Flicker and dimmer challenges in LED-lamp design

The SSL (solid-state lighting) industry is in a state of excitement mixed with trepidation. With the obvious need for lighting more efficient than traditional incandescents, key market drivers are converging in favor of LED-based SSL: environmental concerns, energy costs, and legislation. Lamp designers, however, still have to address issues such as operating life, dimmer compatibility, and the flicker caused by both line frequency and the way some LED drivers interact with dimmers. Incandescent replacement: CFL or LED? For most applications, the only reasonable incandescent bulb replacements are CFLs (compact-fluorescent lamps) or LEDs (light-emitting diodes). Many factors have negatively affected CFL adoption. Perception problems started with the relatively high price, but consumers became more accepting after the introduction of incentives. Economies of scale allowed CFL prices to drop and, with experience, consumers began to appreciate the CFL's benefits including lower operating costs, which balanced some of the high cost of ownership. A start-up delay at turn on and the warm-up period also gave CFL lighting a black eye in the market. Dimmer incompatibility, though, has significantly limited CFL success, particularly in the residential market: On the benign side, most CFLs don't work with dimmers, which can disappoint consumers. On the dangerous side, a few (granted, a statistically small number of) CFLs have caused house fires when interacting with dimmers. Lastly, like all fluorescent lamps, CFLs contain mercury, which makes the lamps' disposal in an environmentally-responsible fashion inconvenient at best and, in some locales, difficult. With CFLs losing the popularity race and LED efficacy continuing to improve, most believe LED-based SSL lighting will grow to represent an increasing share of future lighting applications. As the LED-lighting market continues to develop, companies are jockeying for position and nervously anticipating what could go wrong to derail the SSL market, and that's the source of trepidation. Avoiding CFL pitfalls in LED lighting The famous saying goes, "Those who don't know history are destined to repeat it" (Reference 1). So, with the CFL rollout fresh in everyone's mind, here is the let's-not-repeat-the-mistakes-of-CFL checklist:

• Fast turn-on time
• No warm-up period
• No mercury content
• Work with dimmers

Dimmer compatibility has dominated industry attention. Fortunately, there haven't been any highly publicized house fires, but SSL lights, in less dramatic form than CFL, have not always worked well with dimmers. Some designs are just incompatible with dimmers, and that's fine so long as their packaging makes that fact apparent. Otherwise, the consumer is unpleasantly surprised when their new premium-priced bulb fails to exhibit attributes they took for granted in the incandescent bulb from which they upgraded. The issues have been the total dimming range, dimming linearity, and light flicker. Dimmers aren't easy to work with Most dimmers use TRIACs to phase cut the AC waveform. Many don't do that cleanly or consistently and there are no standards. Some phase cut the leading edge of the waveform, others cut the trailing edge, and many introduce glitches. The circuitry inside the CFL or LED light struggles to handle such behaviors. Note in defense of dimmer manufacturers: They designed their products to drive incandescent bulbs—simple resistive loads. Dimmers have performed that duty efficiently and cost-effectively for years. To keep history from repeating itself, the industry is fixated on dimmer compatibility. It is equally important, however, to factor in SSL's unique challenges. Could flicker derail LED- lighting adoption? Yes, flicker is on the industry's mind but because the issue didn't give CFL a black eye, it might not be getting the attention it deserves. As research continues to surface, however, that's likely to change. Several industry groups are researching flicker, including a US government team at the PNNL (Pacific Northwest National Laboratory), led by senior energy engineer Michael Poplawski (Reference 2). The PNNL is one among ten US DOE (Department of Energy) national laboratories that the agency's Office of Science manages (Reference 3). Speaking at the Design West conference in San Jose on March 27 this year, Poplawski named flicker as one of four problems affecting LED lighting adoption that his group is working to resolve, along with dimming, power-quality, and operating-life problems. He cited the danger of seizures, as well as headaches, fatigue, blurred vision, eyestrain, and job distraction as potential side effects of flicker. Driving an incandescent bulb—a simple resistive load —is far less complex than driving an LED light, which is a solid-state electric device. Poplawski explained that flicker is inherent in every form of electric lighting and it affects everyone differently. He said that LED lighting could reduce flicker, especially compared to CFL, but that flicker is more complicated in SSL and varies substantially, in both amplitude and frequency, in different LED bulbs. Poplawski proposed that researchers should be able to identify and measure qualitatively—in terms of human reaction—the presence and level of flicker in a reportable way that consumers can understand. Another working group, the IEEE PAR1789, is scrutinizing low-frequency (100 and 120 Hz) flicker, which is imperceptible to the human eye. This group published a paper in 2010 and plans a follow-up later this year. The group's 2010 document refers to research that shows that the brain responds to light at frequencies up to and beyond 120 Hz, linking it to headaches. Even worse, some SSL bulbs interact with dimmers in a way that creates light flicker at frequencies known to induce epileptic seizures (Reference 4). SSL flicker: sources and mitigation All AC-powered systems must deal with a line-frequency component. So-called driverless high-voltage LEDs that connect directly to the line voltage provide light that contains a rectified sine-wave artifact. Even sophisticated systems with IC LED drivers don't always completely eliminate the line-frequency and in some cases contain significant light-output ripple at twice the line frequency. One approach to eliminate this line frequency ripple and the resulting output flicker is through a two-stage PFC (power-factor-correction) scheme. A single-stage driver converts and smoothes the AC-rectified line voltage through a flyback transformer and filter and delivers DC current to the LEDs (Figure 1). Unfortunately, the rectified voltage contains ripple at twice the line frequency, 100 or 120 Hz depending on locale. The ripple component survives the voltage transformation and appears on the output as an alternating current perturbation on the LEDs, which can cause flicker.

In two-stage designs, a first-stage chopper circuit implements PFC, supporting power factors greater than 0.9 (Figure 2). 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, which removes 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, more subtle, source of flicker presents itself in dimming systems. The interaction between some LED drivers and dimmers can introduce flicker at lower frequencies that, evidence suggests, can trigger epileptic seizures in some cases. LED drivers with digital control help solve the non-linear attributes of this dimmer problem. While most LED drivers are all-analog, digital drivers analyze and adapt to the dimmer to which they are connected. Intelligent digital algorithms map the operating characteristics of dimmers and digitally filter the LED drive current to eliminate spikes that would otherwise cause flicker. A digital core can also help reduce BOM costs by eliminating components such as the secondary-side controller and opto-isolator from isolated systems. Meanwhile, digital signal control maintains accurate control of the current driving the LEDs on the secondary-side. www.iwatt.com References: 1. Edmund Burke, Irish stateman, 1729—1797. 2. "Future of SSL LED lighting is not dim, but it's flickering," EE Times, March 27, 2012 www.eetimes.com/electronics-news/4369965/Future-of-SSL-LED-lighting-is-not-dim--but-it-s-flickering 3. http://www.pnnl.gov/ 4. "A Review of the Literature on Light Flicker: Ergonomics, Biological Attributes, Potential Health Effects, and Methods in Which Some LED Lighting May Introduce Flicker" http://grouper.ieee.org/groups/1789/comments.html