The Best Technology For LED Dimming

People usually have a misunderstanding about the dimming of LED light, and think that LED dimming is quite easy. But the reality is that the application of LED light dimming technology is often unsatisfactory. Why? Is the dimming technology of LED light immature, or is the technology difficult to master? What kind of technology should be adopted for the dimming of LED light? How do we master it?

To answer the above questions, we should first understand the volt ampere characteristics of LED.

The volt ampere characteristic of LED is the characteristic that the current flowing through the LED p-n junction changes with the voltage. This change can be displayed vividly on the oscilloscope. A complete volt ampere curve includes forward characteristic and reverse characteristic. When the voltage exceeds a certain threshold, the current will rise exponentially, which will break down the LED p-n junction. The forward voltage of LED is also determined by its forward current. From the volt ampere characteristics of LED, the change of forward current will cause the corresponding change of forward voltage. Specifically, the decrease of forward current will also cause the decrease of forward voltage. So when the current is turned down, the LED voltage will also decrease, which will change the relationship between the power supply voltage and the load voltage.

So we can learn from the volt ampere characteristics of LED that the dimming of LED light can not be realized simply by reducing the input voltage or input current of LED. In addition, the sine wave waveform of LED is different from that of incandescent lamp, so we can not change its effective value (effective dimming) simply by changing its conduction angle.

In order to make it easier for you to understand the above points, there is an example as follow:

In a 24 V input LED lamp, eight 1W high-power LEDs are used in series. When the forward current is 350mA, the forward voltage of each LED is 3.3V, so the 8 LEDs in series is 26.4V. Therefore, the load voltage is higher than the input voltage, so a constant current source > 24V should be used. However, in order to adjust the light, the current is reduced to 100mA. At this time, the forward voltage is only 2.8V, and the eight series connection is 22.4v. The load voltage becomes lower than the input voltage, so the constant current source > 24V cannot work at all. Finally, the LED will flash.

Now you may choose a step-down (wide voltage) constant current source, such as 10v-30v constant current source, to dimming, but if this kind of voltage reducing (wide voltage) constant current source is adjusted to a low forward voltage, the load current of LED will also become very low, so the step-down ratio is very large, which is beyond the normal working range of this kind of Buck (wide voltage) constant current source, and it will not work twinkle. In addition, if the buck (wide voltage) constant current source works at low brightness for a long time, its efficiency will be reduced and the temperature rise will be increased and it will not work. Because the efficiency of the buck type (wide voltage) constant current source is related to the step-down ratio, the higher the Buck ratio, the lower the efficiency, and the greater the power consumption on the chip, which will damage the life of the constant current source and LED light source. Because many people do not understand the problem, they always have to go to the dimming circuit to find the problem, which is futile.

Ordinary incandescent lamp and halogen lamp usually use thyristor to adjust the light. Because the incandescent lamp and halogen lamp are pure resistance devices, it does not require that the input voltage must be sine wave, because its current waveform is always the same as the voltage waveform, so no matter how the voltage waveform deviates from the sine wave, as long as the effective value of the input voltage is changed, the dimming can be carried out.

However, the SCR dimming will cause unexpected problems in the adjustment of LED light source, that is, the LC filter at the input end will make the thyristor oscillate, which is indifferent to the incandescent lamp, because the thermal inertia of the incandescent lamp makes the human eye can not see this oscillation, but the LED drive power will produce audio noise and flicker. In addition, thyristor dimming will destroy the sine wave waveform, thus reducing its power factor value (usually less than 0.5), so the SCR dimming greatly reduces the system efficiency of LED. Moreover, the waveform of thyristor dimming increases the harmonic coefficient, and the non sinusoidal waveform will produce serious interference signal (EMI) on the line, which will pollute the power grid and seriously paralyze the power grid.

Then you may ask: "reducing voltage or current and SCR dimming methods are not suitable for LED light source dimming, so what is the most appropriate way?"

Is it analog (1-10V) dimming? No. Analog dimming faces a severe challenge, which is the accuracy of the output current. Almost every LED driver uses a series resistor to identify the current. The tolerance, offset and delay in the analog (1-10V) dimming drive lead to a relatively fixed error, which in turn reduces the accuracy of the output current, and the final output current cannot be specified, controlled or guaranteed. Therefore, to ensure the dimming effect of LED light source, one of the important points is to reduce the output current error and improve the current accuracy in a closed-loop system.

PWM (pulse width modulation) dimming mode can well solve the above problems, because LED is a diode, it can realize fast switching, it can allow the switching speed can be as high as microseconds, which is incomparable to any light-emitting device. Therefore, as long as the power supply is changed into a pulse constant current source, the brightness can be changed by changing the pulse width. This method is called pulse width modulation (PWM) dimming method. This dimming method is like a sluice which switches on and off at microseconds. Because the switching frequency of the sluice is so fast that we can't identify its switching state with naked eyes, the result is that we can only identify the speed of its switching frequency through the amount of water downstream. In addition, since the duty ratio of the output water flow (effective flow) is changed by the sluice, and the instantaneous water pressure and instantaneous flow of the water flow are not changed, so the switching action of the sluice for more than microseconds will not affect the work of hydropower generation, because the instantaneous water pressure and instantaneous flow remain unchanged, what changes is the downstream water volume and the total amount of power generation. Therefore, the PWM dimming method does not change the instantaneous voltage and current of the input led PN junction, but changes the duty cycle of the output current, thus changing its brightness.

So led PWM (pulse width modulation) dimming mode has the following advantages:

1. No spectral shift of the LED will be generated because the LED always operates between full amplitude current and zero.

2. It has very high dimming accuracy, because the pulse waveform can be controlled to a very high accuracy, so it is easy to achieve the accuracy of 1 / 10000.

