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(Figure) LED dimming technology Raiders

In Electronic Infomation Category: - | on April 22,2011

Dimming the light source is very important, not only can get a more comfortable home environment, and 74HC165D datasheet and can be further saving and 74HC165D price and emission reduction, but also for the LED light source, it is over other fluorescent dimming, energy saving lamps, high pressure sodium lamps and 74HC165D suppliers and other more easily achieved, and so it should add all types of LED lighting dimming function.

1. Using DC power LED dimming technology

1.1, with a forward current transfer method to adjust the brightness

To change the LED brightness is very easy to achieve. First thought is to change its drive current, because the LEDs brightness and it is almost directly proportional to drive current. Figure 1 shows the Crees XLampXP-G of the output relative intensity and forward current.


Figure 1: XLampXP-G of the output relative intensity and forward current relationship

Can see from the figure, if the time to 350mA as 100% light output, then output to 200mA when the light is about 60%, 100mA to about 25%. Therefore, the current size can be easily adjusted to achieve the brightness adjustment.

1.1.1 forward current method of regulating Adjust LED current

easiest way is to change and LED load current sense resistor in series (Figure 2a), almost all DC-DC Hengliu chip has a detection current interface, by detecting the voltage and chip Comparison of the internal reference voltage to control the current constant. But the sense resistor value is usually very small, only a few tenths of Europe, if you want to install a few tenths of the wall of the pot in Europe is unlikely to regulate the current, because the lead resistance there will be a few tenths of Europe. Therefore, some chip provides a control voltage interface, change the input control voltage can change the value of output current. Such as Linear Technologys LT3478 (Figure 2b) as long as the change in the ratio of R1 and R2 can also change its output constant current value.


Figure 2: The value of output current adjustment

1.1.2 Chromatography will forward current transfer offset However, the forward current by adjusting

way to adjust the brightness will have a problem, that is the brightness adjustment will also change its spectrum and color temperature. Because the current white light LED is excited by blue LED light generated $ phosphor, when the forward current decreases, the blue LED brightness increase in the thickness of $ phosphor thinning is not in proportion to its growth in the main wavelength spectrum , the specific examples shown in Figure 3.


Figure 3: dominant wavelength and the forward current relationship When the forward current is 350mA

, the main wavelength of 545.8nm; when the forward current is reduced to 200mA, the main wavelength of 548.6nm; when the forward current is reduced to 100mA, the main wavelength of 550.2nm. Change in forward current will cause the color temperature changes (Figure 4).


Figure 4: The color temperature and white LED forward current relationship between the

From Figure 4, when the forward current is 350mA, the color temperature is 5734K, while the forward current increased to 350mA, the color temperature to shift to the 5636K. Current is further reduced, to the warm color temperature change. Of course, these issues when the actual lighting in general might not be a big problem. However, the use of RGB LED system, can cause color shift, while the human eye is very sensitive to the color deviation, and therefore can not be allowed.

1.1.3 will have to adjust the current constant current source can not work a serious problem

However, in the specific implementation, the method used to adjust the forward current dimming may have a more serious problem.

We know that is usually used LED constant current driver DC-DC power supply to drive, and such constant current drive source is usually divided into two step-up or step-down (buck of course, but efficiency is low, prices are not common.) Is the boost or buck LED by the power supply voltage and load voltage relationship between the decision. If the supply voltage is lower than the load voltage to the boost; if the power supply voltage higher than the load voltage to a buck. The LED forward voltage is determined by its forward current. From the LEDs voltage characteristic (Fig. 5) shows that changes in forward current will cause a corresponding change in forward voltage, or rather, the decrease of forward current can also cause the decrease of forward voltage. So, when the current reduction, LED forward voltage will follow the decrease. This will change the supply voltage and load voltage relationship.


