UBM Tech
UBM Tech

Remote dim LEDs with a single-stage off-line driver

Tom Stamm, STMicroelectronics -October 02, 2012

Dimmable LED drivers have stability problems at very low light levels. This article will examine the reasons and propose a solution to the problem. It will not cover triac dimming since instability at low settings is due to different mechanisms. Dimming methods using a communication scheme to set LED current include DALI, 0-10V, Zigbee, and Powerline Carrier control.

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 Problem

Dimming 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:

The capacitor is unfortunately also part of the control loop. The capacitor and LED dynamic resistance set a control pole at about 30Hz. So it adds a phase lag of 45 degrees and reduces loop gain by 6dB at that frequency. We’ll get back to this later… The graph below details gain and phase shifts due only to the LED-capacitor pole.

Notice that the dynamic impedance of the LED increases as current is reduced. Unfortunately, this shifts the control loop pole to the left. At 10% current, the corner frequency is about 3 Hz. At 1% current it’s about 0.3Hz. Note that the typical control loop for a PFC stage has a crossover frequency between 3Hz and 20Hz.

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.

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