February 2011

Doc ID 16815 Rev 2

1/23

AN3111

Application note

18 W single-stage offline LED driver

Introduction

With the rapid development of high brightness LEDs, SSL (solid state lighting) has begun to

move from being a niche market to penetrating residential markets. There is a large

potential market for the residential application of SSL, and CFL (compact fluorescent lamp)

retro-fit is part of it. Standardization of SSL products is helping to lead the growth of the

criteria for SSL products. To meet the ENERGY STAR specifications for SSL products, the

power factor of power supply must be higher than 0.7 for residential applications. For CFL

retro-fit applications, cost, size, and reliability are very important. To achieve a high power

factor, either passive PFC (power factor correction) or active PFC can be used. Typically,

passive PFC requires large passive components, which makes it difficult to maintain a small

size. Traditional active PFC circuits require a two-stage topology, which entails a boost stage

for PFC, and then buck or flyback for current regulation of the LEDs. The cost of the two

stages is high. In this application note, a non-isolated, soft-switched, single-stage high

power factor offline LED driver is introduced. The buck-boost converter is chosen for this

application due to its simplicity and low cost. The converter operates with constant peak

current for constant power control, and in transition mode (boundary mode between CCM

and DCM) to achieve soft switching, using the L6562A controller. High power factor is

achieved by reshaping the peak current near the zero crossing of the input AC line.

Figure 1.

18 W single-stage offline LED driver board

For this particular design, the LED string consists of 18, 1 W white LEDs in series. Isolation

is not required. The goal of the design is high power factor, high efficiency, simplicity, and

low cost.

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