Comprehensive analysis of LED dimming

introduction

As a light source, dimming is very important. Not only to create a more comfortable environment at home, but also to reduce unnecessary electric light to further achieve energy conservation and emission reduction. Moreover, for LED light sources, dimming is easier to achieve than other fluorescent lamps, energy-saving lamps, high-pressure sodium lamps, etc., so the dimming function should be added to various types of LED lamps.

Part 1: Dimming Technology for LEDs Using DC Power

1 Adjust the brightness by adjusting the forward current

It is very easy to change the brightness of an LED. The first thing that comes to mind is to change its driving current, because the brightness of an LED is almost directly proportional to its driving current. Figure 1 shows the relationship between the relative light intensity and forward current of Cree's XLampXP-G output.

Figure 1. Relationship between XLampXP-G output relative light intensity and forward current

As can be seen from the figure, if the light output at 350mA is taken as 100%, then the light output at 200mA is about 60%, and at 100mA is about 25%. Therefore, adjusting the current can easily achieve brightness adjustment.

1.1 Methods for adjusting forward current

The simplest way to adjust the current of the LED is to change the current detection resistor in series with the LED load (Figure 2a). Almost all DC-DC constant current chips have a current detection interface, which compares the detected voltage with the reference voltage inside the chip to control the constant current. However, the value of this detection resistor is usually very small, only a few tenths of an ohm. It is unlikely to install a potentiometer of a few tenths of an ohm on the wall to adjust the current, because the lead resistance will also be a few tenths of an ohm. Therefore, some chips provide a control voltage interface, and changing the input control voltage can change its output constant current value. For example, the LT3478 of Lingte Company (Figure 2b) can also change its output constant current value by changing the ratio of R1 and R2.

Figure 2. Output constant current adjustment

1.2 Adjusting the forward current will cause the color spectrum to shift

However, there is a problem with adjusting the brightness by adjusting the forward current, that is, while adjusting the brightness, its spectrum and color temperature will also change. Because the current white light LED is produced by using blue light LED to excite yellow phosphor, when the forward current decreases, the brightness of the blue light LED increases while the thickness of the yellow phosphor does not decrease proportionally, so that the main wavelength of its spectrum increases. The specific example is shown in Figure 3.

Figure 3. Relationship between dominant wavelength and forward current

When the forward current is 350mA, the dominant wavelength is 545.8nm; when the forward current is reduced to 200mA, the dominant wavelength is 548.6nm; when the forward current is reduced to 100mA, the dominant wavelength is 550.2nm.

A change in forward current will also cause a change in color temperature (Figure 4).

Figure 4. Relationship between color temperature and forward current of white LED

As shown in Figure 4, when the forward current is 350mA, the color temperature is 5734K, and when the forward current increases to 350mA, the color temperature shifts to 5636K. When the current is further reduced, the color temperature changes to a warmer color.

Of course, these problems may not be a big problem in general practical lighting. However, in the RGB LED system, it will cause color deviation, and the human eye is very sensitive to color deviation, so it is not allowed.

1.3 Adjusting the current will cause serious problems that make the constant current source unable to work

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

We know that LEDs are usually driven by a DC-DC constant current drive power supply, and this type of constant current drive source is usually divided into two types: boost type or buck type (of course there is also a buck-boost type, but it is not commonly used due to its low efficiency and high price). Whether to use a boost type or a buck type is determined by the relationship between the power supply voltage and the LED load voltage. If the power supply voltage is lower than the load voltage, a boost type is used; if the power supply voltage is higher than the load voltage, a buck type is used. The forward voltage of the LED is determined by its forward current. From the volt-ampere characteristics of the LED (Figure 5), it can be seen that a change in the forward current will cause a corresponding change in the forward voltage. To be precise, a decrease in the forward current will also cause a decrease in the forward voltage. Therefore, when the current is lowered, the forward voltage of the LED will also decrease. This will change the relationship between the power supply voltage and the load voltage.

