Dimming LED sources

I just happened to find an interesting presentation on light dimming technologies and who they work with LED lighting. Dimming LED sources: what’s working and what still needs fixing is a worth to take a look if you are into LED lighting or light controlling applications. It is a very good overview of the current light dimming practices and how they would work with LED lighting. In some applications dimming is easy and there are many applications where dimming is not so easy.

11 Comments

  1. Tomi Engdahl says:

    Cree won’t compromise on dimming
    http://www.edn.com/electronics-products/other/4422720/Cree-won-t-compromise-on-dimming

    The LMH2 LED module from Cree delivers a natural dimming profile that previously could not be achieved in an energy-saving context. The dimming experience providing by the module is very similar to incandescent lighting, dimming smoothly form 2700K to 1800K, yet achieves more than 80% energy reduction over incandescent bulbs.

    Providing white light and moving to a rich, warm light for restaurants, bars, homes, conference and reception halls and theaters, the one-module form factor addresses a broad range of lighting applications.

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  2. Tomi Engdahl says:

    Over-driving LEDs for brightness
    http://bryanduxbury.com/2013/12/02/over-driving-leds-for-brightness/

    Most of the time when I’m driving an LED, I do it with a simple ballast resistor, and that’s all there is to it.

    EDs have a rated forward current, but they also have a rated peak current, specified at a given pulse width and duty cycle. For instance, these red LEDs are rated for 30mA forward current and 185mA peak in 0.1 ms pulses at 10% duty cycle. Why is this stat useful? As long as you stick to the pulse width and duty cycle parameters, you can intermittently drive an LED at an excess current and get a brighter light without burning it out.

    There is a risk in using this setup, though. If for some reason your PWM signal were to lock up in the “high” state, then the pulse width limitations would be exceeded and you’ll probably fry the LED in an instant. This means you need to be especially cautious during development – crashing your microcontroller at the wrong time can be costly!

    I would say that the PWM-only LED is substantially dimmer, and the overdriven LED is almost but not quite as bright as the steady-on LED. This turned out to be exactly the sort of obvious difference I was looking for. It seems well worth adding the additional circuitry to get a much brighter display.

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  3. Tomi Engdahl says:

    Non-Isolated Buck-Boost TRIAC Dimmable LED Driver
    http://www.eeweb.com/company-blog/power_integrations/non-isolated-buck-boost-triac-dimmable-led-driver

    This is a design example report of a 12 W, high power factor, non-isolated buck-boost, TRIAC dimmable LED driver employing LYTSwitch™-4 LYT4322E. The design is intended for A19 LED driver application. It operates in a wide input range of 190 VAC to 265 VAC and provides an output of 120 V at 100 mA.

    The topology used is a single-stage non-isolated buck-boost that meets high power factor, constant current regulation, and dimming requirements for this design. This document contains the LED driver specification, schematic, PCB details, bill of materials, transformer documentation and typical performance characteristics.

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  4. Tomi Engdahl says:

    A Primer on Buck (and Boost) Converters
    http://hackaday.com/2015/01/24/a-primer-on-buck-and-boost-converters/

    Now, SparkFun Director of Engineering [Pete Dokter] has a tutorial which explains how these mysterious devices work.

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  5. Tomi Engdahl says:

    Perceived Brightness and Dimming Conclusion
    http://www.eeweb.com/company-blog/recom/perceived-brightness-and-dimming-conclusion/

    Having discovered that there is no ideal way to dim LEDs, we hit the next problem: our eyes. Human visual perception of brightness is non-linear. At low light levels, our irises automatically open to let in more light – so we perceive the LED to be brighter than a simple light meter would indicate it to be. To work out the relationship between perceived brightness and measured brightness, you take the square root of the normalized measured light, e.g. a LED dimmed to a quarter (0.25) of the nominal LED current would appear to be 0.5 or half as bright to our eyes.

    So although almost all LED driver manufacturers persevere to make their dimmers dim as linearly and as mathematically accurately as possible, our eyes naturally prefer the non-linear curve of the incandescent lamp as it matches our perception of brightness much more closely than the linear response of the LED. At present, the demand from the LED lighting market is for linearity over naturalness because it makes the matching up of different lights easier, but this may change in the future as the market matures and the demand for more natural dimming increases.

    LED dimming may be represented as a “done deal” by many ballast suppliers who confidently write specifications like 1:1000 dimming ratios in their datasheets even though their output accuracy is only +/- 5% (1:20), but this short discussion shows that accurate, linear and flicker-free LED dimming still cannot be taken for granted, despite the many thousands of different dimmable LED drivers on the market.

