As a light source, dimming is very important. Not only in order to get a more comfortable environment at home, in today’s world, reducing unnecessary electric lighting, in order to further achieve the goal of energy conservation and emission reduction is more important. And for LED light source, dimming is easier to achieve than other fluorescent lamps, energy-saving lamps, high-pressure sodium lamps, so it should be more in various types of LED lamps increase the dimming function of light.
Part A: Adopts the dimming technology of dc power led
1. Adjust the brightness by adjusting the forward current
To change the brightness of led, it is easy. The first thought is to change its driving current, because the brightness of the LED is almost directly proportional to its driving current. The figure shows the relationship between the output relative light intensity and positive current.
According to the figure, if the light output at 350mA is 100%, then the light output at 200mA is about 60% and that at 100mA is about 25%. So adjusting the current can easily realize the brightness adjustment.
1.1 Method of regulating forward current
The easiest way to adjust the LED current is to change the current detection resistance in series with the LED load (figure a). Almost all dc-dc constant-current chips have an interface to detect the current, which controls the constant current by comparing the detected voltage with the reference voltage inside the chip. But the value of this resistance is usually very small, only a few tenths of an ohm. It is not possible to install a potentiometer of a few tenths of an ohm on the wall to regulate the current, because the lead resistance will also have several tenths of an ohm. So some chips provide a control voltage interface, changing the input control voltage can change the output constant current value. For example, LT’s LT3478 (figure b) can also change the constant current value of its output by changing the ratio of R1 to R2.
1.2 Adjusting the forward current will offset the chromatography
However, the problem with adjusting brightness by adjusting forward current is that it also changes its spectrum and color temperature. At present, white led are generated by blue led to stimulate yellow phosphors. When the forward current decreases, the brightness of blue led increases while the thickness of yellow phosphors does not decrease proportionally, so that the main wavelength of its spectrum increases. The specific example is shown in the figure below.
When the forward current is 350mA, the main wavelength is 545.8nm. When the forward current decreases to 200mA, the main wavelength is 548.6nm.When the forward current decreases to 100mA, the main wavelength is 550.2nm. The change of forward current will also cause the change of color temperature (as shown in the figure below).
According to the figure above, when the forward current is 350mA, the color temperature is 5734K, while when the forward current increases to 350mA, the color temperature is shifted to 5636K. As the current decreases further, 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 LED system with RGB, color deviation will be caused, and the human eye is very sensitive to color deviation, so it is not allowed.
1.3 Regulating the current can create serious problems that make a constant current source inoperable
However, a more serious problem may be caused by adjusting the forward current.
We know that LED is usually used DC-DC constant current drive power to drive, and this type of constant current drive source is usually divided into boost type or buck type two (of course, there is a rise and fall type, but because of low efficiency, expensive and not commonly used). Whether to use boost type or step-down type is determined by the relationship between the supply voltage and the LED load voltage. If the power supply voltage is lower than the load voltage, use booster type;If the supply voltage is higher than the load voltage, use the step-down type. The forward voltage of LED is determined by its forward current.According to the volt-ampere characteristics of the LED (as shown in the figure below), a change in the forward current will lead to a corresponding change in the forward voltage. Specifically, a decrease in the forward current will also lead to a decrease in the forward voltage. So when the current is turned down, the forward voltage of the LED decreases. This changes the relationship between the supply voltage and the load voltage.
For example, in a 24V input LED lamp, 8pcs of 1W high-power leds are used in series.When the forward current is 350mA, the forward voltage of each LED is 3.3v. So eight in series is 26.4v, higher than the input voltage. Therefore, the booster type constant current source 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 pieces in series are 22.4v, so the load voltage becomes lower than the supply voltage. In this way, the booster type constant current source cannot work at all, and should use the step-down type. For a boost type constant current source must it work in the step-down is not good, finally LED will appear flashing phenomenon. In fact, as long as it is a booster type constant current source, when using the forward current modulation, as long as the brightness is very low will almost certainly produce flicker phenomenon. Because at that time the LED load voltage must be lower than the supply voltage. Because many people do not understand the problem, but always from the dimming circuit to find the problem, it is useless.
Use buck type constant current source problem will be less, because if the original supply voltage is higher than the load voltage, when the brightness is low, the load voltage is reduced, so still need buck type constant current source.However, if the forward current is set to a very low level, the load voltage of LED will also become very low. At that time, the step-down ratio will be very large, which may also be beyond the normal working range of this step-down constant current source, which will also make it unable to work and generate flicker.