Figure 5. I-V characteristics of LED

For example, in an LED lamp with an input of 24V, 8 1W high-power LEDs are connected in series. When the forward current is 350mA, the forward voltage of each LED is 3.3V. Then the 8 series voltage is 26.4V, which is higher than the input voltage. Therefore, a boost constant current source should be used. However, in order to dim the light, the current is reduced to 100mA. At this time, the forward voltage is only 2.8V, and the 8 series voltage is 22.4V, and the load voltage becomes lower than the power supply voltage. In this way, the boost constant current source cannot work at all, and the buck type should be used. It is not possible for a boost constant current source to work at a buck, and the LED will flicker in the end. In fact, as long as a boost constant current source is used, when dimming by adjusting the forward current, flickering will almost certainly occur as long as the brightness is adjusted to a very low level. Because at that time, the LED load voltage must be lower than the power supply voltage. Many people always look for problems in the dimming circuit because they do not understand the problem, which is futile.

There will be fewer problems with a buck constant current source, because if the power supply voltage is higher than the load voltage, when the brightness is lowered, the load voltage is reduced, so a buck constant current source is still needed. However, if the forward current is adjusted to a very low level, the load voltage of the LED will also become very low. At that time, the step-down ratio is very large, which may exceed the normal working range of this buck constant current source, and it will also cause it to fail to work and produce flicker.

1.4 Long-term operation at low brightness may reduce the efficiency of the buck constant current source and increase the temperature rise, causing it to fail to work.

Most people may think that dimming down means reducing the output power of the constant current source, so it is impossible to increase the power consumption of the buck constant current source and increase the temperature rise. However, when the forward current is reduced, the forward voltage reduction caused by reducing the forward current will reduce the step-down ratio. The efficiency of the buck constant current source is related to the step-down ratio. The larger the step-down ratio, the lower the efficiency and the greater the power consumption on the chip. Figure 6 is the relationship curve between the efficiency and step-down ratio of SLM2842J.

Figure 6. Relationship between efficiency and step-down ratio of a step-down constant current source

The input voltage in the figure is 35V, the output current is 2A, and when the output voltage is 30V, the efficiency can be as high as 97.8%. However, when the output voltage is reduced to 20V, the efficiency drops to 96%; when the output voltage is reduced to 10V, the efficiency drops to 92%. In these three cases, although the output power is 60W, 40W and 20W respectively, the power loss is 1.2W, 1.6W, and 1.6W respectively. The power consumption increases by 33% in the latter two cases. If the heat dissipation system of the constant current module is designed to be very critical, increasing the dissipated power by 33% may increase the junction temperature of the chip, resulting in over-temperature protection and failure to work, and in severe cases, it may also burn the chip.

1.5 Adjusting the forward current cannot achieve accurate dimming

Because the forward current and light output are not completely proportional, and different LEDs have different forward current and light output curves, it is difficult to achieve precise light output control by adjusting the forward current.

2. Use pulse width modulation (PWM) to dim the light

LED is a diode, which can realize fast switching. Its switching speed can be as high as microseconds. It is unmatched by any other light-emitting device. Therefore, as long as the power supply is changed to a pulse constant current source, its brightness can be changed by changing the pulse width. This method is called pulse width modulation (PWM) dimming method. Figure 7 shows the waveform of this pulse width modulation. If the pulse period is tpwm and the pulse width is ton, then its working ratio D (or aperture ratio) is ton/tpwm. Changing the working ratio of the constant current source pulse can change the brightness of the LED.

Figure 7. Changing the pulse width to change the brightness of the LED

2.1 How to implement PWM dimming

The specific method to implement PWM dimming is to connect a MOS switch tube in series in the LED load (Figure 8), and the anode of this LED string is powered by a constant current source.

Figure 8. Using a PWM signal to quickly switch the LED string on and off

Then a PWM signal is added to the gate of the MOS tube to quickly switch the string of LEDs. This achieves dimming. There are also many constant current chips that have a PWM interface that can directly receive PWM signals and then output to control the MOS switch tube. So what are the advantages and disadvantages of this PWM dimming method?

2.2 Advantages of PWM Dimming

1. There will be no color spectrum shift, because the LED always operates between full current and 0.

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

3. It can be combined with digital control technology for control, because any number can be easily converted into a PWM signal.

4. Even if the dimming is within a wide range, flickering will not occur. This is because the working conditions of the constant current source (boost ratio or step-down ratio) will not be changed, and overheating and other problems are unlikely to occur.

