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This paper proposes a transformerless single-phase single-stage high step-up ac-dc converter based on symmetrical Cockcroft-Walton voltage multiplier (CW-VM) with only adding two bidirectional switches and two boost inductors. With the help of the boost structure, not only the voltage gain can be higher than that of the conventional symmetrical CW-VM but also the power factor correction (PFC) methods for conventional boost ac-dc converter can apply to the proposed topology to achieve high quality of line conditions and regulated dc output. In this paper, a commercial average-current controller (ICE1PCS01) based on quasi-steady-state approach by using one cycle control technique is used to implement the PFC function. By this way, the multiplier and line-voltage sensor are unnecessary as compared with the multiplier approach; it simplifies the controller design and reduces the circuit components and cost. A 250W laboratory prototype is built for testing, and experimental results demonstrate the validity of the proposed converter.