1.4 Working in low brightness for a long time may reduce the efficiency of step-down constant-current source and increase the temperature rise
Some people may think that downgrading light reduces the output power of constant current source, so it is impossible to cause the increase of power consumption and temperature rise of step-down constant current source.However, when the forward current is reduced, the forward voltage drop will reduce the step-down ratio. The efficiency of the step-down constant current source is related to the step-down ratio. The higher the step-down ratio is, the lower the efficiency will be, and the higher the power consumption of the loss on the chip will be. The following picture shows the relationship curve between SLM2842J’s efficiency and the step-down ratio.
The input voltage in the figure is 35V and the output current is 2A. When the output voltage is 30V, the efficiency can be as high as 97.8%. But when the output voltage drops to 20V, the efficiency drops to 96%. When the output voltage is reduced to 10V, the efficiency is reduced to 92%. In these three cases, although its output power is 60W, 40W and 20W successively,but its loss power is 1.2w, 1.6w and 1.6w successively.In the latter two cases, the power consumption have increased by 33%. If the design of the heat dissipation system of the constant current module is very critical, increasing the dissipated power by 33% May make the junction temperature of the chip rise high, so that the temperature protection cannot work, and the chip may be burned down seriously.
1.5 Adjusting the forward current does not provide accurate dimming
Because of the positive current and light output are not completely proportional, and different LED will have different positive current and light output relationship curve.Therefore, it is difficult to achieve accurate optical output control by adjusting forward current.
2. Pulse width modulation (PWM) is used for dimming
LED is a diode that can switch on and off quickly.It can be switched on and off at speeds of more than microseconds, which is unmatched by any luminescent device. Therefore, as long as the power source to pulse constant current source, by changing the pulse width method, you can change its brightness.This method is called pulse width modulation (PWM) dimming. The following figure 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 called aperture ratio) is ton/ TPWM. Changing the working ratio of the constant current source pulse can change the brightness of the led also.
2.1 How to achieve PWM dimming
Specific PWM dimming method is to put a MOS switch tube in the load of LED (as shown in the figure below), and the anode of this series of LED is powered by a constant current source.
Then a PWM signal is added to the gate of the MOS tube to quickly switch on and off the string of leds, thus achieving dimming. There are also many constant current chip itself with a PWM interface, which can directly accept PWM signal, and then output control MOS switch tube. Then what are the advantages and disadvantages of this PWM dimming method?
2.2 Advantages of PWM dimming
a: There is no chromatographic shift.Because the LED always works between full amplitude current and 0.
b: Can have extremely high dimming accuracy. Because the pulse waveform can be completely controlled to a very high accuracy, it is easy to achieve the accuracy of 1/10,000.
c: Can be combined with digital control technology to control.Because any number can be easily converted into a PWM signal.
d: Even in a large range of dimming, flicker will not occur. Because it will not change the working conditions of the constant current source (boost ratio or buck ratio), it is more unlikely to occur overheating and other problems.
2.3 Pay attention to the problem of pulse width dimming.
a: Pulse frequency selection
Because the LED is in the fast switching state, if the working frequency is very low, the human eye will feel flashing. In order to take full advantage of the visual residual phenomenon of the human eye, its working frequency should be higher than 100Hz, preferably 200Hz.
b: Eliminate howling caused by dimming
Although more than 200Hz cannot be detected by the human eye, but the range of human hearing is all the way up to 20kHz. There’s a chance you’ll hear something.There are two ways to solve this problem. One is to raise the switch frequency above 20kHz, and get out of the range of human hearing. But too high a frequency can also cause problems, as various parasitic parameters can distort the pulse waveform (front and rear edges). This reduces the accuracy of dimming.Another way is to find the device that makes the sound and deal with it. In fact, the main vocalizing device is the output ceramic capacitor, because ceramic capacitors are usually made of high permittivity ceramics, which have piezoelectric properties. At a pulse of 200Hz, the sound is produced by mechanical vibration. The solution is to use tantalum capacitors instead. However, high voltage tantalum capacitors are hard to get and expensive, which will increase the cost.
Part B: Led dimming with ac power supply
3.Use Triac for Led dimming
Ordinary incandescent lamps and halogen lamps usually use Triac to dimmer. Since incandescent lamp and halogen lamp are pure resistance devices, it does not require the input voltage to be sinusoidal wave, because its current waveform is always the same as the voltage waveform, so no matter how the voltage waveform deviates from the sinusoidal wave, as long as the effective value of the input voltage can be changed, the dimming can be done. Using Triac is to cut the sine wave of alternating current to change its effective value. Its electrical schematic diagram is shown in the figure below. The dotted line is a wall mounted silicon controlled dimmer switch. The resistance between a and b is the incandescent lamp load. So the load is in series with the Triac switch.
