LED lighting products replace the universal incandescent lamp, energy-saving lamp illumination, in addition to dimming, LED lamp's luminous color, light output, luminous efficiency and other related indicators, and LED-driven dimming control scheme (such as 0-10V, DALI and other related control protocols), topology, cooling system, drive scheme and some existing infrastructure are closely related, which affect the working state and dimming control performance of the LED dimming process.
At present, there are several technologies related to LED dimming:
1. Adjust the brightness by adjusting the forward current
LED dimming, as its name suggests, can be understood as adjusting the brightness of the LED and adjusting the brightness of the LED. The simplest measure is to change its operating current. Because according to the characteristics of the LED, its brightness is basically proportional to its operating current. Figure 1 shows the relative light intensity and forward current of a high-power LED chip XlampXP-G from Cree, a well-known American chip company.
Figure 1 XLampXP-G output relative light intensity and forward current relationship
As can be seen from the above figure, when the LED light output is 100%, the operating current is 350mA, then when its operating current is reduced to 200mA, its light output is about 60%, when its operating current is reduced to 100mA, the light output Reduce to about 25%. Therefore, it is easy to adjust the brightness by changing the forward current of the LED.
1.1 Method of adjusting forward current
Analyze the topology of the current LED power supply circuit. Basically, all LED driver chips have an interface for detecting current. The working principle of this interface is to detect the voltage on the output current sense resistor and the reference voltage inside the chip. Compare to achieve the purpose of controlling the output current to be constant. This is also to say that the most convenient way to adjust the current of the LED is to change the current sense resistor in series with the LED load (Figure 2a) to achieve the purpose of dimming. Then, according to the factors considering the overall efficiency of the system, the value of this detection resistor is usually very small, often only a few ohms or even a few tens of ohms. It is not too much to adjust the current by installing a potentiometer of several ohms or even a few ohms on the wall. A realistic approach, because the lead resistance between the LED lamp and the potentiometer will also have a few ohms.
In addition, some chips provide an interface to control the voltage, changing the input control voltage to change its output constant current value. For example, Linear Technology's LT3478 (Fig. 2b) can change the constant current value of its output by changing the ratio of R1 and R2 to change the reference voltage inside the chip. However, due to the particularity of the circuit connection method, this method is difficult to apply to a wide range of practical lighting applications.
Figure 2 Output constant current value adjustment
2, using pulse width modulation (PWM) to dim
Unlike traditional incandescent lamps and other linear impedance sources, LED lamps are a diode that can be quickly switched, and its switching speed is basically above us. This is unmatched by other traditional light source illuminators. Therefore, for this, if the driving 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 dimming, which is PWM dimming. The waveform of this PWM pulse width modulation is shown in Figure 3 below.
Figure 3 to change the brightness of the LED by changing the pulse width
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