An AC/DC LED Driver with Unity Power Factor and Soft Switching

Traditional light-emitting diode (LED) drivers with pulse-width modulation (PWM)-type converters suffer the problem of hard switching, leading to low circuit efficiency and low reliability. LED drivers supplied by alternating current (AC) line source generally require using an additional power-factor correction (PFC) stage to satisfy the regulations on power factor (PF) and total current harmonic distortion (THDi). It results in more circuit losses, especially when the active switch of the PFC stage operates at hard switching. This paper presents an alternating current-to-direct current (AC/DC) converter for driving high-brightness LEDs with the features of soft switching and high PF. The proposed single-stage circuit is formed by integrating a buck–boost converter and a buck converter. By elaborately rearranging the wirings between the circuit components of both converters, the power MOSFETs can be switched on at zero voltage. The operating modes at steady-state are analyzed and the mathematical equations for deriving circuit parameters are conducted. Finally, a prototype circuit for driving 60-W LEDs was built and measured. Based on the experimental results, the feasibility and satisfactory performance of the proposed LED driver are proved.