Main dimming technology of LED lamps and lanterns

LED lights are widely used in mine field of lighting industry, lighting industry, LED stage lights, such as the LED moving head light, LED project-light lamp, LED wash wall light, LED effect lights stage, such as light-emitting diodes (leds) have been recognized and loved by the masses of users. 
 
LED as a light source, the light is very important. Not only to get a more comfortable environment in household, in today, reduce unnecessary electric light, in order to achieve the purpose of energy saving and emission reduction is one of the more important thing. And for the LED light source, dimmer than other fluorescent lamp, energy-saving lamp, high-pressure sodium lamp and so on are easier to implement, so it should be added to raise to the various types of LED lamps and lanterns light function. 
 
First, using dc power led dimming technology 
 
Mixed with forward current methods to adjust the brightness to change the brightness of the LED, is very easy to implement. The first thought is to change its drive current, because of the brightness of the LED is almost and its drive current is directly proportional to the relationship. 
 
1 to adjust forward current method 
 
Adjust LED the simplest method is to change the current resistance and LED in the series of load current detection, almost all DC - DC constant current chips have a test current of interface, the voltage is detected and chip internal reference voltage, to control the current of constant. But the sense resistor values are usually very small, only a fraction of the, if you want to put a zero on the wall a few of the potentiometer to adjust current is unlikely, because there will be a fraction of the lead resistance. So some chip provides a control voltage interface, change the input control can change the output voltage constant current value. 
 
2 the forward current chromatography deviation 
 
However, mingled with forward current methods to adjust the brightness will produce a problem, that is at the same time of adjustable brightness can also change its spectrum and color temperature. Because the white LED is done with lincom LED excitation yellow phosphor, forward current, small blue LED brightness increased with the thickness of yellow phosphor is not in proportion to thinning, making it the dominant wavelength of the spectrum. Such as: when the forward current of 350 ma, color temperature is 5734 k, the forward current increases to 350 ma, colour temperature is offset to 5636 k. The current is further reduced 
 
, color temperature to warm color changing. 
 
Of course, these problems in the general of the actual lighting may not be a big problem. In using the RGB LED system, however, will cause the deviation of color, and the human eye is very sensitive to color deviation, so also is not allowed. 
 
3 adjustable current will produce serious problem make constant current source will not work 
 
However, in the concrete implementation, mixed with forward current methods to move light may lead to a more serious problem. 
 
We know that the LED is usually use DC - DC constant current drive power to drive, and this kind of constant current drive source is usually divided into type step-up or step-down two (of course, lifting pressure type, but due to the low efficiency, is expensive and not commonly used). What use step-up or step-down 
 
Is the relationship between the load voltage by the power supply voltage and LED to a decision. If the power supply voltage is lower than the load voltage using a booster; If the supply voltage is higher than the load voltage using a step-down. And the LED forward voltage is determined by the forward current. From the LED volt ampere 
 
Characteristics, the forward current of change will cause the corresponding change of forward voltage, to be exact, the decrease of the forward current will cause the decrease of the forward voltage. So when the current lower forward voltage of the LED with reduced. This will change the power supply voltage 
 
And the relationship between the load voltage. 
 
For example, in an input for 24 v LED lamps and lanterns, chose eight 1 w high power LED. In the forward current of 350 ma, each LED forward voltage is 3.3 V. Then eight series is 26.4 V, is higher than the input voltage. So you should use booster type constant current source. However, in order to move light, reduce the current to ma, 100 at that time the forward voltage of only 2.8 V, eight series of 22.4 V, load voltage becomes lower than the power supply voltage. This type of pressure constant current source can't work, and buck type should be used. For a booster type constant current source must be it is not working in step-down, finally LED flashing phenomenon will occur. In fact, as long as it is used the booster type constant current source, when mixed with forward current adjustable light, as long as transferred to the very low brightness will almost certainly produce flashing phenomenon. Because at that time the LED load voltage must be below the supply voltage. Because a lot of people don't understand the question, is always to find problems from the dimming circuit, it was all in vain. 
 