2.3 Issues to note when using pulse width dimming

1. Selection of pulse frequency

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

2. Eliminate the howling caused by dimming: Although the human eye cannot detect frequencies above 200Hz, they are within the range of human hearing up to 20kHz. At this time, you may hear a slight hissing sound. There are two ways to solve this problem. One is to increase the switching frequency to above 20kHz, out of the range of human hearing. However, too high a frequency will also cause some problems, because the influence of various parasitic parameters will cause the pulse waveform (front and back edges) to be distorted. This reduces the accuracy of dimming. Another way is to find the sound-generating device and deal with it. In fact, the main sound-generating device is the ceramic capacitor at the output end, because ceramic capacitors are usually made of ceramics with high dielectric constants, and these ceramics have piezoelectric properties. Under the action of a 200Hz pulse, mechanical vibration will be generated and sound will be generated. The solution is to use tantalum capacitors instead. However, high-voltage tantalum capacitors are difficult to obtain and are very expensive, which will increase some costs.

Part 2 LED Dimming with AC Power

3. Use thyristor to dim LED

Ordinary incandescent lamps and halogen lamps usually use thyristors to dim. Because incandescent lamps and halogen lamps are pure resistance devices, they do not require the input voltage to be a sine wave, because their 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, dimming can be achieved. The use of thyristors is to cut the sine wave of the alternating current to achieve the purpose of changing its effective value. Its electrical schematic is shown in Figure 9. The dotted part is the thyristor dimming switch installed on the wall. The resistance between ab is the incandescent lamp load. So the load is connected in series with the thyristor switch.

Figure 9. Circuit diagram and waveform of thyristor dimming

Changing the voltage divider ratio of the variable resistor can change its conduction angle, thereby achieving the purpose of changing its effective value. Usually this potentiometer has a switch connected to the input end of n to turn the light on and off. In addition to thyristors, there are also transistor trailing edge dimming technology, etc., because their basic problems are the same, they will not be introduced here.

3.1 Disadvantages and Problems of Silicon Controlled Dimming

However, thyristor dimming has a series of problems.

1. The thyristor destroys the waveform of the sine wave, thereby reducing the power factor value. Usually the PF is lower than 0.5, and the smaller the conduction angle, the worse the power factor (only 0.25 at 1/4 brightness).

2. Similarly, non-sinusoidal waveforms increase the harmonic coefficient.

3. Non-sinusoidal waveforms will generate serious interference signals (EMI) on the line 4. It is easy to be unstable at low loads, so a bleeder resistor must be added. This bleeder resistor consumes at least 1-2 watts of power.

5. When the ordinary thyristor dimming circuit outputs to the LED driving power supply, there will be unexpected problems. That is, the LC filter at the input end will cause the thyristor to oscillate. This oscillation is irrelevant to incandescent lamps because the thermal inertia of incandescent lamps makes the human eye unable to see this oscillation. However, it will produce audio noise and flicker for the LED driving power supply.

3.2 Advantages of thyristor dimming

Although thyristor dimming has so many disadvantages and problems, it does have certain advantages, that is, it has formed an alliance with incandescent lamps and halogen lamps, occupying a large dimming market. If LEDs want to replace the position of thyristor dimming incandescent lamps and halogen lamps, they must also be compatible with thyristor dimming.

Specifically, in some places where thyristor dimming incandescent lamps or halogen lamps have been installed, thyristor dimming switches and knobs have been installed on the wall, and two connecting wires leading to the lamps have been installed in the wall. It is not so easy to replace the thyristor switch on the wall and to increase the number of connecting wires. The simplest way is to do nothing. Just unscrew the incandescent lamp on the lamp holder and replace it with an LED bulb with a thyristor dimming function. This strategy is like LED fluorescent lamps. It is best to make it exactly the same size as the current T10 and T8 fluorescent lamps. No professional electricians are required. Ordinary people can directly replace it, and it can be popularized quickly. Therefore, many foreign manufacturers of LED driver ICs have developed ICs that are compatible with existing thyristor dimming.

3.3 LED driver IC compatible with thyristor dimming

Currently, there are four types of driver ICs compatible with thyristor dimming on the market: NXP's SSL2101/2, National Semiconductor's LM3445, iWatt's iW3610 and OnSemi's NCL3000. Their features are as follows:

The difference from general flyback ICs is that they can detect the conduction angle of the thyristor to determine the LED current for dimming. We are not going to introduce their working principles and performance in detail because we do not think this is the direction of LED dimming.

