In-depth analysis: How to dim LED?

In terms of LED characteristics, solid-state lighting can be dimmed smoothly, but problems arise in order to be compatible with traditional dimming technology.

　　Dimming is often used in hotels and theaters to create atmosphere, create a sense of intimacy, and delight diners and audiences. For conference rooms and lecture halls, dimming can also reduce energy consumption and enhance the functionality of the space. Despite the widespread use of dimming, there are still some problems when using dimmers with traditional light sources, such as reducing the efficiency of incandescent lamps and damaging the life of fluorescent lamps.

　　Nadarajah Narendran, organizer of the Alliance for Solid-State Lighting Systems and Technologies (ASSIST) and director of the Lighting Research Center (LRC), said: Most dimming systems installed today are phase controllers. They were originally designed for incandescent lamps and reduce light output by cutting the input current during each half cycle of the power frequency. The essence of dimming is that the phase controller temporarily cuts the input voltage to reduce the power input to the light source. Because each cut in the input current occurs during the AC sine wave, they are also called phase-cut dimmers.

　　Phase-cut dimmers cut off 120 times per second, or the frequency at which the AC current is delivered to the power lines. Because the tungsten filament of an incandescent lamp heats and cools more slowly, the light source appears to the human eye as a constant decrease in brightness. The longer the cut-off time, the dimmer the light.

　　Not all phase controllers cut off from the same direction of the AC sine wave. The three-terminal bidirectional thyristor device (TRIAC) used for dimming incandescent and halogen lamps cuts off from the leading phase of the sine wave. At the cut-off point, the input voltage has just passed zero and the input current has just reversed. Therefore, it is also called a leading phase control dimmer. The leading phase control dimmer will generate a current spike when it starts to conduct. This peak current can cause buzzing and increase the stress of the electronic driver.

　　A reverse control dimmer avoids this problem because it turns off the dimmer from the second half of the AC sine wave, or the trailing edge. A reverse control dimmer turns on when the input current just changes direction, allowing the voltage to gradually increase, and then turns it off during the next half cycle. A reverse control dimmer is also called an electronic low voltage (ELV) dimmer and is used to improve the performance of halogen lamps that use electronic transformers.

　　Figure: The TRIAC cuts off the power input to the light source at the leading edge, when the input voltage just crosses zero and the input current just changes direction. The reverse control dimmer (ELV) cuts off at the trailing edge of the AC sine wave.

　　How to dim LED?

　　As constant-current sources, LEDs are inherently dimmable. “The current flowing through the LED determines the light output,” Narendran said. Their brightness can be adjusted simply by controlling the current flowing through a layer of semiconductor material attached to a substrate.

　　James Broderick, manager of lighting projects at the U.S. Department of Energy's (DOE) Office of Building Technologies, said: Unlike traditional light sources, dimming does not affect the efficiency and life of LEDs. In fact, dimming can reduce their operating temperature and extend the life of LEDs.

　　In addition, the dimming range of LEDs is wider than that of compact fluorescent lamps and high-intensity discharge lamps. Compared with the 10% to 30% dimming margin of compact fluorescent lamps (data from the National Electrical Manufacturers Association NEMA) and the 30% to 60% dimming margin of high-intensity discharge lamps (data from the National Lighting Product Information Project NLPIP), the dimming margin of LEDs can be less than 1% of the full load output.

　　Any LED device, whether it is a replacement light source or an LED fixture, requires a driver to achieve dimming. Because LEDs are low-voltage DC sources, they require an electronic driver to convert AC into a usable and adjustable DC current. These drivers are divided into two dimming methods. In pulse-width modulation (PWM), the current through the LED is turned on and off at a very high frequency, "usually thousands of times per second," Narendran said. "The current through the LED is equal to the average current during the LED's switching cycle." By reducing the time the LED is powered on, the average current or effective current can be reduced, thereby reducing the brightness of the LED.

　LEDs , like traditional light sources, can also be dimmed using constant current reduction (CCR), or analog dimming. CCR keeps the light source flowing with a continuous current, but dims it by reducing the current amplitude. "The light output is proportional to the current through the LED device," Narendran said.

　　PWM and CCR each have their own advantages and disadvantages. PWM is more widely used and has a wider dimming range, which can reach less than 1% of the light output, Narendran said, and can avoid color drift regardless of whether the LED is operating at rated current, maximum current, or zero current. Because PWM dimming uses fast switching on and off, it requires more complex and expensive electronic drive equipment to generate current pulses with a high enough frequency to prevent flicker that can be detected by the human eye.

