Design Guide for TRIAC Dimmable LED Driver Using FL7730

introduction

LED lighting is an ideal lighting source that can replace traditional lighting systems such as fluorescent lamps and incandescent lamps. In particular, in the traditional TRIAC dimming lamp system, a lot of research has been invested in trying to develop an LED bulb that is compatible with TRIAC dimmers. Because incandescent light sources consume a lot of power and have a short lifespan, LED bulbs have become an ideal replacement for them, with advantages such as very low power consumption and very long lifespan.

The biggest problem with TRIAC dimming LED bulbs is the compatibility of the dimmer. The original design of the traditional TRIAC dimmer was to handle the power consumed by hundreds of watts of incandescent bulbs. LED bulbs that consume less than 20W of power will interact with the dimmers that use high-power switching devices. If the interaction between the dimmer and the LED bulb is unstable, visible flicker will occur.

To prevent flicker, some special requirements need to be considered. The TRIAC dimmer needs to be able to hold the current after the TRIAC is triggered, and to maintain the current during the conduction period after the trigger. If these two currents are not met, the TRIAC dimmer will be falsely triggered and the LED lighting will flicker. Figure 1 shows the wiring diagram of the TRIAC dimmer and LED bulb. Figure 2 shows the circuit of the TRIAC dimmer blocking the power supply at the beginning of the power cycle and connecting the input power and LED bulb after triggering. As shown in Figure 3, if the holding current and holding current flowing through the dimmer are insufficient, the TRIAC dimmer will shut down.

Figure 1. TRIAC dimmer and LED bulb

Figure 2. Dimmer operation when the holding current and the maintaining current are sufficient

Figure 3. Dimmer operation when the holding current and the maintaining current are insufficient

For different dimmer models, the holding current and the maintaining current will also be different. The typical holding current and maintaining current range is about 5~50mA. Due to the large power consumption, these working requirements will not appear when using incandescent lighting. The power output of a single LED bulb is generally less than 20W, so a single LED bulb cannot maintain the required current during the entire power cycle.

This application note provides practical guidelines for TRIAC dimming LED bulb circuit board design. Passive and active bleeder design guidelines are described in detail to keep the current holding and maintaining the current without flickering. Active damping design can improve efficiency by reducing the number of external components. The input filter design section describes the filtering effect of the filter components in terms of power factor (PF), total harmonic distortion (THD) and EMI.