3.4 Problems and Disadvantages of Compatible thyristor Dimming

Although many multinational chip companies have launched chips and solutions that are compatible with existing thyristor dimming, such solutions are not recommended for the following reasons:

1. The thyristor technology is an outdated technology with a history of more than half a century. It has many disadvantages as mentioned above and is a technology that should have been eliminated long ago. It should be phased out at the same time as incandescent lamps and halogen lamps.

2. Many chips of this type claim to have PFC, which can improve the power factor. In fact, it only improves the power factor of the thyristor load, making them look close to pure resistance incandescent lamps and halogen lamps, but does not improve the power factor of the entire system including the thyristor.

3. The overall efficiency of all thyristor-compatible LED dimming systems is very low. Some of them do not take into account the loss of bleeder resistors required for stable operation, which completely damages the high energy efficiency of LEDs.

4. All thyristor LED dimming systems also adjust the forward current of the LED, and have the disadvantages such as the color spectrum shift mentioned above.

5. The proportion of incandescent lamps and halogen lamps with thyristor dimming is less than one ten-thousandth, and the proportion of thyristor switches installed in the wall is less than one ten-thousandth of the thyristor dimming lamps, because the vast majority of thyristor dimming lamps are table lamps, bedside lamps, and floor lamps. Moreover, there are dozens of different specifications of thyristor and transistor dimming switches on the market. In fact, the developed IC is not compatible with all thyristor switches at all, but only a small part of them.

6. LED is a brand new technology with incomparable advantages. There is no need to sacrifice the advantages of LED to take care of the backward thyristor. It is even more inappropriate to install a new thyristor switch on the wall to achieve LED dimming.

4 Future LED dimming systems

So what kind of dimming system should LED use?

4.1 PWM Dimming

As mentioned above, it is best to use PWM dimming for LED dimming. When using PWM dimming, a simple PWM generator can be installed in the wall switch, and then a potentiometer can be used to control the PWM working ratio to achieve dimming. However, if you want to turn the light on and off, you need to add another pair of wires. Therefore, it is not compatible with the leads of the original thyristor switch in the wall. The original thyristor switch has only two leads, which can be dimmed and switched. This advantage is difficult to be compatible. However, in fact, the most commonly used dimming lamps are table lamps or floor lamps. Those dimming switches are installed on the power cord instead of the wall, so it doesn't matter if the two leads in the wall are used. In other words, PWM dimming can be directly applied to dimming table lamps.

4.2 Segmented switch dimming

A company in Taiwan has launched a GM6182 four-stage switch dimming called EZ-Dimming, which is a good solution. It can achieve 4-stage dimming using only ordinary light switches on the wall. The first switch is full brightness, the second switch is 60% brightness, the third switch is 40% brightness, and the fourth switch is 20% brightness. The advantage of this system is that it can achieve dimming using ordinary wall switches. And its power factor is as high as 0.92 or more. There is no worry about generating interference signals. The disadvantage is that it cannot dim continuously. And the operation is a bit more troublesome.

4.3 Remote Control Dimming

Use infrared remote control to dim LED. This is certainly the most ideal solution. It can realize switching lights and continuous dimming with PWM. The disadvantage is that it is expensive, has no uniform specifications, and can only be used in high-end residences.

In fact, we should think back to what the main purpose of dimming should be. All the dimming purposes mentioned above are to meet the needs of people at home for different light intensities in different occasions. For example, it may be darker when watching TV, and brighter when reading. These are mostly in residential areas. Few offices, shopping malls, factories, and schools are equipped with dimming lamps. Moreover, most of these places are equipped with fluorescent lamps and energy-saving lamps, and it is impossible to dim or difficult to achieve continuous dimming.