3. Even in a wide range of dimming, there will be no flicker. Because it will not change the working conditions of the constant current source (boost ratio or depressurization ratio), overheating and other problems are not likely to occur.

4. It can be combined with digital (Dali / DSI / DMX512) control technology, because the digital control signal can be easily transformed into a PWM signal.

Although led PWM (pulse width modulation) dimming mode has many advantages, the following two problems need to be paid attention to:

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

2. To eliminate the whistling caused by dimming, although it can not be detected by human eyes above 200Hz, it is the range of human hearing up to 20kHz. At this time, it is possible to hear a little noise. There are two ways to solve this problem. One is to increase the switching frequency to more than 20kHz and jump out of the range of human hearing. Another method is to find out the sound producing device and deal with it.

At present, some manufacturers of LED dimming power supply, driver and digital control system have solved the above problems. For example, the LED dimming power supply and driver of Tridonic company adopt PWM (pulse width modulation) dimming technology, and their control signals adopt Dali (digital addressable lighting interface) technology, Combined with the digital lighting control system, the full digital LED control product line is realized. In addition, Tridonic's latest LED light engine based on pl-led Technology. Pl-led refers to Tridonic's innovative technology of LED phosphor. This technology can realize the change of color and color temperature in the same LED light source. At the same time, it can select a fixed color temperature (for example: 2700k-6200k) or color (such as RGB) and carry out dimming control. At present, LED is the highest level of digital dimming technology.

Good dimming technology of LED light source needs good LED control signal technology to match and cooperate, in order to become an effective, stable and reliable system. As mentioned before, the LED PWM dimming mode has a prominent advantage, and the digital control signal can be easily transformed into a PWM signal. Among the digital control signals of lighting, Dali (digital addressable lighting interface) has incomparable advantages over other digital lighting control signals, and it is also the mainstream and open international standard of digital control signals applied in lighting industry. Therefore, the matching of PWM (pulse width modulation) dimming mode and Dali (digital addressable lighting interface) can be described as "good horse with good saddle". Each has its own advantages. PWM (pulse width modulation) dimming technology solves the final dimming problem of LED light source, and Dali (digital addressable lighting interface) technology solves the control, feedback and networking of each LED lamp.

The biggest characteristic of Dali (digital addressable lighting interface) technology is that a single lamp has an independent address. Through Dali system software, the precise dimming and switching control of single lamp or any group of lamps can be carried out, no matter whether these lamps are in the same circuit or different circuits in strong current. In other words, lighting control has nothing to do with strong current circuit. Dali system software can address single or multiple lamps on the same strong current circuit or different circuits independently, so as to realize separate control and arbitrary grouping. This concept brings great flexibility for lighting control. Users can design lighting schemes to meet their needs according to their needs, and even can modify the control requirements at will during the operation after installation without any changes to the circuit.

In 1991, Tridonic introduced the world's first digital dimming technology DSI interface technology. Dali is the latest dimmable digital technology developed on the basis of digital serial interface technology. In 2000, Tridonic and other manufacturers formulated Dali Industrial standards, which were incorporated into iec60929 In 2001, the world Dali association was established. Through the introduction and application of Dali technology, Dali has become the mainstream standard of digital dimming in Europe.

The following are the application advantages of PWM (pulse width modulation) dimming combined with Dali (digital addressable lighting interface)

1. The design is simple and easy to operate

In the design, as long as they are connected to each other through digital signal interface, they are connected in parallel to the 2-core control line. All groups and scenes can be programmed by computer software during installation and debugging, which not only saves the cost of wiring, but also only needs to change the software settings for design modification, re layout and separation without rewiring, which is very simple and easy.

2. Simple and economical installation:

Dali control line has no special requirements for wire rod and polarity during installation. As long as the main power line is isolated from the control line, the control line does not need to be shielded. It should be noted that the voltage drop does not exceed 2V when the control line current is 250mA and the line length is 300m. The control line and power line can be parallel without additional buried wire. The compact design of the control module does not require a special control cabinet, so the installation is simple and economical.

3. The operation is simple and convenient

The PWM LED driver of dal I control interface can automatically handle filament preheating, ignition, dimming, switching, fault detection and other functions. The user interface is very friendly, and users can operate and control without deep understanding of this. For example, if a command to change the current scene is sent, each related LED driver calculates the dimming speed according to the difference between the current brightness and the brightness required by the scene Rate to achieve all LED light sources are synchronized dimming to the required scene brightness.

4. The control is accurate and reliable

Dali is a digital signal, which is different from analog signal. The signal of 1010 can realize undisturbed control and will not distort the control signal due to long-distance voltage drop. Therefore, even if Dali digital signal control line and strong wire run in the same tube, it will not be interfered. Dali signal is two-way transmission, not only can forward transmission control command, but also can feedback led driver status, fault information, switch, actual brightness value information back to the system.

5. Wide range of applications:

Today, Dali interface is not only used for fluorescent lamp ballast dimming, various halogen lamp electronic transformer, gas discharge lamp electronic ballast, led also uses Dali interface dimming; control equipment also includes: radio receiver, relay switch input interface. All kinds of key control panel, including LED display panel, have Dali interface, which will make Dali more and more widely used. The controller will expand from the smallest office to the multi room office building, from a single store to a star hotel.

Although the application of LED dimming at this stage is chaotic, and there are still some problems and obstacles, but it is undeniable that the application prospect of LED dimming is bright. As long as we maintain a rigorous attitude, scientific way and responsible attitude, develop LED dimming technology, promote LED dimming technology, and make good use of LED dimming technology, then the benefits of LED dimming products, lighting industry and human beings will be in the near future.