Choosing the right dimmable LED driver
LED driver designs can deliver flicker-free and fast startup dimming performance while still providing high power factor and efficiency
Dimming performance of LED lamps in legacy TRIAC-based dimmers is one of the biggest challenges for LED lighting designers. In many cases, this causes flickering and reduced dimming range compared to incandescent bulbs.
The problem is that TRIAC-based dimmers were designed for use with incandescent lamps, which have resistive loads. When LEDs started to be used with the dimmers, the low-current requirements, along with the non-resistive nature of the devices, caused the TRIACs to intermittently turn on and off, which creates the flickering effect.
Since LEDs and LED drivers have non-resistive loads, and the current required is lower than the holding current of the TRIAC in the dimmer. Not enough hold-up current when the TRIAC fires, which causes flickering and a limited dimming range.
At issue is that the TRIAC needs a certain amount of latching current and holding current, which aren’t met by an LED bulb, which in turn means the TRIAC will intermittently turn on and off.
To provide the latching and holding current, LED driver IC makers are now adding bleeding and damping circuits to their designs. A bleeding circuit (typically a small resistor with an input capacitor) provides the latching and holding current, and eliminates the misfiring of the TRIAC. It can be implemented as a passive or active bleeder.