Figure 5: LED voltage characteristic

Example, in a 24V input of the LED lamps, using a high-power 1W LED 8 in series. 350mA forward current, each LEDs forward voltage is 3.3V. Then the 8 series is 2 * V, higher than the input voltage. So should the boost current source. However, in order to be dimming, the current to 100mA, this time the forward voltage is only 2.8V, 8 Ke series to 22.4V, the load voltage becomes lower than the supply voltage. This step-up current source simply can not work, but should be a buck. For a step-constant current source must be that it works in step-down is not enough, the last LED will flicker. In fact, as long as the step-up current source used in the forward current by adjusting dimming, just transferred to low light is almost certainly give rise to flicker. Because then the LED load voltage must be below the supply voltage. Many people do not understand the issues, and always going from the dimmer circuit to find the problem, it is futile.

Problems with step-down constant current source will be less, because if the original power supply voltage higher than the load voltage, when the brightness is to low-key, the load voltage is lower, they still need to step-down constant current source. But if you adjust to a very low forward current, LED load voltage becomes low, then step-down ratio is very large, this may be beyond the normal step-down current source scope of work, will make it flicker can not work.

1.1.4 to work long hours at low brightness may cause reduced efficiency step-down constant current source of increased temperature and can not work Most people might think

down just to reduce the current source output power, it can not cause the power step-down current source to increase the temperature increased. Little do they know when the lower forward current Forward voltage drop caused by the step-down ratio will decrease. The efficiency of step-down current source and the step-down ratio is related, step-down ratio the greater the efficiency the lower the power consumption on the chip bigger. Figure 6 is SLM2842J efficiency and step-down ratio curve.


Figure 6: The step-down constant current source of the relationship between efficiency and the step-down ratio

Figure input voltage 35V, output current of 2A, when the output voltage is 30V, the efficiency can be as high as 97.8%. But when the output voltage down to 20V, the efficiency is reduced to 96%; when the output voltage drops to 10V, the efficiency is reduced to 92%. In all three cases, although the output power of the order of 60W, 40W and 20W, but the loss of power was the order of 1.2W, 1.6W, 1.6W. In both cases, after a 33% power increase. If the constant current modules cooling system designed to be very critical, an increase of 33% of the dissipation power is likely to cause the junction temperature rise, so as to cause over-temperature protection are unable to work, may also be serious when the chip burned.

1.1.5 can not be accurately adjust the forward current dimming

As forward current and light output is not entirely proportional relationship will be different and different LED current and light output of the forward curve. Therefore, the method by adjusting the forward current is difficult to achieve precise control of light output.

1.2. Using pulse width modulation (PWM) to adjust light

LED is a diode, it can achieve fast switching. It can be as high switching speed of microseconds or more. Any light-emitting devices can not match. Therefore, as long as the constant current power Gaicheng pulse source, pulse width by changing the method can change its brightness. This method is called pulse width modulation (PWM) dimming method. Figure 7 shows such a pulse width modulated waveform. If the pulse period is tpwm, pulse width ton, then the job than D (otherwise known as porosity ratio) is ton / tpwm. Work to change than the current source pulse can change the LED brightness.


Figure 7: The pulse width by changing the method to change the LED brightness

1.2.1 How PWM dimming

Realization PWM dimming method is to load the string in the LED into a MOS switch (Figure 8), this string of LEDs anode with a constant current source power supply.


Figure 8: PWM signal with a rapid on-off LED string

Then added with a PWM signal MOS gate control to quickly switch to this string of LED. In order to achieve dimming. There are many chip itself with a constant current PWM interface, you can directly accept the PWM signal, then output control MOS switch. Then this PWM dimming method has advantages and disadvantages that what it?

1.2.2 PWM dimming advantage

(1) does not produce any chromatographic shift. Because the LED is always working at full rate between current and 0.

(2) can have very high dimming precision. Because the pulse shape can control to a very high precision, so it is easy to achieve the precision of one ten thousandth.

(3) and digital control technology can be combined to control. Because any number can easily transform into a PWM signal.

(4) even in a wide range dimming, flicker will not occur. Because it would not change the working conditions of constant current source (step-up ratio or step-down ratio), not to overheating and other issues.

