Flicker and dimmer challenges in LED lamp design
Incandescent Replacement: CFL or LED?
For most applications, the only viable solutions to replace incandescent lights are compact fluorescent lamps (CFL) or light-emitting diodes (LEDs). Many factors have negatively impacted CFL market adoption. Perception problems started with the relatively high price, but consumers became more accepting after incentives were introduced, prices dropped and they understood cost-of-ownership benefits. A start-up delay at turn on and the warm-up period also gave CFL lighting a black eye. But the dimmer issue really traumatized the industry. What is the dimmer issue? 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, skeptics have delighted in informing the world that “green” CFL contains non-environmentally-friendly mercury.
With CFL losing the popularity race, and LED efficacy continuing to improve, most believe LED-based SSL lighting will continue to grow to represent an increasing share of future lighting applications. So as the LED lighting market continues to grow, companies are jockeying for position to take their chunk of the market and nervously anticipating what could go wrong to rob them, 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.” So with the CFL roll-out fresh in everyone’s mind, here’s the “let’s not repeat the mistakes of CFL” checklist:
- Fast turn-on time – CHECK!
- No warm-up period – CHECK!
- No mercury content – CHECK!
- Work with dimmers – CHECK!
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 just aren’t designed to work with dimmers, and that’s fine so long as the packaging clearly states it, so the consumer isn’t unpleasantly surprised when their new premium-priced bulb doesn’t come with attributes they took for granted in the incandescent bulb they traded up from.
The issue has been in the total dimming range, the linearity of the dimming and light flicker.
Dimmers aren’t easy to work with
Most dimmers use TRIACs to phase-cut the A/C waveform. Many don’t do that cleanly or consistently and there are no standards. Some phase-cut the leading edge of the waveform, others the trailing-edge, and many introduce “glitches.” The circuitry inside the CFL or LED light struggles to handle such variables. But, in defense of dimmer manufacturers, their products were designed to drive incandescent bulbs; a simple resistive load. They have effectively, efficiently, and cost-effectively performed that duty for years.
To keep history from repeating itself, the industry is fixated on dimmer compatibility. But, it is equally important to factor in today’s unique challenges.
Flicker: the gotcha that could impact LED lighting adoption?
Yes, flicker is on the industry’s mind but because it didn’t give CFL a black eye, maybe it’s not getting the attention it deserves. But as research continues to surface, that’s likely to change.
Flicker is being researched by several industry groups, including a US government team at the Pacific Northwest National Laboratory (PNNL), led by senior energy engineer Michael Poplawski (reference 1). The PNNL (https://www.pnnl.gov/) is one among ten U.S. Department of Energy (DOE) national laboratories managed by DOE’s Office of Science.
Speaking at the DESIGN West conference in San Jose on Tuesday, March 27, Poplawski named flicker as one of four problems impacting 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, eye strain and job distraction as potential side-effects of flicker.
Driving an incandescent bulb, which is a simple resistive load, is far less complex than driving an LED light, which is a solid-state electric device. Poplawski explained to the DESIGN West audience 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, both in amplitude and frequency, in different LED bulbs.
Poplawski went on to say that his lab is looking to develop LED solutions with minimal flicker. Furthermore, he proposed that researchers should be able to identify and measure qualitatively (in terms of human reaction) the presence and level of flicker in a way that can be reported and understood by consumers.
Another working group scrutinizing low-frequency (100Hz/120Hz) flicker, which is imperceptible to the human eye, is the IEEE PAR1789. This group published a paper in 2010 and is scheduled to publish a follow-up later this year. The group’s 2010 document refers to research that shows the brain responds to light at frequencies up to and beyond 120Hz, linking it to headaches and migraines. Even worse, some SSL bulbs interact with dimmers in a way that creates light flicker at frequencies known to induce epileptic fits (reference 2).
Two primary sources of flicker in an SSL system and possible methods to eliminate it
Firstly, all AC-powered systems must deal with a line-frequency component. “Driverless” high-voltage LEDs that 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 a significant ripple at two times the line frequency in the light output.
One approach to eliminate this line frequency ripple and the resulting flicker at the output is via a two-stage power-factor-correction (PFC) scheme.
Let’s first review issues with a one-stage (Figure 1) approach. An AC-rectified line voltage is converted to the required DC current through a flyback transformer, filtered and applied to the LEDs. Unfortunately, the rectified voltage contains ripple at two times the line frequency (100Hz or 120Hz). The frequency component also gets transformed and appears on the output of the LEDs as an alternating current on the LEDs, 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. Unfortunately, the rectified voltage contains ripple at two times the line frequency (100Hz or 120Hz).
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 and the boosting of 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.
Figure 2: Two-Stage Power-Factor-Correction (PFC): A front-stage chopper circuit (shown below stage 1) boosts the input voltage as it improves power factor (PF > 0.9), followed by a flyback circuit that converts the output of the chopper circuit to the required DC current on the secondary side. (For full resolution click here)
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 that have been shown to trigger epileptic fits 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 they are connected to. 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, where digital signal control maintains accurate control of the current driving the LEDs on the secondary-side.
Reference 1
“Future of SSL LED lighting is not dim, but it’s flickering,” EE Times, March 27, 2012
Reference 2
“A Review of the Literature on Light Flicker: Ergonomics, Biological Attributes, Potential Health Effects, and Methods in Which Some LED Lighting May Introduce Flicker” https://grouper.ieee.org/groups/1789/comments.html
About the author: Scott Brown joined iWatt in October 2011 with over 20 years of experience in the global analog semiconductor industry. Brown has semiconductor marketing, business and functional management experience, as well as a deep knowledge of the Power Management market. Prior to iWatt, Brown held marketing and management positions at National Semiconductor, Micrel, ON Semiconductor and Catalyst Semiconductor. He holds a BSc in Electrical and Electronic Engineering from Brunel University in the UK.
Article first appeared in EE Times.
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