Really good article on LED dimming, controls, and drivers and how they interact, written by Alice Liao in 2014.
Retailers and restaurants have used lighting as a way to create atmosphere and foster intimacy for years. Dimming reduces energy consumption and enhances function, as in the case of a seminar room or lecture hall. But despite their ubiquity, dimmers used with conventional sources can still have problems, including a reduction in efficacy for incandescent lamps, and a reduction in longevity for fluorescent lamps.
The majority of dimming systems are phase control devices. Designed originally for the incandescent lamp, they reduce light output by interrupting the current during each alternating current half-cycle. Essentially, phase-control devices temporarily shut off power to the light source many times during a second, ultimately dimming voltage. They’re called phase cut dimmers because the interruptions in current creates cuts in the AC sine wave.
The interruptions happen at a rate of about 120 times per second, or twice the frequency at which AC delivers electricity over a power line. But because the filament in an incandescent lamp takes time to heat up and cool down, the human eye simply sees the resulting output as a constant level of decreased brightness. The longer the interruption, the dimmer the light.
Not all phase control devices cut from the same part of the AC sine wave. A triode semiconductor for alternating current (TRIAC), which is used to dim incandescent and halogen lamps, cuts from the forward phase, which begins just as the current changes polarity and the voltage running through the circuit is zero. Also referred to as forward-phase control dimmers, TRIACs can produce spikes in current that cause dimmed lamps to buzz and add stress to electronic drivers.
Reverse-phase control dimmers avoid these problems by cutting from the latter portions, or trailing edges, of the AC waveform. By switching the light circuit on just as the current changes direction, they allow the voltage to rise gradually before turning it off later in the half-cycle. Also called electronic low-voltage (ELV) dimmers, reverse-phase control dimmers were developed to enhance the performance of halogens that use electronic transformers.
TRIACs cut power to the source in the forward phase, just as the current changes polarity and the voltage in the circuit is zero. Reverse-phase control dimmers cut from the trailing edges of the AC waveform.
As a constant-current source, the LED is dimmable. “The amount of current flowing through an LED device determines the light output,” Narendran says. Their level of brightness is adjusted by controlling the current passing through the stacked layers of semiconductor materials mounted on a circuit board.
Unlike conventional sources, dimming does not affect the efficacy or longevity of LED chips. Dimming can actually extend the lifespan of LED chips by lowering their operating temperature.
The dimming range of LED chips is also broader than that of CFL and HID lamps. They can turn down to less than 1 percent of full output, compared to 10 to 30 percent of measured light output for CFLs, according to the National Electrical Manufacturers Association (NEMA), and 30 to 60 percent of lamp power for HID lamps.
All LED chips require a driver in order to dim. Because the chips are low-voltage, direct current (DC) sources, LEDs need drivers to convert AC flowing through power lines into usable DC form.
These drivers dim LEDs in one of two ways: In pulse-width modulation (PWM), the current sent through an LED is switched on and off at a high frequency; often several thousand times per second. The current flowing through the LED is the time-averaged value of the current when the LED is on and when it is off. Reducing the amount of time that the LED is actually on decreases the time-averaged current that is delivered to the device and, as a result, the output or brightness that the eye recognizes.
LED chips can also be dimmed through constant current reduction (CCR), or analog dimming. CCR is a continuous current to the source, but it reduces its amplitude to achieve dimming. The light output is directly proportional to the amount of current flowing through the device.
Both PWM and CCR have advantages and disadvantages. PWM offers a broad dimming range, can decrease light output to values of less than 1 percent, and avoids color shift by operating the chip at its rated level of current. However, because PWM involves rapid switching, the drivers are often more sophisticated and expensive in order to produce the current pulses at a frequency high enough to prevent a perceptible flicker.
CCR dimming is more efficient and simpler to implement because of its less complex and less expensive electronic requirements. It does not have the potential to generate electromagnetic interference, which can result from high-frequency switching. CCR dimming also allows drivers to be located remotely from the light source, which is helpful in the case of LED replacement lamps or in smaller fixtures where space is an issue. However, CCR is not suitable for applications where dimming light levels below 10 percent is desired. “At very low currents, LEDs do not perform as well and the light output can be erratic.
Compatibility issues can occur and the the driver’s performance depends on its compatibility with the dimming device, such as a phase control device or ‘wall dimmer’. The driver must be designed to understand and interpret the signal that the dimming device is providing in order for dimming to occur.