1.2.3 PWM dimming should pay attention to the problem

(1) pulse frequency selection

Because the LED is in a state of fast switching, if the operating frequency is very low, will feel the human eye blink. To take full advantage of the phenomenon of human visual residues, and its frequency should be higher than 100Hz, preferably 200Hz.

(2) eliminate the dimming caused by the howling:

While the human eye can not detect more than 200Hz, but they are all up to 20kHz range of human hearing. This time it is possible to hear the slightest sound. There are two ways to solve this problem, first, the switching frequency up to 20kHz or more, out of the range of human hearing. But the frequency is too high can cause some problems, due to various parasitic effects, will pulse waveform (before and after the edge) produces distortion. This reduces the accuracy of dimming. Another method is to find the sound device and to be addressed. In fact, the main sound output device is a ceramic capacitor, as ceramic capacitors are usually made of high dielectric constant of the ceramic, such ceramics have piezoelectric properties. Under the pulse in the 200Hz will produce mechanical vibration and sound. The solution is to replace the use of tantalum capacitors. However, a high voltage tantalum capacitors difficult to obtain, and the price is very expensive, will increase some costs.

2. Using AC power LED dimming

2.1, with the SCR on the LED dimming

Ordinary incandescent and halogen lamps commonly used to SCR dimming. Because incandescent and halogen lamps is a pure resistance device, it does not require the input voltage must be a sine wave, because its current waveform and voltage waveform never the same, so no matter how the deviation from the sine wave voltage waveform, as long as the RMS input voltage change, to be dimming. Use of SCR is to be cut on the AC sine wave to achieve the purpose of changing its effective value. The electrical schematic shown in Figure 9. Dashed part is installed in the wall of the SCR dimmer switch. a-b is the resistance between the incandescent lamp load. So the load and the thyristor switch is connected in series.

Control switch


Figure 9: SCR dimming circuit and waveforms

Change the variable resistor-divider ratio can change the conduction angle, in order to achieve the purpose of changing its effective value. Usually this potentiometer with a switch, connected to the n input, for switching lights. In addition to SCR, there are trailing edge dimming technology transistors, etc., because their basic problem is the same, not in this introduction.

2.1.1 SCR dimming the shortcomings and problems

However, there is a series of problems SCR dimming.

(1) SCR destroyed the sine wave, which reduces the power factor, PF is usually less than 0.5, and the conduction angle power factor worse the more hours (1 / 4 brightness is only 0.25).

(2) Similarly, non-sinusoidal waveforms increased harmonic coefficients.

(3) non-sinusoidal waveform on the line will have a serious interference signals (EMI).

(4) at low load is easy to unstable, this must be added a discharge resistor. The resistance of the discharge at least 1-2 watts of power consumption.

(5) in the ordinary thyristor dimming circuit output to the LED drive power will be generated when unexpected problems, that is, the input LC filter will oscillate SCR, this oscillation for the incandescent does not matter, because the thermal inertia of incandescent lamps do not see eye makes this oscillation. However, the drive power for the LED will produce audio noise and flashing.

2.1.2 SCR dimming advantage Although SCR dimming

there are so many shortcomings and problems, but it has certain advantages that it has formed an alliance halogen and incandescent lamps, dimming occupy a large market. If you want to replace the SCR dimming LED incandescent and halogen lamps position to have and the SCR dimmer compatible.

Specifically, SCR has been installed in some dimming of the incandescent or halogen place, the wall has been installed SCR dimmer switches and knobs, the walls have also been installed in leading light The two cables. SCR to replace the wall switch and to increase the number of cable is not so easy, the easiest way is to change anything, as long as the incandescent lamp unscrew and replace with compatible SCR LED light bulbs can function. Such a strategy as LED lamp, the best made and now T10, T8 fluorescent lamps exactly the same size, no electrician, ordinary people can directly replace, it can quickly spread. Therefore, many foreign manufacturers producing LED driver IC are developed can be compatible with existing IC to SCR dimming.

