LED dimming Engineering Technology

Author: Mao Yuhai

As a light source, dimming is very important. Not only in order to get a more comfortable environment in the home, in today's case, to reduce unnecessary electro optical cable, in order to further achieve the purpose of energy-saving emission reduction is more important thing. And for the LED light source, dimming is also easier than other fluorescent lamps, energy-saving lamps, high pressure sodium lamp, etc., so it should be in various types of LED lamps and light up the function.

The first part of the DC power supply LED dimming technology

A method of adjusting the forward current to adjust the brightness

To change the brightness of LED, is very easy to achieve. The first thing to think about is to change the drive current, because the brightness of the LED is almost directly proportional to the driving current. Figure 1 shows the relationship between the output light intensity and the forward current of Cree's XLampXP-G.

Output Figure 1 XLampXP-G intensity and forward current

As can be seen from the figure, if the light output at 350mA is 100%, the light output of the 200mA is about 60%, and 100mA is about 25%. So it is easy to adjust the brightness.

1.1 method of regulating forward current

Method of adjusting the current of the LED is the most simple change and LED load current detection resistor in series (Figure 2a), almost all of the DC-DC constant current chip has a current detection interface is detected by the internal voltage and the reference voltage chip, constant current control. However, the value of the detection resistor is usually small, only a few tenths of a few euros, if it is necessary to install a zero on the wall of the potentiometer to adjust the current is unlikely, because the lead resistance will be a few zeros. So some chips provide a control voltage interface, change the input control voltage can change its output constant current value. For example, the LT3478 Corporation (Figure 2b) as long as the change ratio of R1 and R2, also can change its output constant current value.

(a) (b)

Figure 2 the output value of the constant current regulation

1.2 the forward current will cause the shift of the chromatogram

However by adjusting the forward current to adjust the brightness will have a problem, that is in the brightness adjustment will also change its color temperature and spectrum. Because the current white LED is blue light excitation LED yellow fluorescent powder produced when the forward current decreases, and increase the brightness of blue LED yellow phosphor thickness does not proportionally thinner, so as to increase the main wavelength of the spectrum. Examples are shown in figure 3.

Figure 3.. Relationship between the main wavelength and forward current

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

The change in the forward current also causes a change in color temperature (Figure 4).

Fig. 4 white LED color temperature and forward current

As can be seen from Figure 4, when the forward current is 350mA, the color temperature is 5734K, while the forward current is increased to 350mA, the color temperature is offset to 5636K. If the current is further reduced, the color temperature will change to the warm color.

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

1.3 the current will produce serious problems that the constant current source can not work

However, in the specific implementation, the dimming of the forward current can produce a more serious problem.

We know that LED is usually driven by constant current drive power DC-DC, and this kind of constant current source is usually divided into two kinds of buck boost type or (of course, lifting the pressure type, but because of low efficiency, the price is expensive and not used). Whether the boost or buck is determined by the relationship between the supply voltage and the LED load voltage. If the supply voltage is lower than the load voltage on the use of step-up type; if the supply voltage is higher than the load voltage. The forward voltage of LED is determined by the forward current. According to the characteristic of LED (Figure 5), the change of forward current will cause the corresponding change of forward voltage. So when the current is lowered, the forward voltage of the LED is also reduced. This changes the relationship between the supply voltage and the load voltage.

Figure 5 LED volt ampere characteristic

For example, in a 24V input LED lamps, the use of 8 1W high-power LED series. When the forward current is 350mA, the forward voltage of each LED is 3.3V. Then the 8 series is 26.4V, higher than the input voltage. Therefore, the constant current source should be used. However, in order to dimming, the current down to 100mA, this time the forward voltage is only 2.8V, 8 series of 22.4V, the load voltage becomes lower than the supply voltage. This type of boost constant current source can not work at all, but should be based on step-down. For a boost type constant current source must be working on the buck is not enough, and finally the LED will flicker. In fact, as long as the use of the boost type constant current source, in the use of positive current dimming, as long as the transfer to a very low brightness will almost certainly flicker. Because at that time the LED load voltage must be lower than the supply voltage. Because a lot of people do not understand the problem, but also to go from the circuit dimming to find problems, it is no use.

