Design considerations of a high power factor SEPIC converter for high brightness white LED lighting applications

Abstract

A current-shaper SEPIC converter for high brightness LEDs is presented. Due to the recent advancement in the light emitting diode (LED) technology, high brightness white LED becomes feasible in residential, industry and commercial applications to replace the incandescent bulbs, halogen bulbs, and even compact fluorescent light bulbs. In these offline applications, high power factor, and low harmonics are of primary importance. It is also important to step up/down voltage with simple single-stage conversion for low cost and high efficiency. A SEPIC converter is particularly suitable for non-isolated application since it is single-stage, and can step up/down, and of high power factor as it is run in discontinue conduction mode. In this paper, we proposed a high power factor SEPIC converter for the high brightness LED lighting applications with universal input voltages and dimmable LED light. The condition of discontinuous conduction mode is given. The harmonics of the input line current is reduced and power factor is high because the input inductor current follows the input voltage. A current feedback loop is proposed to control the LED brightness. The gate drive signal of the switch is generated by comparing the saw-tooth carrier signal of the PWM controller with the current feedback signal. This circuit has the advantages of one stage of power conversion, no need to sense the input voltage, simple feedback control, voltage step-up and down, high power factor and dimmable LED current. It is particular suitable for the offline LED applications. The critical design constraints and equations for both the power stage and control loop are highlighted and detailed. A practical evaluation board is developed to verify the proposed design.