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And the LED with temperature and hours of work change,display signs, change curve shown in Figure 1.2. Figure 1.2 The relative change of the temperature curve can be seen from Figure 1.2, with the temperature rise will gradually become smaller, on the contrary,menu board, when the temperature decreases, will increase. When the LED's temperature rose to 85 degrees, has become a 3.25V (3.4V-0.15V),LED outdoor sign, the opposite when the temperature dropped to -40 degrees LED becomes 3.6V (3.4V +0.2 V).
Therefore, LED dimming, to get transferred to a fixed brightness LED, it is necessary to ensure a fixed, constant current control to be used is to say. There is another way: constant power control. It is also currently on the market generally used ASIC dimming way. But the constant power control has the inherent disadvantage.
2 Constant power control LED dimming 2.1 theoretical basis for constant power control using single-stage FLYBACK general topology, the basic block diagram shown in Figure 2.1. Figure 2.1 Diagram of constant power control can be seen from the diagram of switching power supply FLYBACK general than constant power program more than a dimmer signal control circuit for detecting the input SCR conduction angle and input voltage, thus gives the corresponding dimming signal.
Meanwhile, the program is no feedback signal, completely open-loop control, the size of the original edge of the control duty cycle to control the size of the output power. Theoretical basis for its control are as follows: Where: constant power control mode are the first to predict the output power of LED P,LED billboard, in theory, once after a good transformer design, have determined that as long as the change in the size of the primary current, you can change the output power size.