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  6. Tomi Engdahl says:

    LED Dimming
    http://www.eeweb.com/company-blog/recom/led-dimming/

    However LEDs are dimmed – be it by 1 – 10 V analogue voltage, mains phase angle, power-line, digital inputs such as DALI, or a WLAN link, there is in fact only two ways to actually dim the output of a LED; either by linearly reducing the current through the LED (analogue dimming) or by switching it off and on very quickly with different mark/space ratios (PWM dimming). Although both methods achieve the same effect, there are important differences in the way they work in practice, which makes the right choice of dimming method critical to many applications.

    Analogue dimming can give very linear dimming curves apart from the extremes of adjustment at almost full brightness or almost total darkness. At the brightest dimming levels, saturation effects in the comparator can generate non-linear responses; while at the dimmest light levels the current through the shunt resistor is so low that the input offset voltages in the measuring amplifier become a significant source of error. The overall result is unavoidable non-linear dimming in the bottom 3% and top 3% of the dimming range for even a well-designed analogue dimming circuit.

    An alternative to analogue dimming is PWM dimming.

    PWM dimming is not as linear as analogue dimming. When the PWM control input goes low the output voltage does not switch off immediately as the output capacitance needs to discharge through the LED load. When the PWM input goes high, the voltage regulator has a delayed reaction time to the enable input as if first needs to powers up. These switch-on and switch-off delays mean that relatively low frequency PWM signals need to be used (a few hundred Hz) and the dimming responses is non-linear. In many designs, these delays mean that PWM dimming below 10% is not possible because the driver cannot react in time to the brief input signal.

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  7. Tomi Engdahl says:

    30 W Isolated Flyback 1-10 V Analog Dimming LED Driver
    http://www.eeweb.com/company-blog/power_integrations/30-w-isolated-flyback-1-10-v-analog-dimming-led-driver/

    This design example report is a 30 W isolated flyback, 1 to 10 V analog dimming LED driver employing LYTSwitch™-4 LYT4315E. This design operates at 90 VAC to 132 VAC input voltage range and delivers an output of 30 V to 60 V at 0.5 A current output. It features wide output voltage range with accurate constant current regulation, single-stage pwoer factor correction, consistent dimming peroformance across output and input voltage range, energy efficient at 115 V, constant voltage open load protection, and integrated protection.

    The key design goals were to achieve high efficiency, 1 V to 10 V dimming and constant current regulation across the output range.

    The LYTSwitch-4 driver IC, combines the PFC function which both meet power factor and harmonics requirements.

    The topology used is an isolated flyback operating in continuous conduction mode. Constant current and dimming regulation are achieved through a secondary feedback control

    The document contains the power supply specification, schematic, bill of materials, transformer documentation, printed circuit layout, design spreadsheet and performance data.

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  8. Tomi Engdahl says:

    More Than Efficient Lighting: An Effective LED Driver Using an 8-Bit MCU
    An 8-bit microcontroller can create an effective LED driver and add advanced features that make the lighting solution even more attractive.
    https://www.designnews.com/electronics-test/more-efficient-lighting-effective-led-driver-using-8-bit-mcu/126476129147250?cid=nl.x.dn14.edt.aud.dn.20170112.tst004t

    In today’s energy-conscious environment, LEDs are often favored over conventional light sources. This is because of their inherent low power and long life. In addition to this, since LEDs are solid-state lighting (SSL), they can be dimmed, allowing the user to create fantastic lighting effects while reducing the overall power consumption.

    Obtaining these benefits from LEDs requires an effective LED driver. The LED driver’s effectiveness is linked to its ability to provide an efficient energy source, to ensure LED’s optimal performance and to maintain the long life of LEDs, even both as the driver keeps the LED output intensity constant and while changing intensity. Also, an LED driver that is intelligent and has advanced capability can make lighting solutions even more attractive.

    Although an effective LED driver can offer many advantages, there are also challenges in its implementation. This article will show how an 8-bit microcontroller (MCU) can be used to alleviate design challenges and create high-performance LED driving solutions with capabilities beyond that of traditional solutions.

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  9. Tomi Engdahl says:

    Careful design delivers halogen-like LED dimming (MAGAZINE)
    http://www.ledsmagazine.com/articles/print/volume-10/issue-10/features/careful-design-delivers-halogen-like-led-dimming-magazine.html?cmpid=enl_leds_architecturalillumination_2017-07-24

    Enabling LEDs to follow the black-body radiation curve isn’t black magic, and Uwe Thomas explains a successful approach to the challenge of dimming SSL products to warm CCTs.

    People are comfortable with the familiar, uncomfortable with the unexpected. When a halogen or incandescent lamp is dimmed, less current passes through the lamp filament. The filament cools down, producing a warmer light with a greater proportion of radiation at the red end of the spectrum. As a result, we are conditioned to expect that dimming a lamp will produce a warm, relaxing ambience. LEDs produce light through a different physical mechanism — electroluminescence rather than incandescence. Here there is no significant color temperature shift when the current that passes through an LED die is reduced in order to lower its lumen output. You must design LEDs and solid-state lighting (SSL) systems to dim like halogen lamps.

