AN-9745 Datasheet, PDF (1/11 Pages) Fairchild Semiconductor – Design Guide for TRIAC Dimmable LED Driver Using FL7730

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AN-9745
Design Guide for TRIAC Dimmable LED Driver Using FL7730
Introduction
An LED has become a promising light source for replacing
conventional lighting systems, such as fluorescent and
incandescent lights. Especially in the conventional TRIAC
dimmer infrastructure, there has been much research into
development of an LED bulb compatible with TRIAC
dimmers. Because the incandescent light source consumes a
hundred watt with short life time, an LED bulb can be the
excellent substitute with considerably less power dissipation
and longer life.
The biggest recent issue of TRIAC dimmable LED bulb is
dimmer compatibility. The conventional TRIAC dimmer
was originally designed to handle hundreds of watts
induced by incandescent bulbs. An LED bulb consuming
less than 20 W should interact with those dimmers
composed of high-power devices. If the interaction between
dimmer and LED bulb is not stabilized, visible flicker is
perceptible.
To manage the interaction without flicker, some
requirements for dimmer operation need to be considered.
TRIAC dimmer needs latching current at firing and holding
current during TRIAC turn-on after firing. If those two
currents are not met, TRIAC dimmer misfires and LED
light flickers. Figure 1 shows the connection of TRIAC
dimmer and LED bulb. As shown in Figure 2, the TRIAC
dimmer blocks input line in the beginning of line cycle, then
connects input line and LED bulb after firing. The TRIAC
dimmer turns off if latching or holding current flowing
through the dimmer is inadequate, as shown in Figure 3.
The latching and holding currents are different from dimmer
models. The typical range of latching and holding currents
is around 5 ~ 50 mA. Those operating requirement do not
cause problems using incandescent bulbs due to high power
consumption. An LED bulb with less than 20 W output
power cannot maintain this amount of current over the
whole line cycle.
This application note provides a practical guideline of
TRIAC dimmable LED bulb board design. Passive and
active bleeder design guides detail how to maintain latching
and holding current without visible flicker. Active damper
design improves efficiency by minimizing the count of
external components. The input filter design section covers
the effect of filter components on PF, THD, and EMI.
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.2 • 10/11/12
Figure 1. TRIAC Dimmer and LED Bulb
Figure 2. Dimmer Operation with Adequate
Latching / Holding Current
Figure 3. Dimmer Operation with Inadequate
Latching / Holding Current
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