LED lighting must work with legacy dimming technologies (MAGAZINE)

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This article was published in the June 2011 issue of LEDs Magazine.
View the Table of Contents and download the PDF file of the complete June 2011 issue.

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Many potential applications for LED-based solid-state lighting (SSL) will require dimming capabilities. Dimming not only provides mood lighting, such as in a restaurant or living room, but also increasingly serves to reduce energy consumption by only providing the light level actually needed at any given moment. For example, lights in a parking garage or along a residential street could be dimmed to a low level late at night and only brought up to full brightness if a motion sensor detects activity in the area. Lights over a wide area may be linked using wireless technology to form a local network, with sophisticated sensing, control and monitoring capability through software to optimize the light levels under all conditions. To succeed broadly, SSL luminaire makers must deliver dimmable fixtures that also work with legacy incandescent dimming-control technology.

The fact that LEDs react instantaneously to changes in power input makes SSL especially appropriate for dimming scenarios. Indeed LED lighting offers the potential for dramatic improvements in energy consumption due to the combination of light-source efficiency and compatibility with efficient dimming schemes. In contrast, previous lighting technologies such as metal halide (MH) or high-pressure sodium (HPS) react extremely slowly, and it is not technically feasible to control the brightness of such lights in real time.

Dimming capability can therefore be considered as a key advantage that can be a factor in growing the general acceptance and use of LED lighting. But LED luminaire makers face a challenge in designing products that work with a variety of legacy dimming-control technologies and in some cases that offer the ability to operate in emerging wireless-network-control scenarios.

LED dimming technology

Luminaire designers must first understand the related but separate concepts of the mechanism used to feed the dimming information into the luminaire, and the technique used to actually reduce the brightness of the LEDs. First let’s discuss the dimming techniques for LEDs. There are two basic alternatives that can be used to reduce the light output of the LEDs – analog dimming and pulse-width-modulation (PWM) dimming.

FIG. 1. Analog dimming simply controls the drive current fed to the LEDs. Full brightness uses the full current. The driver electronics linearly reduces the current to dim the LEDs. Analog dimming can be simple to implement but may not deliver the best overall performance. The efficiency of the LEDs tends to increase at lower currents, but the LEDs may not produce a consistent color at lower drive currents.

For PWM dimming, the driver electronics supplies pulses of full-amplitude current to the LEDs. The driver varies the duty cycle of the pulses to control the apparent brightness. PWM dimming relies on the capability of the human eye to integrate the average amount of light in the pulses. Provided the pulse rate is high enough (typically about 200 Hz), the eye does not perceive the pulsing but only the overall average.

PWM dimming requires the addition of a PWM controller and a MOSFET switch in the driver electronics at the output of the DC power supply. PWM dimming is generally more complex to implement than analog dimming, but PWM dimming maintains high efficiency and ensures the LED light output does not vary in color.

Now let’s discuss how dimming control information is conveyed to the driver electronics in an LED-based luminaire. We’re all familiar with the wall dimmers sold for home use, but SSL will be used in a range of scenarios from retrofit luminaires to networked lighting systems with programmatic controls. Luminaire designers must understand the ways in which customers will deploy their products and support the appropriate control schemes.

Dimming information can be carried through the AC wiring, a dedicated analog input, a dedicated digital input, or a wireless interface or network. Each of these options has some advantages and some drawbacks, and different options are appropriate for different applications.

In this article we will focus on dimming signals carried through the AC wiring and how a luminaire can support such dimmers. Table 1 summarizes these scenarios. In a second article planned for a future issue of LEDs Magazine, we will cover dimming controls that rely on dedicated analog or digital interfaces, and networks.

Generally speaking, dimming control technologies including signaling schemes and control-information formats are common to any lighting technology – they are not specific to LED lighting. Indeed most of the schemes predate SSL. The majority of LED luminaires will be retrofitted into existing installations and they must be able to interact with the existing lighting controls.

AC phase control

A widely used form of brightness control is the familiar triac-based dimmer that is present in many residential applications. Triac dimmers operate by cutting out a portion of the AC waveform. The most common type cuts out a portion of the leading edge of the AC waveform, as shown in Fig 1a. The dimmer senses each zero-crossing of the AC input, and waits for a variable delay period before turning on the triac switch and delivering the AC to the load. The AC input to the light therefore has a bite out of the leading edge of each half sine wave.

A second similar type of dimmer operates in the reverse manner, by cutting a portion of the trailing edge of each half sine wave, as shown in Fig. 1b. This type of dimming is sometimes called reverse phase control, and is designed for use in electronic low voltage (ELV) applications.

Phase-control dimmers were originally developed for incandescent lighting, where the lamp brightness is directly dependent on the average power in the AC input. By cutting out a portion of the waveform, the power is reduced and the lamp becomes dimmer. However, this is not the case with LED lighting, because LED luminaires contain a power supply and driver whose primary function is to supply constant current to the LEDs regardless of the AC input voltage.

If you connect a constant-current or constant-voltage power supply to the output of a phase-control dimmer, the power supply will attempt to compensate for the missing portions of the AC waveform. As the amount of phase cut increases, the power supply will maintain its output voltage by drawing higher input current, and the LEDs will remain at normal brightness. Eventually, when the dimmer setting is very low, the power-supply feedback circuits will no longer be able to compensate and the power supply output will collapse.

FIG. 2. For an LED luminaire to respond correctly to a phase-control dimmer, it is necessary to add several functional blocks into the driver electronics, shown in orange in Fig. 2. A sensor monitors the AC input waveform before the power-factor-correction (PFC) stage and generates an output signal proportional to the amount of phase cut. The design in Fig. 2 passes the output signal through an isolation circuit for safety to the secondary side of the power supply and serves as an input to the PWM controller. The PWM controller drives the MOSFET switch connected at the output of the DC-DC converter. The MOSFET produces constant-voltage pulses that are converted to constant-current pulses by the LED driver. When no AC phase-cut is detected the output is driven at 100% duty cycle to give full brightness.

AC input voltage control

Another approach that can be used for dimming most lighting technologies including incandescent, MH, and HPS lamps is to simply reduce the AC input voltage. This is particularly suitable for street lighting, to reduce energy consumption during off-peak hours. For example, the AC voltage can be reduced by 10% in late evening, by 20% after midnight, and then brought back to normal in the early morning. The technique is most effective when a single voltage controller can be used at a central point with the output distributed to multiple lights in the area.

But voltage control is yet another technique that isn’t inherently compatible with standard LED lighting. As with phase control, the LED power supply will compensate for the reduction in voltage and will maintain a constant-current output to the LEDs. SSL luminaire designers can accommodate voltage control by adding an AC voltage sensor to control the output duty-cycle.

The design for such a system is almost identical to the phase-control example in Fig. 2, except the phase-sensor block is replaced by a voltage-sensor block. Because an incandescent filament behaves approximately as a pure resistive load, a drop in input voltage of 10% results in a drop in power (brightness) of approximately 20%. The AC voltage sensor in an LED power supply would be designed to simulate this behavior rather than providing a linear control, so that the variation of LED brightness more closely matches that of an incandescent lamp.

In this first article of a two-part series, we have discussed dimming control through the AC wiring. In the next issue, we will look at dimming using a separate analog, digital or wireless input. Whichever technique is used, the virtually instantaneous response of LEDs to changes in drive current allows great flexibility and will support ongoing and future initiatives to reduce energy use.