LED lights are widely used in various fields of the lighting industry, the stage lighting industry LED stage lights such as LED moving head light, LED flood light, LED wall washer, LED pattern effect lights and other stage LED lights have been recognized and loved by the majority of users .

LED as a light source, dimming is very important. Not only for a more comfortable environment at home, it is even more important today for the purpose of reducing unnecessary electric light to further save energy and reduce emissions. And for LED light sources, dimming is also easier than other fluorescent lamps, energy-saving lamps, high-pressure sodium lamps and so on, so it should be more in all types of LED lamps with dimming function.

First, the use of DC power led dimming technology

Adjust the brightness with the method to adjust the current To change the brightness of the LED is very easy to achieve. The first thought is to change its drive current, because the brightness of the LED is almost directly proportional to its drive current.

1.1 method to adjust the forward current

The easiest way to adjust the LED current is to change the current sense resistor in series with the LED load. Almost all DC-DC current-sense ICs have a current-sense interface that compares the sensed voltage to the internal reference voltage on the chip Constant current. However, the ＆#118alue of the sense resistor is usually very small, only a few tenths of Europe, if you want to install a potentiometer on the wall of a few tenths of a euro to adjust the current is not possible, because the lead resistance will have a few tenths of a euro. So some chips provide a control voltage interface, change the input control voltage can change its output constant current ＆#118alue.

1.2 Adjusting the forward current will shift the chromatogram

However, adjusting the forward current to adjust the brightness will have a problem, that is, while adjusting the brightness will change its spectrum and color temperature. Because the white LEDs are produced by using yellow LED to excite the yellow phosphor, when the forward current decreases, the brightness of the blue LED increases and the thickness of the yellow phosphor is not proportionally thinned, so that the dominant wavelength of the spectrum is increased. For example, when the forward current is 350mA, the color temperature is 5734K, and when the forward current is increased to 350mA, the color temperature is shifted to 5636K. When the current decreases further, the color temperature will change to warm color.

Of course, these issues may not be a big problem in general actual lighting. However, in the use of RGB LED system, it will cause the color shift, and the human eye is very biased color deviation, it is not allowed.

1.3 Adjusting the current will produce a serious problem that the constant current source can not work

However, in actual implementation, adjusting the forward current by adjusting the current may cause a more serious problem.

We know that LEDs are usually driven by DC-DC constant-current drive power. Such constant-current drive sources are usually classified as either boost or buck (and of course buck-boost, but due to low efficiency , Expensive but not commonly used). What is the use of step-up or step-down by the power supply voltage and LED load voltage relationship between the decision. If the power supply voltage is lower than the load voltage step-up type; if the supply voltage is higher than the load voltage step-down type. The LED forward voltage is determined by the forward current. From the LED's volt-ampere characteristics, we can see that the change of the forward current will cause the corresponding change of the forward voltage. To be exact, the decrease of the forward current will also cause the decrease of the forward voltage. So when the current down, the LED's forward voltage also will be reduced. This will change the relationship between supply voltage and load voltage.

For example, in a 24V input LED lamps, the use of eight 1W high-power LED in series. When the forward current is 350mA, the forward voltage of each LED is 3.3V. Then 8 series is 26.4V in series, higher than the input voltage. So should adopt the step-up type constant current source. However, in order to dim, the current dropped to 100mA, this time the forward voltage is only 2.8V, 8 in series to 22.4V, the load voltage becomes lower than the supply voltage. This step-up constant current source simply can not work, but should be used step-down type. For a step-up constant current source must work in step-down is not enough, the last LED will appear flashing phenomenon. In fact, as long as the use of step-up constant current source, when using the forward current dimming, as long as transferred to a very low brightness almost certainly will produce flicker phenomenon. Because at that time the LED load voltage must be lower than the supply voltage. Many people because they do not understand the problems, but also always go to find the problem from the dimming circuit, it is futile.

The use of step-down constant current source will be less problems, because if the original power supply voltage is higher than the load voltage, when the brightness is to low profile, the load voltage is reduced, so still need step-down constant current source. However, if the transfer to a very low forward current, LED load voltage has become very low, then the step-down ratio is very large, it may go beyond the normal operating range of this step-down constant current source, it will make it Failed to work causing blinking.

