Fairchild FL7730 8.4W调光LED驱动方案 - 消费电子 - 电子工程网
A 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 20W 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. 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. The latching and holding currents are different from dimmer models. The typical range of latching and holding currents is around 5 ~ 50mA. Those operating requirement do not cause problems using incandescent bulbs due to high power consumption. An LED bulb with less than 20W 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.
The following user guide supports the demonstration kit for the FL7730.
Compatible with traditional TRIAC control
Cost effective solution without input bulk capacitor and feedback circuitry
Line voltage compensation
Linear frequency control for better efficiency and easy design
Constant current regulation vs output voltage change (10~28V) : +/- 3.1%
Constant current regulation vs line voltage change (180~265Vac) : +/- 1.9%
Output open short circuit protection with auto restart
Flicker-free TRIAC dimming control
System efficiency up to 84.5%
pF and THD at nominal voltages (220 ~ 230Vac): PF( 0.92), THD( 18%)
All components’ temperature (at room temp.): less than 55 ordm;C