Many of the other dimming technologies used for conventional sources can also work with LED chips. These include zero-to-10V analog, DALI (Digital Addressable Lighting Interface), DMX (Digital Multiplex), and other technologies that separate the signal from the AC Mains voltage.
Installing dedicated wiring that relays dimming information to the dimming device can alleviates compatibility issues because it enables the dimmer and source to operate with little or no interference from each other. The most common phase control device is the TRIAC dimmer. NEMA estimates that there are 150 million of these installed in U.S. homes, and that these legacy devices will represent the bulk of dimming devices for replacement LED fixtures as incandescent sources are phased out. Unfortunately, the compatibility of LEDs with TRIAC dimmers is problematic.
One reason for this stems from the difference in how incandescent lamps and LEDs are powered. Incandescents produce light through simple resistive loads that draw electricity directly from the AC grid. The relationship between current, voltage, and brightness is linear and straightforward. A change in the voltage affects the current proportionally.
That is not the case with LED chips. Because the diodes rely on drive circuitry to ensure constant current and to adapt power and voltage for their use, their interactions with TRIAC dimmers are less predictable. At low dimming levels, for example, an LED driver designed to supply constant current or constant voltage may try to compensate for the phase cut interruptions in the AC sine wave by drawing in more current, causing the LED to stay bright or to flicker.
Different driver circuitry means different ways of drawing power, converting it, and outputting it. Consequently, pairing a TRIAC with an LED product can be hit or miss. One lamp on a single dimmer might work but when several lamps are added in parallel, like in a chandelier, it may not dim very well.
Many manufacturers advise against mixing different LED lamps on the same dimmer because of the variety of driver designs.
The wiring for a TRIAC dimmer further exacerbates matters. Many existing and installed dimmers are two-wire devices; that is, the same wire that provides power to the light source also provides the dimmed voltage, or dimming signal. This can interfere with the performance of both the LED device and the dimmer. Dimmers, particularly those with additional features such as nightlights and light level indicators, have internal circuitry that require constant, albeit minimal, power even when the light source is turned off. With incandescents, this can be done without triggering illumination of the lamps. However, because LED chips don’t require much to power up, this is a little trickier for drivers and LED chips, and can sometimes be the cause for flicker.
The incompatibility between LED drivers and TRIAC dimmers can cause a host of problems. Six such problems are: pop-on, in which the LED source suddenly turns completely on as the dimmer switch is raised from fully off; drop-out, in which the light source shuts off completely as it is dimmed; dead travel, which occurs when changing the dimmer setting produces no visible shift in the light level; ghosting, where light is still visible when the dimmer switch is fully off; audible noise; and flicker.
The lighting industry has developed protocols to bring uniformity to the marketplace. Ledotron is an open digital standard, launched in Europe, that aims to stabilize dimming performance in systems designed for CFL and LED lamps. The standard results from collaborations between several European manufacturers, including Osram and Schneider Electric.
In North America, the ZigBee Alliance’s ZigBee Light Link, is a standard for wireless dimming and the control of LED products. Created for consumer convenience, Light Link certification ensures that lighting and light control products have plug-and-play functionality and interoperability; those that qualify bear the Zigbee Certified seal.
In practice, LED dimming problems can be minimized by taking certain precautions. Designers should specify dimming control devices that are designed for LED. Look for LED source and dimmer combinations that are recommended by the manufacturer of either product, or both.
When using LED lamps with phase control dimmers, designers should decrease the maximum load rating of the dimmer, usually given in watts, to minimize stress to dimmer electronics. Although LEDs are considerably more efficient than their incandescent counterparts, determining the number of LED sources that can be connected to a dimmer is not as simple as dividing its maximum load rating by watts per source.
Instead, a decrease is needed to accommodate small spikes in power caused by driver functioning. Typical de-rating percentages should be in the range of 25 to 30 percent of the dimmer-rated power. A dimmer with a maximum load rating of 1,000w would therefore be de-rated to 250W. This could then be used to calculate the maximum number of sources that the dimmer could accommodate.
Manufacturers are working to enhance circuitry in both LED drivers and dimming devices for better compatibility with TRIACs. Some drivers incorporate adaptive control processing, allowing drivers to synchronize with any type of dimmer. So, advancements in dimming are being made. However, optimal performance still requires a little more time, energy and research.