Flicker and dimming challenge LED lamp design

Because flicker didn’t give CFL a black eye, perhaps it’s not getting the attention it deserves. But as further research on the topic surfaces, that’s likely to change.

Flicker is being researched by several groups, including a U.S. government team at the Pacific Northwest National Laboratory (PNNL) led by senior energy engineer Michael Poplawski [1]. Speaking at Design West in March, Poplawski named flicker as one of four barriers (along with dimming, power quality and operating life) to LED lighting adoption that his group is working to resolve.

Poplawski told his Design West audience that flicker is inherent in every form of electric lighting (including incandescent bulbs, where it has been reliably managed) and affects everyone differently, with potential side effects including headaches, fatigue, blurred vision, eyestrain, task distraction and even seizures. He said LED lighting could reduce flicker compared with CFLs but noted that flicker is more complicated in SSL systems and that it varies substantially, both in amplitude and frequency, among different LED bulbs.

Poplawski went on to say that his lab is looking to develop LED solutions with minimal flicker. He argued that researchers should be able to identify and measure qualitatively (that is, in terms of human reactions) the presence and level of flicker in a way that can be reported and understood by consumers.

The IEEE PAR1789 working group has been scrutinizing low-frequency (100- or 120-Hz) flicker, which is imperceptible to the human eye. The group published a paper on its findings in 2010 and is scheduled to publish a follow-on report later this year. The 2010 document refers to research that shows the brain responds to light at frequencies up to and beyond 120 Hz, sometimes leading to headaches and migraines. Some SSL bulbs interact with dimmers in a way that creates light flicker at frequencies known to induce seizures [2].

All ac-powered systems must deal with a line-frequency component. “Driverless” high-voltage LEDs, which connect directly to the line voltage,provide light that contains the same rectified sine-wave component. Even sophisticated systems with IC LED drivers don’t always eliminate the line frequency completely and, in some cases, contain significant ripple, at two times the line frequency in the light output. 

One approach for eliminating line-frequency ripple and the resulting flicker at the output is a two-stage power factor correction (PFC) scheme.

In a one-stage approach (Figure 1), an ac-rectified line voltage is converted to the required dc current through a flyback transformer, filtered and applied to the LEDs. The rectified voltage contains ripple at two times the line frequency (100 Hz or 120 Hz). The frequency component also gets transformed and appears on the output of the LEDs as an alternating current, which can cause flicker.

Figure 1. One-stage approach. An ac-rectified line voltage is converted to the required dc current through a flyback transformer, filtered and applied to the LEDs, but the rectified voltage contains ripple at two times the line frequency (100 Hz or 120 Hz). Click on image to enlarge.

In two-stage designs (Figure 2), a front-stage chopper circuit handles PFC, supporting power factors greater than 0.9. A chopper circuit is essentially a boost converter; boosting the incoming rectified ac provides a higher, dc voltage to the input of the flyback converter, removing the ac frequency component. In the second stage, the flyback converter converts the dc voltage on the primary side of the transformer to the required dc current on the secondary side. 

The second source of flicker, more subtle to understand and quantify, presents itself in dimming systems. The interaction between some LED drivers and dimmers can introduce the aforementioned flicker at lower frequencies. LED drivers with digital control help solve the nonlinear attributes of the dimmer problem.

Figure 2. Two-stage power factor correction. A front-stage chopper circuit (shown below stage 1) boosts the input voltage as it improves power factor (PF > 0.9). A flyback circuit then converts the output of the chopper circuit to the required dc current on the secondary side. Click on image to enlarge.