　　CCR dimming is more efficient and simple because the driver equipment it requires is simpler and cheaper. Unlike PWM, CCR does not generate electromagnetic interference EMI caused by high-frequency switching. In addition, CCR allows the driver to be placed farther away from the light source, which is beneficial for LED replacement light sources or compact lamps with limited internal space. However, CCR is not suitable for applications where dimming requirements are below 10%. "At very low currents, LEDs cannot work properly and the light output is unstable." Narendran said.

　　Compatibility issues

　　Although the driver determines whether the LED product can be dimmed, the performance of the driver mainly depends on the compatibility with the dimmer, such as the phase control dimmer. If dimming is required, the driver must be designed to match the dimming signal of the dimmer.

　　Many of the dimming techniques used in traditional lamps can also be applied to LEDs, including 0 to 10V analog dimming, DALI (Digital Addressable Lighting Interface) dimming, DMX dimming, and "other technologies that separate the dimming signal from the AC bus voltage," Brodrick said.

　　Installing dedicated lines to dimmers that carry dimming information can improve dimming compatibility because the dedicated lines allow for little or no crosstalk between the dimmers and the luminaires. However, such dimming systems are more complex and expensive, which is why they are more common in commercial applications than in residential applications.

　　The most common phase controller is the TRIAC dimmer. NEMA estimates that there are 15 million such dimmers installed in homes in the U.S., and with the ban on incandescent lamps, that means there are a lot of old dimmers waiting for LED fixtures to replace incandescent lamps. Unfortunately, compatibility between LEDs and TRIACs is a problem.

　　One of the reasons for the poor compatibility is the difference in the way incandescent lamps and LEDs are powered. Incandescent lamps generate light by obtaining electricity from the grid through a simple resistive load, and the relationship between current, voltage and brightness is linear and direct, with changes in voltage being proportional to current.

　　LEDs are different from incandescent lamps. LEDs rely on the driver circuit to provide a constant current and ensure the appropriate power and voltage, and their interaction with the TRIAC is unpredictable. For example, at low dimming levels, a constant-current or constant-voltage LED driver may compensate for the phase shedding - or interruption of the AC sine wave - by drawing more current from the grid, causing the LED to remain bright, or causing it to flicker.

　　Furthermore, not all driver architectures are created equal. Different circuits mean different ways of extracting power, converting it, and delivering it. So matching a TRIAC to an LED “may work or it may not,” Narendran said. Also, “it may work fine with one lamp connected to a dimmer, but when multiple lamps are connected in parallel to a dimmer, like in a chandelier, it may not dim well.”

　　This phenomenon does exist, said Jan Kemeling, founder and chief marketing officer of Ledzworld, a Dutch LED lighting product manufacturer. He recommends avoiding connecting different LEDs to the same dimmer because the driver designs of these LED lamps are different.

　　The compatibility issue is further exacerbated by the way TRIACs are wired. Most dimmers currently on the market or already installed are two-wire; that is, one wire carries both the power and the dimming voltage or signal. This can interfere with the function of the LED device and the dimmer, Brodrick said. Dimmers, especially those with additional features such as night lights and light-level displays, still need to be powered, albeit at a low power level, even when the light is off. For incandescent lamps, this can be achieved without triggering the light source. However, LEDs do not require much power to light up, so even a small amount of power can cause LEDs to flicker, said Michael Skurla, senior manager of product and marketing for Philips Global Interior Lighting Systems Americas.

　　Incompatibility between LED drivers and TRIACs can lead to many problems, six of which are listed below:

　　Sudden light: When the dimmer knob is slowly turned up from fully off, the LED light source is suddenly fully turned on.

　Dip: When dimming, the light source turns off completely.

　　Dead zone: When the dimmer conduction angle is changed, the light output does not change accordingly.

　　Afterimage: The light remains dimmed when the dimmer is turned all the way down.

　　noise.

　　Flashing.

　　Flicker, dimming and color drift are the main reasons why experts and consumers are cautious about solid-state lighting . However, the lighting industry is addressing dimming issues from all sides. Last year's NEMA SSL 7A-2013 Phase-cut Dimming for Solid-State Lighting: Basic Compatibility attempts to minimize LED phase-cut dimming compatibility issues by establishing design and testing guidelines for dimmers and LED products . However, the standard is only for future technology, rather than trying to regulate dimming and lighting equipment in the past.