1.4 working for a long time at low brightness may make the step-down constant current source to reduce the efficiency of low temperature rise and can not work

Most people may think downward light is to reduce the output power of constant current source, it is impossible to cause the step-down constant current source power consumption increases and temperature increases. As everyone knows when reducing the forward current caused by the forward voltage drop will make the buck ratio decreased. The buck-type constant current source efficiency and buck ratio related, the buck ratio is greater, the lower the efficiency, the greater loss of power consumption on the chip.

1.5 Adjusting the forward current can not be precisely dimming

Because the forward current and the light output is not completely proportional, and different LED will have different forward current and light output curve. Therefore, it is difficult to achieve accurate light output control by adjusting the forward current.

LED is a diode, it can achieve fast switching. Its switching speed can be as high as microseconds. Unmatched by any light emitting device. Therefore, as long as the power into a pulse constant current source, with the change of pulse width method, you can change its brightness. This method is called pulse width modulation (PWM) dimming method. If the period of the pulse is tpwm and the pulse width is ton, the duty ratio D (or porosity ratio) is ton / tpwm. Changing the duty ratio of the constant current source pulse can change the brightness of the LED.

2.1 How to achieve PWM dimming

The specific method of PWM dimming is to string into a MOS switch in the load of the LED. The anode of the string of LEDs is powered by a constant current source.

Then a PWM signal is applied to the MOS tube gate, in order to quickly switch the string of LEDs. In order to achieve dimming. There are many constant current chip itself with a PWM interface, you can directly accept the PWM signal, and then output control MOS switch. So what are the advantages and disadvantages of this PWM dimming method?

2.2 pulse width modulation dimming advantages

1. Does not produce any chromatographic shift. Because the LED always work in full-scale current and 0 between.

2 can have a high dimming accuracy. Because the pulse waveform can be controlled to a very high accuracy, it is easy to achieve one thousandth of the precision.

3. Can be combined with digital control technology to control. Because any number can be easily transformed into a PWM signal.

4. Even in a wide range of dimming, it will not flicker. Because it will not change the working conditions of constant current source (boost ratio or step-down ratio), less likely to overheat and other issues.

2.3 pulse dimming should pay attention to the problem

1. Pulse frequency of choice: Because the LED is in a fast switching state, if the operating frequency is very low, the human eye will feel flashing. In order to make full use of the visual residual phenomenon of the human eye, its operating frequency should be higher than 100Hz, preferably 200Hz.

2. Eliminate howling caused by dimming: Although the human eye can not be detected above 200Hz, but until 20kHz but it is the range of human hearing. At this time there may be heard slightest voice. There are two ways to solve this problem. One is to raise the switching frequency above 20kHz and jump out of the range of human hearing. But the high frequency can also cause some problems, because of the influence of various parasitic parameters, will make the pulse waveform (front and back) distortion. This reduces the dimming accuracy. Another way is to find the sounding device and handle it. In fact, the main sounding device is the output of ceramic capacitors, because the ceramic capacitors are usually made of high dielectric constant ceramics, these ceramics have piezoelectric properties. Under the action of the pulse of 200Hz, it will produce mechanical vibration and sound. The solution is to use tantalum capacitors instead. However, high-voltage tantalum capacitors are hard to come by, and expensive, adding some cost.

Second, the use of AC power LED dimming

3, LED dimming with thyristor

Ordinary incandescent and halogen lamps are usually SCR to dimming. Because incandescent and halogen lamps are purely resistive devices, it does not require the input voltage to be sinusoidal because its current waveform is always the same as the voltage waveform, so no matter how the voltage waveform deviates from the sine wave, simply change the rms ＆#118alue of the input voltage, Can dimming. The use of SCR is the AC sine wave to be cut to achieve the purpose of changing its RMS. The load is in series with the thyristor switch.

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

3.1 SCR dimming shortcomings and problems

However, there are a number of problems with thyristor dimming.

1. Thyristor disrupts the sine wave waveform, reducing power factor ＆#118alues, typically PF below 0.5, and the poorer the power factor (a quarter of the brightness is only 0.25) at lower conduction angles.

Similarly, the non-sinusoidal waveform increases the harmonic coefficient.

