on .Thus different standards have been imposed to restrict these harmonics. To meet these standards, a type of converter must be used in the input stage of rectifiers to increase the power factor which is called power factor correction converter.In this thesis different types of power factor correction converters have been examined. In general, these converters are divided into two groups of isolated and non-isolated power factor correction converters. In this group isolated converters (converters with DC transformer) are commonly used in low power applications, because of the isolation between input and output, the ability to have multiple outputs and also to increase the range of input to output voltage ratio.Two-stage power factor conversion converters are a general solution for improving the power factor. This type of converters have high power factor and fast output voltage response because of using two separate controllers for each stage. Main disadvantage of this type is its high cost because of its control circuits and power stages. In order to reduce the cost, the single-stage converter idea is developed, which merge the PFC stage with a dc/dc converter into one stage. In most of them the PFC stage switch is replaced by some elements like auxiliary winding, diode or passive components.Hard switching of power switch of the rectifier could make extra energy loss and therefore decreasing the efficiency of converter. Thus, with soft-switching the efficiently can be increased and the switch loss could be minimized. Some simple and basic topologies from each group are presented and analyzed. Moreover, problems, advantages and disadvantages of each group are described separately. After analyzing these converters, some solutions such as: achieving soft-switching conditions, reducing storage capacitor voltage and number of semiconductor elements on the power transition path (to reduce the conductivity loss), have been made to improve their performance. The first proposed converter is a single-stage single-switch soft-switching (S^6) converter which is based on a boost-flyback sheme. In this converter soft switching condition is achieved without using any additional switch. It also has fewer elements compared to similar converters. Also a solution for improving the performance of this converter is presented. The second proposed converter is improved ,quasi boost flyback converter. In quasi boost converter , the boost switch is replaced with a magnetic switch (combination of passive elements, auxiliary windings and diodes).The proposed converter reduces the storage capacitor voltage and increases hold up time of output voltage. Furthermore, the number of semiconductor elements in power transition path are reduced, thus the efficiency is improved. Because of reducing the storage capacitor voltage and possibility of increasing its capacity, this converter is appropriate for specific applications such as computer and digital applications. Operation principles of the proposed converters are fully analyzed and a prototype of each converter is realized. Experimental data verify the theorical and simulation results. Keywords: 1) power factor correctio 2) single stage converter 3) isolated converters 4) non-isolated converter 5) AC–DC converter 6) soft switching