Remote dim LEDs with a single-stage off-line driver
Tom Stamm, STMicroelectronics - October 2, 2012
A signal received at the LED driver sets a reference current, and a control loop adjusts scaled LED current to match the reference. Tight control is required so that the light from adjacent fixtures appears the same. We were puzzled by flicker and “shimmer” that appear at very low light levels.
Single-stage power factor correction
If a two-stage power converter is used, the low light level instability is never seen. The first stage (boost or PFC-Flyback) establishes a relatively steady voltage, and the second stage (usually inverse buck) closely regulates the current in the LEDs. The two-stage approach uses more parts and is typically less efficient than a single-stage converter. For cost reasons, the single-stage PFC-Flyback converter is usually selected.
The ProblemDimming over at least a two-decade range is desired. Incandescent lamps have no problem delivering this range--they become dramatically less efficient at low power levels, so the power range needed for the two-decade light range is fairly narrow. If 40% of voltage or current is delivered, light output drops to about 1%. Unfortunately, this sets the market’s expectations for LEDs.
LEDs have a much more linear response, and their efficacy actually increases at low currents. The eye can discern 5% differences between adjacent sources, and it responds to the percentage difference, not the absolute light level. This will require very tight control of the current, and the accuracy required gets even tighter at low light levels. Primary side control cannot be used if dimming to 1% of full output is required.
LEDs have no self-filtering mechanism like incandescents. The thermal mass of the filament in a light bulb is a satisfactory filter for the AC line, but LEDs require external filtering. The usual solution is a large electrolytic capacitor directly across the LEDs, and it works well.The size of the electrolytic is set by the requirement for optical ripple. If the current ripple is less than about 10% rms (about 28% p-p) the light quality is perceived the same as for pure DC. (Also, the Energy Star label requires a statement on the lamp if ripple exceeds 10%.)
LEDs have a dynamic resistance (slope resistance) of about 1/10 of the apparent V/I resistance. Figure 2 shows the V-I curve for a typical LED.
So, for ripple less than 10% RMS, the capacitor must hold the voltage across the LEDs within about 1%. The value needed is:
It’s not reasonable to design a control loop with a movable pole of this range. The only possible solution is to design for a crossover in the 0.03 - 0.1 Hz range--and the loop would be very, very sluggish.