AN2711 Datasheet, PDF (1/36 Pages) STMicroelectronics – The cost of power LEDs is rapidly decreasing

AN2711
Application note
120 V_AC input-Triac dimmable LED driver based on the L6562A
Introduction
The cost of power LEDs is rapidly decreasing, while performance is improving. Their
efficacy (light out per watt in) is now competitive with compact fluorescent lamps, while their
life cycle is significantly longer at 100,000 full-power hours (11+ years) at half brightness.
Dimming LEDs can extend the life cycle further, and also reduce power drain. By far the
most common dimming method is the Triac phase-control dimmer, which directly replaces
the AC On-Off switch, but can be used only with incandescent lamps and specially designed
fluorescent ballasts.
If the dimmer's thyristor current falls below its holding current, it stops conducting until it
receives another pulse from the dimmer's triggering circuit. The power converter does not
receive input power except for a very brief pulse each time the thyristor fires, until the
thyristor turns off because of a lack of holding current. Performance of the converter is very
erratic, and the LED load flickers badly.
This application note presents a low-cost driver for LEDs that is compatible with Triac phase-
control dimmers. The design gives luminaire manufacturers a low-cost, commonly available
dimming option for home fixtures. A side benefit is that, when not wired to a dimmer, the
unit's power factor is over 0.9. The physical design can be made small enough to power a
PAR38 lamp having 65 W equivalent illumination.
The design is based on the ST L6562A transition-mode PFC controller driving a single stage
PFC-flyback power converter.
The device features extremely low consumption (60 µA max. before startup and <5 mA
operating) and includes a disable function suitable for IC remote on/off, which makes it
easier to comply with energy saving requirements (Blue Angel, ENERGY STAR^®,
Energy2000, etc.).
The L6562A's totem-pole output stage, capable of a 600 mA source and an 800 mA sink
current, is suitable for driving high current MOSFETs or IGBTs.
September 2010
Doc ID 14425 Rev 3
1/36
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