The use of low voltage constant current source will be less, because if the supply voltage is higher than the load voltage, when the brightness is low, the load voltage is reduced, so still need to buck type constant current source. But if to very low forward current, load voltage LED becomes very low, when the voltage ratio is very large, it may be beyond the normal operating range of this step-down constant current source, will make it unable to work due to flicker.

1.4 working for a long time at low brightness may reduce the efficiency of the constant current source and increase the temperature rise

It is generally believed that the downward dimming is to reduce the output power of the constant current source, so it is impossible to cause the power consumption of the constant current source and the temperature rise. However, it is not known that the forward voltage drop caused by the decrease of forward current will reduce the voltage ratio. However, the efficiency of the buck constant current source is related to the ratio of the buck to the buck. The lower the voltage ratio is, the lower the efficiency is. Figure 6 is the relationship between the efficiency of SLM2842J and the ratio of the buck.

The relationship between efficiency and buck Figure 6 buck constant current source ratio

The input voltage is 35V, the output current is 2A, 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 is reduced to 96%, when the output voltage is reduced to 10V, the efficiency is reduced to 92%. In these three cases, although the output power is 60W, 40W and 20W in turn, but the loss power is in turn 1.2W, 1.6W, 1.6W. Power consumption increased by 33% in the last two cases. If the cooling system of the constant current module design is very critical, increase the power dissipation of 33% is likely to make the chip junction temperature increased, resulting in over temperature protection and can not work, serious may also make the chip burned.

1.5 adjust the forward current can not get precise dimming

Because the forward current and the light output are not directly proportional to each other, and different LED will have different forward current and light output curve. Therefore, it is very difficult to realize precise output control by adjusting the forward current.

Two. Pulse width modulation (PWM) is used to adjust light

LED is a diode which can realize fast switching. Its switching speed can be up to microseconds. Is any light-emitting devices can not match. Therefore, as long as the power source is changed to 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. Figure 7 shows the pulse width modulation waveform. If the pulse period is TPWM, the pulse width is ton, then the work is more than D (or hole ratio) is ton/tpwm. Changing the operating ratio of the constant current source pulse can change the brightness of the LED.

Figure 7.. The method of changing the pulse width to change the brightness of LED

2.1 how to achieve PWM dimming

The specific way to realize the PWM dimming is to load a MOS switch in the LED load (Figure 8). This series of LED is supplied with a constant current source.

Figure 8 PWM signal with a fast break LED string

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

2.2 the advantages of pulse width modulation dimming

1 does not produce any chromatographic shift. Because LED always works at full amplitude current and between 0. 2 can have a very high dimming accuracy. Because the pulse waveform can be controlled to a very high accuracy, it is easy to achieve the accuracy of 1/10000.

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 large range of dimming, flicker will not occur. Because it does not change the working conditions of the constant current source (step-up ratio or step-down ratio), it is impossible to overheat.

2.3 pulse width modulation should pay attention to the problem

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

2 the elimination of dimming caused by howling: Although 200Hz more than the human eye can see, but until 20kHz is the scope of the human hearing. At this time it is possible to hear the slightest sound. There are two ways to solve this problem, one is to improve the switching frequency to 20kHz above, out of the range of human hearing. However, the frequency is too high will cause some problems, because the impact of various parasitic parameters, the pulse waveform (front and rear) distortion. This reduces the accuracy of dimming. Another way is to find out the audible device and process it. In fact, the main sound device is the output of the ceramic capacitor, because the ceramic capacitor is usually made of high dielectric constant ceramic, this kind of ceramics have piezoelectric properties. Under the action of 200Hz pulse will produce mechanical vibration and sound. The solution is to replace tantalum capacitors. However, it is difficult to obtain high - voltage tantalum capacitors, and the price is very expensive, will increase some costs.

The second part uses the LED dimming of AC power supply

Three. LED dimming with silicon controlled rectifier

Ordinary incandescent and halogen lamps are usually controlled by silicon to light. Because incandescent lamp and halogen lamp is a pure resistance device, it does not require the input voltage is sinusoidal current waveform, because it is always the same and the voltage waveform, so no matter how the deviation from sine wave voltage waveform, as long as the effective value change of the input voltage, it can adjust the light. Silicon controlled rectifier is used to cut AC sine wave to achieve the purpose of changing its effective value. The electrical schematic is shown in figure 9. The dotted line is a silicon controlled dimmer switch mounted on the wall. A-b