Changing the partial voltage ratio of variable resistance can change its conduction Angle, so as to change its effective value. Usually this potentiometer has a switch, connected to the input of n, for turning on and off the light.In addition to Triac, there are transistor back edge dimming technology and so on, because their basic problems are the same, not here.
3.1 Defects and problems of Triac controlled dimming
a: The Triac destroys the waveform of sinusoidal wave, thus reducing the value of power factor, normally PF is less than 0.5, and the smaller the conduction Angle, the worse the power factor (only 0.25 at 1/4 brightness).
b: Similarly, the non-sinusoidal waveform increases the harmonic coefficient.
c: Non-sinusoidal waveforms can produce significant interference signals (EMI) on the line.
d: It is easy to be unstable at low loads and must be coupled with a discharge resistance.This discharge resistance consumes at least 1-2 watts of power.
e: When the ordinary Triac dimming circuit is output to the driving power of LED, there will be unexpected problems, that is, LC filter at the input end will make the Triac oscillate, which is indifferent to incandescent lamp, because the thermal inertia of incandescent lamp makes the human eyes cannot see this kind of oscillation at all. However, the drive power of LED will generate audio noise and flicker.
3.2 Advantage of Triac dimming
Although Triac dimming has so many shortcomings and problems, it has certain advantages, that is, it has formed an alliance with incandescent halogen lamps, occupying a large dimming market.If leds want to replace Triac dimming incandescent and halogen lamps, they also need to be Triac dimming compatible.
Specifically, in some places where Triac dimming incandescent or halogen lamps have been installed, Triac dimming switches and knobs have been installed on the walls, and two connecting wires leading to the lamps have been installed in the walls. It is not easy to replace the Triac switch on the wall and increase the number of connecting wires. The easiest way is to keep everything the same. Just unscrew the incandescent lamp on the lamp holder and replace it with an LED bulb with compatible thyristor dimming function. This kind of strategy is like LED fluorescent lamp, had better make it and current T10, T8 fluorescent lamp size is exactly the same, do not need professional electrician, common people can be replaced directly, that can be popularized very quickly. Therefore, many foreign manufacturers of LED driver IC have developed IC compatible with the existing Triac controlled dimming.
3.3 Compatible Triac controlled dimming problems and disadvantages
Although many multinational chip companies have launched the existing Triac controlled dimming chip and solutions. But such solutions are not recommended for the following reasons:
a: Triac technology is more than half a century old technology, it has many disadvantages as mentioned above, is a long overdue technology.It should go out of business at the same time as incandescent and halogen bulbs.
b: Many of these chips claim to have PFC, which improves the power factor. In fact, it only improves the power factor that ACTS as a load on the Triac, making them look close to the pure resistance of incandescent and halogen lamps, without improving the power factor of the whole system, including the Triac.
c: The overall efficiency of all silicon-compatible LED dimming systems is very low, and some have not considered the loss of discharge resistance needed for stable operation, which completely damages the high efficiency of LED.
d: All Triac LED dimming systems also regulate the positive current of LED, which has disadvantages such as chromatographic deviation mentioned above.
e: The proportion of incandescent lamp and halogen lamp that install Triac dimming is less than 1/10000, and the proportion of Triac switch installed in the wall is less than 1/10000 in the Triac dimming lamps, because the vast majority of Triac dimming installation is lamp, bedside lamp, stand lamp.What’s more, there are dozens of different specifications of Triac and transistor dimming switch on the market. In fact, the IC developed is not compatible with all thyristor switches, but only a small part of them.
f: LED is a new genesis technology, it has incomparable advantages. There is no need to sacrifice the advantages of leds in order to take care of the backward thyristor.Not to go to the newly installed wall thyristor switch to achieve LED dimming.
4. So what kind of dimming system of leds should be used?
4.1 PWM dimming
It has been said that LED dimming is best to use PWM dimming, using PWM dimming, you can install a simple PWM generator in the wall switch, and then use potentiometer to control the work ratio of PWM to achieve dimming. But if you want to turn the lights on and off, you need to add another pair of wires.So it is not compatible with the lead of the Triac switch in the original wall.The original Triac switch has only 2 leads, which can be dimmed and switched.This advantage is not compatible.But the most commonly used dimmers are desk lamps or standing lights, and those dimmers are mounted on power cords rather than walls, so it doesn’t matter if you use the two leads in the wall. That is to say, PWM dimming can be directly applied to dimming lamp.