    Directional halogen lamps are popular in hospitality environments. But in these applications, the well-documented benefits of LED lighting over halogen lamps are desirable. In particular, LED light sources are far more efficient at converting electricity into light, so they save energy and run cooler. However, making an LED source dim with a similar color shift to a halogen source, maintaining color quality along the way, has presented significant technical challenges to designers of LED emitters and fixtures.

    The aim has been to find an LED emitter that closely follows the idealized black-body curve as it dims. Better still would be one that follows the curve even more closely than halogen sources.

    The light emitted from the tungsten filament follows the idealized black-body curve quite closely but does deviate somewhat from the ideal black-body curve, producing a greenish tinge at some temperatures. Color quality, defined in terms of color rendering index (CRI), is well maintained by halogen lamps as they dim.

    The light from an LED is not created by thermal radiation. LEDs create light through electroluminescence. Light is emitted when electrons and holes recombine in a material, most commonly a semiconductor. The spectrum, or color, of light emitted is determined primarily by the constituent materials of the semiconductor and by phosphors — chemicals used to coat the LED die. As a result, when an LED dims as less current is passed through it, the color temperature shift is very small because thermal radiation represents a negligible portion of the total light emitted. In fact, the hue change as an LED dims is hardly discernible to the human eye.

    We’re accustomed to halogen-like dimming, and to the high CRI of halogens being maintained as they dim. CRI is most noticeable in skin tones.

    We identify detail through the green and red regions of the spectrum and perceive luminance changes primarily within the green part. Incidentally, pure white is, by definition, 76% green, 22% red, and 12% blue light.

    We’re used to halogen dimming; we feel familiar and comfortable with the effect and if a light dims without appearing to create a warmer white, it feels unnatural — something that’s very undesirable in a hospitality environment such as a restaurant, bar, or hotel.

    Which vital characteristics count?

    If we are going the change the color of an LED light source along the black-body curve or other profile as it dims, we must mix the light from at least three types of die to produce a range of white tones, or color temperatures. To make a white LED emitter, you coat a blue LED die with a combination of red and yellow phosphors. Most commonly, die that produce light at 445–455-nm wavelength are used, but those that produce longer wavelengths may be adopted. The combination of die wavelength and yellow/red phosphor recipe is used to achieve the desired color points.

    Phosphors may be sprayed onto the LED wafer before it’s sliced up to create the individual die, or printed directly onto the die.

    Combining multiple different die/phosphor configurations can produce color temperatures ranging from 1800K to 5500K when mixed within a single high-density package.

    In order for light to be mixed effectively, the LED die must be closely packed on the substrate.

    At 3000K, a CRI of 90 and R9 of 80 can be achieved and across the dimming range the CRI average is 85; the red component, R9, averages 70.

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  10. Tomi Engdahl says:

    Control Thy LED
    https://hackaday.com/2017/08/29/control-thy-led/

    The idea is to be able to effectively control the brightness of the LED and prolong their life while doing it. An efficient driver can make all the difference if you plan to deploy them for the long-haul. Let’s take a look at the problem and then discuss the solutions.

    The easiest thing to do is add a potentiometer in series with the LED. Simple! Essentially when you vary the resistance, Ohm’s Law kicks in and voila! Variable resistance equals variable current equals variable brightness.

    Next easiest is to create a constant current circuit. There are a number of ways to create a simple constant current source

    The Digital Method

    The next circuit involves the use of a set of pulses to switch ON and OFF the current through the LED. It’s like flicking the power switch quickly enough that it seems like the light is dimmed. Commonly known as PWM or Pulse Width Modulation, a series of pulses with variable duty cycles or ON and OFF times can be employed for the task.

    For generating the pulses, the humble 555 is a good choice.

    You can use a BJT or a FET or a MOSFET depending upon your budget and state of mind. BJTs are simpler creatures and require very few additional components. A 2N2222 can safely deal with 800 mA of current which is good for many applications.

    MOSFETs are an LED’s Best Friend

    A MOSFET has a very low ON resistance of the order of a few milliohms which means that in such a state, it will dissipate very small amounts of heat as per P = I2R.

    LED Drivers

    Dedicated LED driver chips enable you to control LEDs effectively without having to think about all the parameters. A good example is the TPS92512 which allows for control of high brightness LEDs using PWM which is internally controlled. Current control is implemented internally and external signals including PWM as well as analog signals can be used to control the brightness linearly. No need for lookup tables.

    Conclusion

    So how do you drive an LED? The answer lies in your application area. For small LED current draws, BJTs are simpler and the least expensive. For medium current draws, MOSFETs are a better fit and if you want solutions that offer great out-of-the-box experiences, dedicated driver chips are the way to go.

    Know Thy LED
    https://hackaday.com/2017/08/08/know-thy-led/

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