Using step-down type constant current source problem will be less, because if the power supply voltage is higher than the load voltage, when the brightness is low, the load voltage is lower, so still need to buck type constant current source. But if transferred to the forward current is very low, the LED load voltage also become very low, then step-down ratio is very big, may also beyond the normal work of the step-down type constant current source, also can make it unable to work and produce flashing. 
 
4 to work long hours in the low light could make step-down type constant current source higher efficiency to reduce temperature rise and unable to work 
 
Ordinary people may think to cut just to reduce the output power of the constant current source, it is not possible cause step-down type constant current source temperature increased by the increasing power consumption. But when reduce the forward current caused by the forward voltage lower depress step-down ratio. And buck type constant current 
 
Source efficiency is associated with step-down than, step-down ratio, the greater the efficiency is lower, the greater the power consumption of loss on a chip. 
 
5 forward current cannot get accurate adjustment 
 
Because the forward current and optical output is not fully proportional relationship, and different leds have different forward current and optical output curve. So it's difficult to use the method of adjusting the forward current to achieve precise control of light output. 
 
LED is a diode, it can realize fast switch. The switching speed of it can be as high as above microseconds. Is unmatched by any light emitting device. Therefore, as long as the power supply into a pulse constant-current source, use the method of pulse width, you can change the brightness. 
 
This method is called pulse width modulation (PWM) dimming method. If pulse cycle for TPWM, pulse width is ton, so its work than D (or called hole degree than) is ton/TPWM constant-current pulse change work than you can change the brightness of the LED. 
 
Second, using the ac power LED dimming 
 
1 how to implement a PWM dimming 
 
Concrete implementation method of PWM dimming is in LED series with the load of a MOS switch tube, the list of the LED anode with a constant current source power supply. 
 
And then use a PWM signal to the MOS tube grid, in order to quickly switch the string of leds, so as to realize the dimmer. There are many constant current chip itself with a PWM interface, can be directly to PWM signal, and output control MOS switch tube. Then the PWM dimming 
 
What are the advantages and disadvantages for the method? 
 
Pulse width modulation (PWM) dimming the advantages of the two: 
 
1. Does not produce any chromatographic deviation. Because the LED always work in full range of current and between 0. 
 
2. Can have high accuracy that move light. Because the pulse waveform can completely control to high precision, so it's easy to achieve the accuracy of one over ten thousand. 
 
3. Can be merged and digital control technology to control. Because any number can be easy to transform into a PWM signal. 
 
4. Even in a wide range of light, flashing phenomenon will not occur. Because does not change the working conditions of constant current source (step-up or step-down ratio), the overheating of the more unlikely. 
 
 
 
Three, pulse width adjustable light must pay attention to the problem 
 
1. The choice of pulse frequency: because the LED is in rapid switch state, if the working frequency is low, the human eye will be flashing. In order to make full use of human visual residue phenomenon, its working frequency should be higher than 100 hz, it is best to 200 hz. 
 
2. Howl caused dimming: although more than 200 hz human eye cannot perceive, but has been to 20 KHZ is the human ear hearing range. At that time may hear the sound of the bit. There are two ways to solve this problem, one is to increase the switching frequency to more than 20 KHZ, jump out of the human ear hearing range. But high frequency can also cause some problems because of various kinds of parasitic parameters, the influence of pulse waveform (along) before and after the causes distortion. This reduces the precision of the dimmer. Another way is to find the voice device 
 
And processing. In fact, the main voice device is the output of ceramic capacitors, because ceramic capacitors are usually made by the high dielectric constant of ceramics, this kind of ceramic piezoelectric properties. In 200 hz pulse is produced under the action of mechanical vibration and sound. 

----------Archled Technology Co.,Limited