2.1.3 compatible SCR dimming LED driver IC There are currently on the market

NXP SSL2101 / 2, National Semiconductors LM3445, iWatt the iW3610 and OnSemi the NCL3000 four SCR dimming compatible driver IC. Its characteristics are as follows:


And general flyback

the IC difference is that they can detect the SCR conduction angle to determine the LED current to be dimmed, we are not prepared to detail how they work and performance, as We do not think this is the direction of LED dimming.

2.1.4 SCR dimmer compatible with the problems and disadvantages

Although multinational companies have launched a large chip is compatible with existing silicon chips and dimming solutions. However, such a solution is not recommended, the main reasons are:

(1) SCR technology is old technology with over half a century, it has many of the shortcomings mentioned above, is a long overdue elimination of the technology. It should, and incandescent lamps, halogen lamps at the same time from the historical stage.

(2) many of these chips claiming to have PFC, can improve the power factor, in fact, it only improves as the SCR load power factor, so that they appear close to the pure resistance of the incandescent and halogen lamps, and did not improve the whole system, including silicon, including the power factor.

(3) all compatible LED SCR dimming the overall efficiency of the system are very low, and some do not consider the work needed to stabilize the loss of discharge resistance, completely damaged the LED energy efficient.

(4) all of the SCR system are also LED dimming LED forward current regulation, there is a previously described chromatographic offset other shortcomings.

(5) installation of SCR dimming of incandescent and halogen lamps account for less than one ten thousandth, while the proportion of the wall switch installed in the SCR SCR dimming even in extreme lighting not one that, because the vast majority are installed SCR dimming lamps, bedside lamps, floor lamp. Moreover, the market there are dozens of different specifications of the SCR and the transistor dimmer switch, in fact, impossible for the development of the IC is compatible with all of the thyristor switch, but only a small part of which is compatible.

(6) LED is a new Genesis technology, it has incomparable superiority. There is no need to take care of the expense of backward SCR LED advantages. We should not go to the new switch installed on the wall of the SCR to achieve the LED dimming.

2.2 LED dimming system for the future So what should be used

what LED dimming system?

2.2.1 PWM dimming

Said earlier it is best to use LED dimming PWM dimming, the use of PWM dimming, you can switch on the wall to install a simple PWM generator, then use the potentiometer to control the PWM of the work than to achieve dimming. But if the bright lights have to switch off, then you need to add a pair of lines. So not compatible with the original wall of the thyristor switch leads. SCR switch the original lead is only two to connect but also can switch dimmer. This advantage is very difficult compatible. But in fact the most common dimming really is a table lamp or floor lamp lights, dimmer switches that are installed in the power line instead of the wall, that just do not care to take advantage of the two leads of the wall. In other words, PWM dimming can be directly applied Dimmable lamp.

2.2.2 sub-switch dimmer

Taiwan has launched a company called EZ-Dimming of the GM6182s four paragraphs dimmer switch after all, a good program. Only the use of ordinary light switch on the wall 4 dimming can be achieved, for the first time open to full light, the second opening is 60% brightness, and the third opening of 40% brightness, the fourth opening is 20% brightness. The advantage of such a system can use an ordinary wall switch dimming. And its power factor above 0.92. Does not interfere with the signal of concern. The disadvantage is not continuous dimming. There are a number of operational problems.

2.2.3 remote control dimmer

Infrared remote control LED dimming. This of course is the ideal solution. Can switch lights, and the use of continuous PWM dimming. The disadvantage is high cost, there is no uniform standard can only be used for high-end residential.

In fact, we think we should come back to the main purpose of dimming should be. Dimming all the previous mentioned goal is to meet people in different home situations require different light intensities. For example, when watching television may be darker, when the reading may be brighter. These are mostly in the residence. Few offices, shopping malls, factories, schools to install dimmer lights. And most of these places is installed fluorescent lamps, energy-saving lamps, and can not be dimmed or difficult to achieve continuous dimming.