4.2 Segmental switch dimming
Using the ordinary light switch on the wall can achieve 4 dimming, the first time for full bright, the second time for 60% brightness, the third time for 40% brightness, the fourth time for 20% brightness.The advantage of this system is that it can be dimmed by ordinary wall switches.And its power factor is as high as 0.92 above.No interference signals are generated. The disadvantage is the inability to continuously dimming.And it’s a little tricky to operate.
4.3 Remote control dimming
Infrared remote control is adopted to realize dimming of LED. This is certainly the ideal solution.Can achieve on and off lights, and with PWM continuous dimming. The disadvantage is high cost, no uniform specifications, can only be used for high-end housing.
In fact, we should think back to what the main purpose of our dimming should be.All the above mentioned dimming purposes are to meet the needs of people at home in different occasions different light intensity. For example, it might be darker to watch TV and brighter to read. Most of these are in houses. Few offices, shopping malls, factories and schools have dimmers. And the vast majority of these places are installed fluorescent lamps, energy-saving lamps, it is impossible to carry out dimming or difficult to achieve continuous dimming.
5. Dimming for energy saving
Since the realization of mankind must do everything possible to save energy and reduce emissions, in order to solve the urgent problem of atmospheric warming, how to reduce lighting electricity consumption as an important issue on the agenda. Because lighting accounts for 20% of total energy consumption. Fortunately, the emergence of efficient energy-saving LED, LED itself than incandescent lamp energy saving more than 5 times, than fluorescent lamp, energy-saving lamp also want to save about one time, but not like fluorescent lamp, energy-saving lamp that contains mercury. If you can use dimming to save energy, then it is also a very important means of energy conservation.But in the past all light sources are very difficult to achieve dimming, and easy dimming is a big advantage of LED. Because there are many occasions when you don’t really need to turn on the light, or at least not that bright, but the light is very bright, such as: street lamps between midnight and dawn;Lights in subway cars as they move from underground to suburban floors;more often, when the sun is shining, the fluorescent lights of offices, schools, factories, etc., are still on and the fluorescent lights of factories, etc., are still on. There is no way to know how much electricity is wasted in these places every day. In the past, high pressure sodium lamps, fluorescent lamps, ceiling lamps and energy-saving lamps could not be dimmed at all, so they had to be calculated. After switching to LED now, you can adjust the light freely, and all this power can be saved.So for lamps dimming, dimming on the family wall is not the main application occasions,the market is small.On the other hand, it is the on-demand dimming of street lamps, offices, shopping malls, schools and factories that is more important, which not only has a huge market, but also saves considerable energy. These occasions need not manual dimming but automatic dimming, intelligent dimming.
5.1 The dimming of street lamps
Generally, street lamps are useless after midnight, so the usual practice is to turn them off or turn them on at half the brightness. But the most reasonable approach is to control the brightness of street lamps according to the traffic flow, or even to control the brightness completely adaptively.
In order to achieve this intelligent dimming, the plans are actually very simple.As long as the traffic flow statistics of this area into a single chip microcomputer, according to this curve to give PWM dimming signal to the constant current drive source can be achieved.
5.2 Automatic sensor dimming LED lamp
To minimize unnecessary lighting in high daylight, use light-sensitive auto-dimming LED fluorescent lamps (or any other LED lamps).
The function of the photosensitive element is to sense the surrounding sunlight. If the sunlight is stronger, it will output a PWM signal to all LED lamps (such as LED fluorescent lamps) close to the sunlight, and dim their brightness.A dimming signal generator can adjust many LED lamps, as long as their constant current drive source has PWM dimming control interface.The dimming system itself is more than 92 percent efficient.And there is no compatibility problem with the wall Triac dimming circuit.This automatic self-adaptive energy-saving dimming can not be achieved by any fluorescent lamp, energy-saving lamp, high-pressure sodium lamp and other gas discharge tube, but it is the LED lamp is best at.
iPHD Lighting Co., Ltd, which professional in research & production & Selling led series light since 2009. We offering outdoor led lighting & indoor led lights, outdoor fixture is included: outdoor wall pack light, led lawn light, led underwater light, led underground light, led floodlight, led wall washer, led street light, swimming led pool light, etc; indoor lamp is included: led track light, led panel light, led down light, etc.
Looking forward to service your company in the short time, Contact Us Now!
Leave A Comment