Why do LEDs need constant current?

LEDs have different power drive requirements from traditional light sources. Let's take a look at a special curve of various LED work. As shown in Figure 1, after the voltage is applied to the LED, when the voltage crosses a critical value, the current starts to flow, emit light, and the LED starts to work. LEDs of different colors have different critical voltages, which is consistent with the band gap we mentioned earlier. For example, red light has a long wavelength and low energy, and its corresponding LED critical voltage is also relatively low, and blue light has a short wavelength and high energy, and its bow tie voltage is also high. This voltage is an extremely important parameter of the LED, the forward voltage: VF, forward voltage. We also noticed that these voltages are relatively low, only a few volts, such as the infrared light of about 1.1V, the highest blue light is only less than 3V, even with relatively large currents such as 30-40mA, the voltage value is only 3.5 Around V. This voltage has a certain distance from our daily use voltage, so it is necessary to transform the power supply voltage to meet the LED driving requirements. Another important phenomenon is that once the voltage exceeds VF, the current changes faster, and the relationship is not linear, that is, the current increase is not proportional to the voltage increase.

Figure 1: Working curve of different color LEDs

Let's take a look at another one. Figure 2 shows the LED produced in the same batch. It can be seen from the figure that even in the same batch, its forward voltage will be 200-300mV. Although 200-300mV is only equivalent to a 10% voltage change, it will cause the flow The current through the LED is more than doubled. And we know that the luminous intensity of the LED is determined by the recombination number of electrons and holes on both sides of the PN junction, that is, the current flowing through the PN junction. This means that even under the same voltage conditions, the brightness of the same batch of LEDs will be twice as bad, which will seriously affect our vision. Therefore, the LED is a current-driven device. If you want to ensure a certain brightness of the LED, the most direct and easiest way is to input a certain current to the LED instead of controlling the voltage across the LED.

Figure 2: The relationship curve between the forward voltage and current of the same batch of LEDs. The solid and dashed lines are from different factories.

 If LEDs do not use constant current driver, not only the illuminance will be unstable, but also affect its wavelength and shorten its life. Figure 3 shows the relationship between the forward current of the blue LED and the emission wavelength. It can be seen from the figure that as the current increases, the center wavelength of the LED light will shift to a shorter wavelength. When the current is increased from 0 to 500mA, the wavelength shift is nearly 10nM, which will bring obvious color drift. What's more serious is that if the LED is operated for a long time beyond the maximum rating, it may be damaged. Figure 4 is the experimental result of Philips, which shows the premature LED decay, which is manifested as a decrease in brightness and a lower average life expectancy. As shown in the figure, at the same junction temperature, the greater the current, the shorter the life expectancy: at 140 degrees Under the junction temperature of 1.5A (B50, L70) is 17000 hours, 1A is 36000 hours, 700mA and 350mA, both will exceed 60000h.

Figure 3 The relationship between LED wavelength and current

Figure 4 LED life, drive current, junction temperature and average life (B50, L70)

Therefore, as a current-type device LED, constant current drive is very important, not only can ensure a certain brightness, but also extend its life time.