Nadarajah Narendran, head of the Solid State Lighting Systems and Technology Alliance (ASSIST), director of the Lighting Research Center (LRC), said that most of the dimming systems installed today are phase controllers. Originally designed for incandescent lamps, they reduce the light output by cutting off the input current for each half-frequency cycle. The essence of dimming is that the phase controller temporarily cuts off the input voltage to reduce the power input to the light source. Because each cut-off of the input current occurs in an AC sine wave, they are also known as phase-cut dimmers.
The phase cut dimmer is turned off 120 times per second, or the number of cuts is equal to the frequency at which the alternating current transfers power to the power line. Since the tungsten filament of the incandescent lamp is heated and cooled at a relatively slow rate, the brightness of the light source seen by the human eye is lowered at a constant level. The longer the cut, the darker the light.
Not all phase controllers are switched off in the same direction as the AC sine wave. Triacs are used for dimming of incandescent and halogen lamps. They are switched off from the front phase of the sine wave. At the cut-off point, the input voltage just crosses zero and the input current is just reversed. Therefore, it is also called the pre-phase control dimmer, and the front-phase control dimmer will generate a current spike when it starts to conduct. This spike current will cause a buzzer and increase the pressure of the electronic driver.
The inverting control dimmer avoids the above problem because it cuts off the dimmer from the second half or the trailing edge of the AC sine wave. The inverting control dimmer turns on when the input current has just changed direction, allowing the voltage to gradually rise and then cutting it off during the next half cycle. Inverting control dimmers are also known as electronic low voltage (ELV) dimmers to improve the performance of halogen lamps that use electronic transformers.
Figure: The triac (TRIAC) cuts off the power input to the source at the front phase, which occurs when the input voltage has just zeroed out and the input current has just changed direction. The inverting control dimmer (ELV) is switched off at the trailing edge of the AC sine wave.
How to dim LED?
As a constant current source, LEDs can be dimmed by nature. Narendran said: "The current flowing through the LED bead determines the output of the light." Their brightness can be adjusted very simply by controlling the current flow of the layer of semiconductor material attached to the substrate.
James Broderick, manager of lighting projects at the US Department of Energy's (DOE) Building Technology Office, said: LEDs are not like traditional light sources, and dimming does not affect the efficiency and longevity of LEDs. In fact, dimming can lower their operating temperature and thus extend the life of the LED.
In addition, LEDs have a wider dimming range than compact fluorescent lamps and high-intensity discharge lamps. 10%~30% dimming surplus compared to compact fluorescent lamps (data from NEMA), 30%~60% dimming surplus of high intensity discharge lamps (data from NLPIP) The dimming remaining of the LED can achieve less than 1% of the full load output.
Any LED device that requires a replacement light source or LED fixture requires a driver for dimming. Because LEDs are low voltage DC sources, LEDs require an electronic driver to convert the 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 very high frequencies, "usually thousands of times per second," says Narendran. "The current through the LED is equal to the average value of the current during the LED switching period." By reducing the power-on time of the LED, 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 by constant current reduction (CCR), or analog dimming. The CCR keeps the source of continuous current, but dimming by reducing the current amplitude. “The light output is proportional to the current through the LED device,†says Narendran.
Both PWM and CCR have advantages and disadvantages. PWM is more widely used, and the dimming range is wider, which can achieve less than 1% of the light output. Narendran says, regardless of whether the LED is operating at rated current, maximum current, or zero current, color drift can be avoided. Because PWM dimming uses fast switching on and off, it requires more complex and expensive electronic drive devices to generate current pulses of sufficiently high frequency to prevent flicker that is detectable by the human eye.
The CCR dimming method is more efficient and simpler because it requires a simpler and cheaper drive. Unlike PWM, CCR does not generate electromagnetic interference EMI caused by high frequency switching. In addition, the CCR allows the drive to be placed further away from the light source, which is advantageous for LED replacement of the light source or for compact fixtures with limited internal space. However, CCR is not suitable for applications where the dimming requirement is less than 10%. “At very low currents, the LEDs don't work properly and the light output is not stable,†says Narendran.
Compatibility issue
Although the driver determines whether the LED product can be dimmed, the performance of the driver depends primarily 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 dimming techniques for conventional luminaires can also be applied to LEDs. Includes 0 to 10V analog dimming, DALI (Digital Addressable Lighting Interface) dimming, DMX dimming, and "other techniques for separating dimming signals from AC bus voltage," Brodrick said.
Installing a dedicated line for a dimmer carrying dimming information can improve dimming compatibility because the dedicated line has little or no interference between the dimmer and the luminaire. However, such dimming systems are more complex and expensive, which is why they are common in commercial applications rather than residential applications.
The most common phase controller is the TRIAC dimmer. NEMA estimates that 15 million homes in the United States install such dimmers, which means that a large number of old dimmers are waiting for LED lamps to replace incandescent lamps due to the ban on incandescent lamps. Unfortunately, the compatibility of LEDs with TRIAC is a problem.
One of the reasons for poor compatibility is the difference between incandescent and LED power supply methods. Incandescent lamps obtain electricity from the grid through a simple resistive load. The relationship between current, voltage and brightness is linear and direct, and the change in voltage is proportional to the current.
