March 2009

Rev 1

1/21

AN2928

Application note

Modified buck converter for LED applications

Introduction

The use of high power LEDs in lighting applications is becoming increasingly popular due to

rapid improvements in lighting efficiency, longer life, higher reliability and overall cost

effectiveness. Dimming functions are more easily implemented in LEDs, and they are more

robust and offer wider design flexibility compared to other light sources.

Applications suitable for the use of LEDs include lighting for streets, stadiums, fairs and

exhibitions, shops, interiors, as well as for decorative lighting, outdoor wall lighting and

consumer lighting such as lamps and ballasts. Therefore, LED use for lighting is likely to

represent an increasingly large proportion of the lighting market in the future. To assist

engineers in their design approach, the STEVAL-ILL013V1 80 W offline PFC LED driver

demonstration board has been developed. This application note describes, step-by-step,

all the principles and calculations used for a modified buck converter intended for high

brightness LED applications.

The converter is designed as a constant current source to achieve the best lighting

performance from the LEDs. A “modified buck" topology was chosen because the power

switch is connected to ground rather than the high side switch, as in a standard buck

topology, so with this solution it is easier to control the switch. The design uses a fixed off-

time (FOT) network operating in continuous conduction mode (CCM), rendering the overall

solution simple and cost-effective. The modified buck converter described in this document

can be used for lighting applications from low power and low voltage, to high power and high

voltage. This allows designers to cover a wide range of different LED systems using a single

topology.

Additionally, in lighting applications where the input active power is higher than 25 W and

a high power factor is required, the high PF converter can be connected as the first stage,

before the modified BUCK converter. The STEVAL-ILL013V1 shows this design concept.

The STEVAL-ILL013V1 demonstration board is an 80 W offline dimmable LED driver with

high power factor (PF) intended for 350 mA, 700 mA and 1 A LEDs, and is based on

STMicroelectronics’ L6562A transition-mode PFC controller. The design is complaint with

standard EN61000-3-2 (limits for harmonic current emissions). The order code is STEVAL-

ILL013V1 and the complete design, including schematic diagram, bill of material,

calculations, measurements, etc. is described in user manual UM0670 (see Section 3:

Reference and related materials).

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