Basic characteristics of LED and its working conditions

LED lighting system is an electro-optical conversion system. Its electro-optical conversion process starts from the power supply part, and includes the original power supply ("power source"), power management and conversion, sensing and control, driver, thermal management, LED and its light mixing, scattering and optical extraction. Among them, the LED power supply system composed of the original power supply (such as battery) and the control and drive circuit is a necessary condition for the operation of LED. In a broad sense, the LED power supply system is the "power supply" of LED.

1. Basic characteristics of LED

LED is an electro-optical conversion device that does not emit light by itself but can only emit light when appropriate voltage and current are applied.

In order for LED to work properly, its basic characteristics must be understood. LED has many characteristics, the most important of which are as follows:

①LED, like an ordinary diode, is a semiconductor device containing a PN junction and has unidirectional conductivity.

②LED has a threshold voltage. Only when the voltage across the LED is higher than this threshold voltage, the LED will conduct. The conduction threshold of ordinary silicon diodes is 0.5~0.7V, while the threshold voltage of LEDs is usually 1.5~3.5V. The threshold voltage of LEDs and the forward voltage drop during normal operation are related to the light color of the LED. The forward voltage drop (VF) of red, green, yellow LEDs is usually .4~2.6V, while the forward voltage drop of white LEDs is usually 3~4.2V.

③LED has a nonlinear volt-ampere characteristic curve, and the current passing through the LED is not proportional to the voltage applied across it.

④The luminous flux output of LED increases with the increase of current flowing through LED, but it is not proportional. When the luminous flux increases to a certain extent, the amount of increase with the increase of current is very small, showing a trend of obvious slowdown.

⑤LED is a device that is sensitive to temperature. When its junction temperature rises, the light output will decrease and the forward voltage will also decrease.

⑥ Even for LED devices of the same model or even produced in the same batch, the discreteness of their parameters is relatively large.

2. Basic working conditions of LED

LED is a current-driven, low-voltage, unidirectional conductive device. To ensure the normal operation of LED, the following basic requirements must be met.

(1) The input DC voltage must not be lower than the forward voltage drop of the LED, otherwise the LED will not turn on and emit light.

(2) Using DC current or unidirectional pulse current drive, when driving parallel LEDs or LED strings, constant current rather than constant voltage power supply is required.

Figure 1 shows the relationship curve between the forward current IF and the forward voltage TIE of 6 randomly selected white light LEDs (3 of them are brand A and 3 of them are brand B). If these 6 (parallel) LEDs are driven with a constant voltage, the forward currents between them will differ greatly. For example, if driven with 3.4V, the forward current ranges from 10 to 44mA, which makes the brightness and chromaticity of each LED vary greatly.

The rated current (30mA) allowed by LED decreases with the increase of temperature, as shown in Figure 2. It can be seen from the figure that when the ambient temperature rises to 50C, the rated current drops to 20mA. In this case, to prevent the LED from burning, the drive current must be limited to 20mA. Therefore, in order to avoid the LED drive current exceeding the maximum rated value and affecting its reliability, and to obtain the expected brightness requirements and ensure the consistency of brightness and color of each LED, a constant current drive method should be used instead of a constant voltage method.

Figure 1 The relationship between the forward current and forward voltage of 6 white LEDs

Figure 2 Characteristic curve of self-illuminating LED rated current changing with temperature

When the LED is used as a flash, a forward pulse can also be used to drive the LED.

(3) To prevent LED damage, the current flowing through the LED should be limited. There are currently three main methods for achieving LED current limitation, each with its own advantages and disadvantages, as shown in Table 1.

Table 1 Comparison of three LED current limiting methods (white LED: VF=4V, IF=350mA, ViN=12V)

(4) Due to the nonlinear relationship between LED current and its light energy, the LED should be operated at a current value with a relatively high light efficiency.

(5) It is best to add a heat sink to high-power LEDs to prevent the device from overheating and damage.

In short, in order to use LED correctly and ensure its normal operation, it must be provided with appropriate working conditions.