2.3 epoch-making for energy-saving and dimming

Since human beings have to do everything possible to realize energy savings in order to solve the pressing problems of atmospheric warming, how to reduce electricity for lighting as an important issue on the agenda. Because electricity for lighting for 20% of the total energy consumption. Fortunately, the emergence of energy-efficient LED, LED itself is 5 times more energy than incandescent bulbs than fluorescent lamps, energy saving lamps have to be doubled, but unlike fluorescent lamps, energy saving lamps that contain mercury. If you have to use dimming to energy conservation, energy conservation is very important that means. But in the past are not easy to achieve all the light dimming, and easy LED dimming is a great advantage. In fact, on many occasions because not turn on the lights or at least not so bright, but it was very bright light, such as street lights at midnight to dawn hours; subway trains to the suburbs from the ground to open ground in the car lights; more common is near a sunny window when offices, schools, factories, fluorescent lamps have also opened factories there are still lit fluorescent lamps. These places do not know every day how much wasted energy! Over the past because the high pressure sodium lamps, fluorescent lamps, ceiling lamps, energy saving lamps can not dimming, and can only forget. Now after the switch to LED, you can freely adjust the light, the energy can be saved!

So for dimming lights, home wall dimmer than the main applications, the market is very small. Instead, lights, offices, shopping malls, schools, factories dimming demand is more important occasion, not only a huge market, and saving considerable. These occasions do not need a manual but the auto-dimming light, intelligent light!

2.3.1 dimming lights

In general, the lights after midnight to useless, so the usual practice is to turn off the lights or the opening of 12 points after half brightness. But the most reasonable approach is based on traffic flow to control the brightness of lights, and even fully adaptive control of brightness. Figure 10 is a statistical value based on local traffic lights to regulate the brightness of an example.


Figure 10: According to the statistics of traffic flow to intelligently adjust the lamp brightness

In order to achieve this intelligent dimming, the measures actually very simple. As long as the traffic flow in this region of the curve statistics entered into a single chip, according to this curve gives the PWM dimming signal to the constant current drive source can be achieved.

2.3.2 sensitive auto-dimming LED lamp

In strong sunlight in order to reduce unnecessary lighting, you can use sensitive auto-dimming LED lamp (or any other LED lamps.) Its block diagram shown in Figure 11.



Figure 11: sensitive auto-dimming LED lamp block diagram and physical map

Photosensitive element is the role of feelings around the sun, if the sun is stronger then output a PWM signal to the LED lighting all around the sun (such as LED lamp), dim the brightness of them. A dimmer signal generator can adjust a lot of LED lamps, as long as the lighting source of constant current driver with PWM dimming interface. The dimming system itself, more than 92% efficiency. And there are no walls of SCR dimming circuit and compatibility issues. This automatic energy saving dimming is adaptive to any fluorescent lamp, energy saving lamps, high pressure sodium lamps and other gas discharge tube can not be achieved, while the LED lighting is best.

3. Conclusion

Installed across the country the number of fluorescent lamps and energy-saving lamp is very striking large, according to the Ministry of Industry statistics, in 2008 Chinas production of fluorescent lamps in 2008 over four billion, with exports as high as 3.86 billion. According to China Lighting Association, the number of annual domestic consumption of fluorescent lamps is about 400 million. Assume that Chinas actual use of fluorescent lamps for the 1 billion (mostly installed in offices, shopping centers, factories). Assumes 4 hours each day on average lights, each average power 25W (1.2 Mi T8 fluorescent lamps rated at 36W, 40W or more power consumption. However, the actual power of the lower domestic fluorescent lamp, it is assumed to 25W), daily consumption 0.1 degrees, 36.5 degrees each power. Remove the holiday is 30 degrees. 1 billion is 300 million degrees. May be replaced with LED energy-saving fluorescent lamp at least half later, is 150 million degrees. Automatic dimming can be at least another 10% energy saving. That is 15 million degrees. 0.7 yuan per kilowatt basis, is to save 1.05 billion. This is a very impressive figure! This figure does not include the soon to be replaced by energy-saving LED energy saving lamps and incandescent dimmer included. So vigorously develop energy-saving adaptive dimming LED dimming is the focus of direction!

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