AN3111 Datasheet, PDF (1/23 Pages) STMicroelectronics – 18 W single-stage offline LED driver

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
market. In September 2007, the US department of energy (DOE) issued ENERGY STAR^®
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.
February 2011
Doc ID 16815 Rev 2
1/23
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