LEDs are different from incandescent lamps. LEDs rely on drive circuits to provide constant current and ensure proper power and voltage, and their interaction with TRIAC is unpredictable. For example, at low dimming levels, a constant current or constant voltage LED driver may compensate for the phase cut—or the interruption of the AC sine wave—to keep the LEDs bright or cause flicker by getting more current from the grid.
In addition, not all drive architectures are the same. Different circuits mean different ways of extracting power, converting it, and outputting it. Therefore, TRIAC and LED matching "may be just as effective and may not work," Narendran said. In addition, "maybe a lamp works fine when connected to a dimmer, but when multiple lamps are connected in parallel through a dimmer, such as in a chandelier, it may not be well dimmed."
This phenomenon exists. Jan Kemeling, founder and chief marketing officer of Ledzworld, a Dutch LED lighting manufacturer, said. He suggested avoiding connecting different LEDs to the same dimmer because the driver design of these LEDs is different.
The wiring of the TRIAC further exacerbates the compatibility issue. Most of the dimmers currently on the market or already installed are two-wire; that is, one wire is supplied with a dimming voltage or a dimming signal while being powered. This can interfere with the functionality of LED devices and dimmers, Brodrick said. Dimmers, especially those with additional features, such as nightlights and light level displays, require internal power to be energized even when the lights are off, albeit at low power. For incandescent lamps, this can be done without triggering the light source. However, LEDs do not require much power to illuminate, so even a small amount of power can cause LEDs to flicker. Michael Skurla, senior manager of products and markets for the Americas Division of Philips Global Interior Lighting Systems, said.
Incompatibility between LED drivers and TRIAC can cause many problems, six of which are as follows:
Highlight: When the dimmer knob slowly rotates up from full closure, the LED light source is suddenly fully turned on.
Dips: When dimmed, the light source is completely turned off.
Dead zone: When changing the conduction angle of the dimmer, the light output does not follow the change.
Afterimage: The light is still slightly lit when the dimmer is completely off.
noise.
Flashing.
Flashing, 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 in all aspects. Last year's NEMA SSL 7A-2013 Solid Phase Lighting Cut-In Dimming: Basic Compatibility attempted to minimize LED phase-cut dimming compatibility by developing design and test guidelines for dimmers and LED products. However, the standard is only for future technologies, not trying to regulate past dimming and lighting equipment.
Figure: The rate of change of the light output of the dimming curve in this example is a function of the dimming range. All of the above elements do not have to appear in the product dimming profile. (Source: Solid State Lighting Systems and Technology Alliance)
The correct way to dim
The lighting industry has also developed protocols to promote market unification. Ledotron is an open digital standard introduced in Europe to enable stable dimming performance for systems designed for compact fluorescent CFLs and LED light sources. This standard stems from cooperation between several European manufacturers, including OSRAM and Schneider Electric.
In North America, the ZigBee Alliance's ZigBee light is the standard for wireless dimming and control of LED products. For consumer convenience, lighting and lighting control products with plug-and-play functionality and interoperability can be certified by ZigBee to certify their qualification.
The Lighting Research Center (LRC) 2013 released "ASSIST Recommendations... Dimming: A Technology-neutral Definition" suggests that regardless of the type of light source, the performance criteria for dimming should ensure the end user's visual comfort and satisfaction. It sets minimum and maximum light levels (5% and 90%, respectively), dimming evaluation files, and covers issues such as deadband, flicker, and system efficiency.
In practical applications, LED dimming can minimize problems by taking some precautions. First and foremost, LED product designers should specify a dimming controller for the LEDs they design. Look for a combination of light source and dimmer for each LED product recommended by the manufacturer, or both. Installed in the wall, Brodrick recommends a NEMA SSL-7A-compliant dimmer and LED source.
The designer should simulate all dimming conditions during the design process. “All LED sources and dimmers should be tested over the entire dimming range.†If the simulation is not implementable, an LED source and adjustment is required. Proof of the combination of optics, and this proof of information must be guaranteed no more than 6 months.
When the LED lamp is dimmed with the phase controller, the designer should derate the dimmer's maximum rated load (usually expressed in watts) to minimize the dimmer's pressure. Although LEDs are more efficient than their corresponding incandescent lamps, the number of LEDs connected to a dimmer cannot be simply divided by the maximum rated load of this dimmer by the single lamp power.
Instead, derating is required to accommodate spikes caused by the drive. “The typical derating percentage should be 25% to 30% of the maximum rated load of the dimmer,†says Ledzworld's Kemeling. If the maximum rated load of a dimmer is 1000W, it should be derated by 250W. This can be used to calculate the maximum number of light sources a dimmer can accommodate.
Narendran said: Manufacturers are also working to improve the circuit of LED drivers and dimmers to achieve better compatibility with TRIAC. Some drivers have adaptive control, Kemeling said. This makes the LED driver compatible with any dimmer, but the cost is also increased. Therefore, although dimming technology is advancing, it takes some time and effort to achieve optimal performance.
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