5. Revolutionary dimming for energy saving

Since humans realized that they must do everything they can to save energy and reduce emissions in order to solve the urgent problem of global warming, how to reduce electricity consumption for lighting has been put on the agenda as an important issue. Because electricity consumption for lighting accounts for 20% of total energy consumption. Fortunately, high-efficiency and energy-saving LEDs have emerged. LEDs themselves save more than 5 times more energy than incandescent lamps, and about twice as much energy as fluorescent lamps and energy-saving lamps, and do not contain mercury like fluorescent lamps and energy-saving lamps. If you can also use dimming to save energy, it will also be a very important energy-saving method. But in the past, it was very difficult to achieve dimming for all light sources, and easy dimming is a great advantage of LEDs. Because in many occasions, it is actually not necessary to turn on the lights or at least not so bright, but the lights are turned on very brightly, such as street lights from midnight to dawn; the lighting in the subway car when it goes from underground to the ground in the suburbs; more commonly, when the sun is shining, the fluorescent lights in offices, schools, factories, etc. near the windows are still on. I don’t know how much electricity is wasted in these places every day! In the past, because high-pressure sodium lamps, fluorescent lamps, ceiling lamps, and energy-saving lamps could not be dimmed at all, they could only forget about it. Now that we have switched to LED, we can adjust the light freely, and the electricity can be saved! Therefore, for the dimming of lamps, home wall dimming is not the main application occasion, and the market is also very small. Instead, the on-demand dimming of street lamps, offices, shopping malls, schools, and factories is a more important occasion. Not only is the market huge, but the energy saving is also considerable. These occasions do not need manual dimming but automatic dimming and intelligent dimming!

5.1 Dimming of Street Lights

Generally speaking, street lights are useless after midnight, so the usual practice is to turn off the lights or turn on half the brightness after 12 o'clock. However, the most reasonable approach is to control the brightness of street lights according to traffic flow, or even to control the brightness completely adaptively. Figure 10 is an example of adjusting the brightness of street lights based on local traffic flow statistics.

5.2 Photosensitive automatic dimming LED lamp

In order to reduce unnecessary lighting in strong sunlight, a light-sensitive automatic dimming LED fluorescent lamp (or any other LED lamp) can be used. Its block diagram is shown in Figure 11.

Figure 11. Block diagram and actual image of a photosensitive automatic dimming LED lamp

The function of the photosensitive element is to sense the surrounding sunlight. If the sunlight is stronger, a PWM signal will be output to all LED lamps (such as LED fluorescent lamps) close to the sunlight to dim their brightness. A dimming signal generator can adjust many LED lamps as long as the constant current drive source of these lamps has a PWM dimming control interface. The efficiency of this dimming system itself is as high as 92%. And there is no compatibility problem with the thyristor dimming circuit on the wall. This fully automatic adaptive energy-saving dimming is simply impossible to achieve with any fluorescent lamp, energy-saving lamp, high-pressure sodium lamp or other gas discharge tube, but it is what LED lamps are best at.

6 Conclusion

The number of fluorescent lamps and energy-saving lamps installed in the country is very astonishing. According to statistics from the Ministry of Industry and Information Technology, my country's production of fluorescent lamps in 2008 exceeded 4 billion, of which 3.86 billion were exported. According to statistics from the China Lighting Association, the number of fluorescent lamps consumed in China each year is about 400 million. Assuming that the actual use of fluorescent lamps in China is 1 billion (mostly installed in offices, shopping malls, and factories). Assuming that each lamp is turned on for an average of 4 hours a day, and the average power of each lamp is 25W (the rated power of a 1.2-meter T8 fluorescent lamp is 36W, and the power consumption is more than 40W. However, the actual power of domestic fluorescent lamps is lower, so it is assumed to be 25W), the daily power consumption is 0.1 kWh, and the annual power consumption is 36.5 kWh. Excluding holidays, the power consumption is 30 kWh. 1 billion lamps is 30 billion kWh. After switching to LED fluorescent lamps, it is possible to save at least half of the energy, which is 15 billion kWh. Using automatic dimming can save at least 10% more energy. That is 1.5 billion kWh. Calculated at 0.7 yuan per kWh, it means saving 1.05 billion yuan. This is a very impressive number! This number does not include the energy-saving dimming of energy-saving lamps and incandescent lamps that will be replaced by LEDs. Therefore, vigorously developing energy-saving adaptive dimming is the key direction of LED dimming!

Figure 10. Traffic flow statistics to intelligently adjust the brightness of street lights

In order to realize this intelligent dimming, it is actually very simple. Just input the curve of the traffic flow statistics value in this area into a single chip microcomputer, and give the PWM dimming signal to the constant current drive source according to this curve.