Electronic Design

Dimmable LEDs Have A Bright Future

The compact fluorescent lamp (CFL) has not been the success it ought to have been. It is so unlike conventional incandescent lamps that consumers have trouble accepting it, despite the significant energy savings it offers. Then again, CFLs have several disadvantages:

  • CFLs cost five to 10 times as much as incandescent lamps.
  • CFLs take time to reach full brightness.
  • The CFL color spectrum is discontinuous, and it might not render colors the way the consumer expects.
  • CFLs don’t always work with home control systems, and they aren’t easily dimmable.
  • Their long discharge tubes and bulky electronics module keep CFLs from being truly “compact.” Miniature decorative lamps are (at present, anyway) out of the question.
  • A CFL looks like what it is—a coiled-up fluorescent tube. This might be acceptable in a bathroom, but not a dining room.

Laws banning the sale of conventional incandescent lamps, starting with 100-W bulbs, will only encourage people to buy and hoard them. For most consumers, familiarity and a lower initial price outweigh the significant long-term savings of energy-efficient lighting. There will be no significant changeover until consumers are offered light bulbs that look and act like the incandescent lamps they already know.

LED bulbs could be what they’re looking for. The LEDs and their drive electronics are relatively compact, and they can fit inside an envelope that resembles a conventional bulb. LED lighting comes on instantly. Operation with existing home-control systems (including dimming) is possible. And, color balance can be varied to fit taste or need.

The Dimming Dilemma

Inexpensive electronic dimmers using silicon controlled rectifiers (SCRs) or triacs have been available since the early 1960s. Turning the knob or moving the slider varies the voltage at which the triac turns on. The higher the turn-on voltage, the less of each ac cycle is available to power the lamp, and the less brightly it glows. These dimmers require only a few inexpensive components, and brightness varies smoothly over an incandescent lamp’s full range.

Unfortunately, this sort of dimmer doesn’t work well with CFLs. The principal problem is that, when dimmed, a CFL might not draw the holding current needed to keep the triac conducting. This can result in flicker or erratic operation. And because a minimum voltage is needed to make the gas in the fluorescent tube conduct, only the very top of the dimmer’s range is usable. Below that, the CFL flickers out.

Similar problems occur when dimming LED lamps. But LEDs are driven through converters. Without additional circuitry, they can’t convert the output of variable-duty-cycle dimmers into a varying drive for the LEDs.

Testing LED Bulbs

Though many LED lighting products are marketed as dimmable, some nevertheless exhibit flicker and poor performance when tested. One product we tested flickered severely and became very hot when left on for one hour at full brightness. It probably wouldn’t last long under normal service.

A proper definition of a “dimmable” LED would require performance and behavior similar to those of an incandescent lamp’s. It wasn’t surprising that those bulbs that were dimmable according to this definition were the most expensive, ranging from $40 to $60 (U.S.).

We disassembled and analyzed several LED bulbs. They used a variety of converter designs, based on controller ICs from different manufacturers. All used a flyback converter to provide an isolated, current-regulated supply. (Figure 1 illustrates a typical flyback design.) All had additional circuitry to ensure a power factor of at least 0.9.

1. Many LED bulbs use a basic flyback converter.

Practical Design Considerations

The mechanism of dimming in an LED bulb is quite different from that for dimming an incandescent lamp. The variable duty-cycle output of most dimmers works with incandescent lamps, but not other kinds.

When dimming an incandescent lamp, the effect is direct. A longer or shorter duty cycle increases or decreases the power delivered to the lamp, making it brighter or dimmer.

But LED bulbs use a converter to drive the LEDs. Because the converter regulates the LEDs’ drive, changing the duty cycle of the ac feeding the converter doesn’t automatically result in a change in the LEDs’ brightness. (The converter is not much different from a switching power supply that automatically adjusts over a wide range of line voltages.)

For an LED converter to support true dimming capability, it must:

  • Provide a load that allows the dimmer’s triac to fire and remain on until the end of each half cycle of the power-line ac
  • Detect the firing angle (that is, the point along the ac half-cycle where the triac was triggered) and use this information to vary the LED drive.

Keeping The Triac On

Once the triac fires, it must remain on until the end of the ac half cycle. Unfortunately, several things can cause it to shut off prematurely.

The capacitors and inductors that suppress electromagnetic interference (EMI) can “ring” from the rapid voltage rise when the triac turns on. The oscillating current generated adds to and subtracts from the triac’s current. If the net current drops below the holding value, the triac shuts off. Preventing this requires holding the ringing at a level that doesn’t reduce the triac’s current below the holding point.

Triac current also varies with the instantaneous line voltage. Maximum current is drawn at the peak of each half cycle, dropping as the voltage falls. If the current drops below the holding value (typically 20 to 50 mA), the triac shuts off prematurely. If the trigger voltage is still present, the triac might turn on again briefly, then shut off again. The cycle repeats until the end of the half cycle, making the LED flicker.

There are ways to minimize this problem. One method is to switch in “holding” or “bleed” resistors at the appropriate time to draw additional current. This wastes energy and increases the amount of heat the driver circuitry must dissipate. Efficiency can be improved a bit by using a “strong” bleeder to draw the initial latching current and a “weak” holder to draw the lower holding current for the remainder of the half cycle (Fig. 2). This system is not ideal for 120-V power, and it wastes too much energy to be useful in 220-V systems.

2. A dual-level bleeder circuit keeps the dimmer’s triac from shutting off, while minimizing current draw.

A more efficient approach is to add an additional switching regulator stage (a “chopper”) at the front end. This regulator draws the required triac current when necessary, then transfers the energy that was stored in the Lboost inductor and in CIN to the dc bus that supplies the main flyback circuit (Fig. 3).

3. A boost pre-regulator makes dimming practical.

Though a chopper adds cost and complexity, it provides greater efficiency, which is important if the product needs to comply with Energy Star standards. The chopper design also dissipates less heat than a bleeder/holder design. The resulting reduced operating temperature should mean a longer operating life. (CFLs are notorious for premature failure, due to their control circuitry overheating.)

Before the converter can adjust the LEDs to the desired brightness level, it has to determine what that level is supposed to be. This requires measuring the duty cycle of the dimmer’s ac output. This measurement is converted to a dc voltage that’s compared with the LED drive current. The resulting error signal varies the converter’s output.

The response time for drive-level changes is slowed sufficiently to minimize flicker (LEDs lack the thermal inertia of incandescent lamps), without being so slow that the bulb’s response visibly lags the user’s dimmer adjustments.

Advanced Dimming-Control ICs

In some of the bulbs we tested, opto-isolators were used to read the LED current. However, sophisticated new hybrid control ICs combining analog and digital functions can perform all the operations required for dimming without the need for separate op amps and isolators.

These operations include determining the duty cycle; sensing the voltage across and the current through the primary of the drive transformer; and generating the error signal. Putting these functions on a single chip reduces the component count and cost.

The Light Bulb Of The Future?

The Philips LED bulb that won the United States Department of Energy’s L Prize consumes 9.7 W, producing 93.4 lumens per watt, with an output equivalent to a 60 W incandescent bulb (Fig. 4). Its L70 lifetime (the point at which output drops to 70% of its original value) is estimated at 25,000 hours. Our tests in the International Rectifier lighting laboratory showed that this bulb offered good dimming performance.

4. The Philips LED bulb that won the Department of Energy’s L Prize consumes 9.7 W, producing 93.4 lumens per watt, with an output equivalent to a 60 W incandescent bulb.

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