With progress of power electronics domain in new generation aircrafts, hydraulic and mechanical systems have replaced with electrical systems. By increasing the power consumption, the importance of Power Factor Correction (PFC) subject in aircraft electrical system has increased and stringent standards such as DO-160D and ISO-1540 have introduced on the power factor and harmonic currents of aircraft electrical system AC-DC converters. Boost converter with continuous input current, high power factor and low EMI is the most commonly used front end PFC converter. For driving the low voltage loads, a second stage step-down DC-DC converter is used to decrease the high output voltage of the boost converter. The high output voltage of the boost converter increases the switching losses and the voltage stress of the second stage switches and reduces the isolation transformer efficiency and converter reliability. Recently, the use of the buck PFC converter instead of the boost converter in the front stage has been considered with the aim of reducing voltage stress of the elements, reducing switching losses and increasing reliability. But the presence of dead angle in the input current of this converter decreases the power factor and increases input current harmonics. The aim of this study is to present buck PFC converters to reduce the input current dead angle and provide unity power factor and low harmonic input current. The proposed converters have bridgeless structures to reduce conduction losses and simultaneously have soft switching features to reduce switching losses. In this study, three new structures for the Buck PFC converter have been introduced. In the first proposed converter, a step-down PFC converter with unity power factor and bridgeless structure operating in discontinuous conduction mode (DCM) has been presented. In the positive half-line cycle, a buck converter and an auxiliary series flyback converter are used to decrease the input current dead angle. In the negative half-line cycle, a flyback converter has used. The element counts of this converter has reduced respect to same ones that provides a simple structure and low cost. By reducing the input bridge diodes, only two semiconductor components are at the power path so, the conduction losses of this structure have been decreased. Then, using the auxiliary circuit with passive components, the soft switching feature has also been provided. In the second proposed converter, the method of using series auxiliary flyback converter is applied to two buck converters so that the leakage inductance of the flyback converter is resolved and continuous conduction mode (CCM) of operation can be applied. Then, to simplify the second proposed converter structure, the idea of unidirectional switches is introduced so that the dead angle removing circuit can be applied for both half-line cycles. In the third proposed converter, by using an auxiliary resonant circuit and its merging with the auxiliary flyback, zero voltage switching of switches has been provided. Also, this converter has unity power factor and bridgeless structure. Keywords : Single phase step-down PFC converter, aircraft electrical system, bridgeless AC/DC converter, soft switching
