Flicker and dimming challenge LED lamp design

There’s a measure of trepidation in the air of excitement surrounding the solid-state lighting (SSL) industry.

Given the obvious need for more-efficient lighting than traditional incandescent bulbs provide, key market drivers—including environmental concerns, energy costs and legislative mandates—are converging in favor of SSL solutions based on light-emitting diodes. But LED lamp designers still must address such issues as operating life, dimmer compatibility, and the flicker caused by both line frequency and the way some LED drivers interact with dimmers.

For most applications, the only viable alternatives to incandescent lights are LED lamps or compact fluorescent lamps. CFLs have been around longer, but many factors have hampered their acceptance and adoption.

CFLs’ perception problems started with the lamps’ relatively high price, though consumers became more accepting after incentives were introduced, prices fell and the cost-of-ownership benefits became clear. CFL price tags weren’t the only hurdle; the startup delay at turn-on and the warm-up period before the lamps achieve full brightness also left consumers cold. Cleantech skeptics, meanwhile, have delighted in informing the world that “green” CFLs contain mercury. But the dimmer issue really traumatized the CFL industry.

There are two aspects to the dimmer problem. Most CFLs don’t work with dimmers, which is disappointing to some consumers but is a relatively benign problem. More concerning is the potential danger posed by some CFL/dimmer interactions. A few CFLs have caused house fires when interacting with dimmers; the number of such incidents is statistically small but has dealt a blow to user confidence in the lamps’ safety.

With CFLs making little headway in battling their image problem and LED efficacy continuing to improve, most market watchers believe LED-based SSL solutions will claim a steadily increasing share of lighting applications. And as the LED lighting market continues to grow, companies are jockeying for position—and nervously imagining what might go wrong to rob them.

Intent on avoiding a replay of the mistakes made with CFLs, LED lamp makers are working to ensure their technology offers fast turn-on, with no warm-up time to full brightness; contains no toxins; and can be designed to work, safely, with dimmers. Indeed, dimmer compatibility has monopolized the industry’s attention. There haven’t been any highly publicized house fires associated with LED SSLs, but the lamps have not always worked well with dimmers.

Some LED lamps don’t work with dimmers by design. That’s fine, as long as the packaging clearly states the fact, so consumers aren’t unpleasantly surprised when the new, premium-priced bulbs to which they’ve “traded up” fail to offer all the features they took for granted in the cheap incandescent bulbs they’ve replaced.

For those LED lamps that are intended to work with dimmers, the design hurdles have been managing the total dimming range, the linearity of the dimming and light flicker.

Most dimmers use triacs to phase-cut the ac waveform. Many don’t do so cleanly or consistently, and there are no standards. Some solutions phase-cut the leading edge of the waveform, others the trailing edge; many introduce glitches. The circuitry inside a CFL or an LED light struggles to handle such variables. (In defense of dimmer manufacturers, their products were designed to drive an incandescent bulb’s simple resistive load, and the devices 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.