3. Non-sinusoidal waveform in the line will produce a serious interference signal (EMI)

4. In the low load is very easy to instability, this must also add a drain resistor. This drain resistor consumes at least 1-2 watts of power.

5. In the ordinary thyristor dimming circuit output to the LED drive power will produce unexpected problems, that is, the LC filter input will make the SCR oscillation, this oscillation is indifferent to the incandescent lamp, Because of the thermal inertia of incandescent lamps makes the human eye can not see this oscillation. But for the LED driver power will produce audio noise and flashing.

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3.2 SCR dimming advantages

Although there are so many shortcomings and problems with thyristor dimming, however, it has certain advantages that it has formed an alliance with incandescent halogen lamps and occupies a large dimming market. If the LED is to replace the position of thyristor-dimmable incandescent and halogen lamps, it should also be compatible with thyristor dimming.

Specifically, where some silicon-controlled dimmable incandescent or halogen lamps have been installed, the silicon-controlled dimmer switches and knobs have been installed on the wall and two Cable. To replace the wall of the thyristor switch and to increase the number of connections are not so easy, the simplest way is to change anything, as long as the incandescent lamp on the screw unscrew, put on with a compatible SCR Light function LED bulb can be. This strategy is like led fluorescent, the best made and present T10, T8 fluorescent lamp size exactly the same size, do not need professional electrician, ordinary people can be directly replaced, it can quickly universal. Therefore, many foreign manufacturers of LED driver IC have developed a compatible with existing SCR dimming IC.

Unlike general flyback ICs, which detect the conduction angle of the thyristor to determine the LED current for dimming, we are not going to detail how they work and because we do not Think this is the direction of LED dimming.

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3.4 compatible SCR dimming problems and shortcomings

Although many multinational chip companies have introduced chips and solutions that are compatible with existing thyristor dimming. However, such solutions are not worth recommending for the following reasons:

1. SCR technology is more than half a century old technology, it has many of the shortcomings described earlier, is a technology that should be eliminated. It should exit the stage of history at the same time as incandescent lamps and halogen lamps.

2. Many such chips claim to have a PFC that improves the power factor. In fact, it only improves the power factor as a triac load, making them look nearly purely resistive to incandescent and halogen lamps without improving SCR, including the entire system of power factor.

3. The overall efficiency of all thyristor-compatible LED dimming systems is very low, and some have not considered the loss of the bleeder resistors needed for stable operation, completely damaging the LED's energy efficiency.

4. All SCR LED dimming system are also adjust the LED forward current, there is the aforementioned chromatographic bias and other shortcomings.

5. The installation of SCR dimmable incandescent and halogen lamps accounted for less than one per cent, while the proportion of installed in the wall of the thyristor SCR dimming lamps with even one ten thousandth Not to, because most of the installation of SCR dimming lamps, bedside lamps, lamp. Not to mention the market there are dozens of different specifications SCR and transistor dimming switch, in fact, the development of the IC simply can not be compatible with all the SCR switch, but only compatible with a small part.

6. LED is a brand new Genesis technology, it has unparalleled superiority. There is absolutely no need to sacrifice the advantages of LED in order to take care of the backward SCR. Not to install the new thyristor switch on the wall to achieve LED dimming.

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4, the future of LED dimming system

So what kind of LED dimming system should be used?

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4.1 PWM dimming

As mentioned above, it is best to use LED dimming PWM dimming, PWM dimming, the wall switch can be installed in a simple PWM generator, and then use the potentiometer to control the PWM than in order to achieve dimming. But if you want to turn the light off, then you need to add a pair of lines. So can not be compatible with the original wall of the SCR switch leads. The original SCR switch leads only 2, you can dimming and switch. This advantage is hard to be compatible. But in fact the most commonly used dimming lamps are lamp or lamp, those dimming switches are installed in the power line is not the wall, it does not matter to use the two leads in the wall. In other words, PWM dimming can be directly applied to dimming type lamp.

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4.2 Sectional switch dimming

A Taiwanese company has introduced a four-switch dimming called GM6182 EZ-Dimming is a good solution. It uses only ordinary light switch on the wall can be achieved 4 dimming, the first open for the whole light, the second open to 60% brightness, the third open to 40% brightness, the fourth open