Lighting design beginners must see: LED science knowledge

Semiconductor light-emitting devices, including semiconductor light-emitting diode (LED), digital tube, symbol tube, meter word tube and dot matrix display (matrix). In fact, the digital tube, the symbol tube, the meter tube and the matrix of each light-emitting unit is a light-emitting diode.

(1) LED luminescence principle

Light emitting diode is made up of class III compounds, such as GaAs (Shen Huajia), GaP (), (GaAsP) and other semiconductors, the core of which is PN junction. It has the characteristics of general I-N P-N junction, the forward conduction and reverse breakdown characteristics, as in. In addition, under certain conditions, it also has luminescent properties. Under the forward voltage, the electrons are injected into the P region from the N region, and the holes are injected into the N region from the P region. The minority carrier (minority) in the region of the other side is combined with the majority carrier (multiple daughter), which is shown in figure 1.

It is assumed that the light emission occurs in the P region, and then the injected electrons are directly combined with the valence band hole, or are first captured by the luminescent center and then combined with the hole. In addition to this light composite, some electrons are non luminescent center (the center between the conduction band, lies near the middle) capture, and then recombine with holes each release of energy is not, can not form a visible light. The larger the proportion of the luminescence of the composite with respect to the amount of the non luminescent composite, the higher the efficiency of light quantum. Due to the recombination of light in the small sub diffusion region, the light is only in the vicinity of PN junction within the number of mu m.

Theory and practice show that the band gap Eg semiconductor material and light emitting light wavelength peak area, i.e. lambda = 1240/Eg (mm) in Eg units of electron volts (eV). If you can produce visible light (wavelength of 380nm ~ 780nm red light), the semiconductor material Eg should be between 3.26 ~ 1.63eV. Longer than red light. Now there are infrared, red, yellow, green and blue light-emitting diodes, but the cost of blue light-emitting diode, the price is high, the use is not widespread.

(two) characteristics of LED

1 the significance of limit parameters

(1) the maximum allowable power Pm: allows the forward voltage of the LED to be applied to the sum of the current flowing through it. Above this value, LED heat, damage.

(2) the maximum forward current IFm: the maximum forward current is allowed to be added. More than this value can damage the diode.

(3) maximum reverse voltage VRm: maximum permissible reverse voltage. Beyond this value, the LEDs may be damaged by breakdown.

(4) ambient temperature range of topm: light emitting diode working environment. The light emitting diode can not work normally, and the efficiency is greatly reduced.

2 significance of electrical parameters

(1) the spectral distribution and the peak wavelength: the light produced by a light emitting diode is not a single wavelength, and the wave growth is shown in figure 2. It can be seen from the figure that the light intensity of a certain wavelength lambda 0 is the largest, and the wavelength is the peak wavelength.

(2) the luminous intensity IV: the luminous intensity of the light-emitting diode is usually the direction of the fingering line (on the axis of the cylindrical luminous tube) in the direction of the luminous intensity. If the radiation intensity in the direction of (1/683) W/sr, then the luminescence 1 Candela (symbol CD). Because of the general LED light-emitting two intensity is small, so the luminous intensity of commonly used Candela (MCD) as a unit.

(3) the spectral width of half a wavelength: it represents the spectral purity of the light emitting diode () is defined as the difference between the corresponding wavelengths of 1/2 peak intensity in Figure 3

(4) the half value angle theta 1/2 and the angle of view: theta 1/2 is the angle between the direction of the luminous intensity value and the axial direction of the luminous intensity. Half of the value of the angle of 2 times (or half power angle).

Figure 3 shows the distribution of the luminous intensity angle of two different types of leds. Midperpendicular (normal) AO coordinates for the relative luminous intensity (the luminous intensity and the maximum luminous intensity ratio). Obviously, the relative luminous intensity in the normal direction is 1, and the larger the angle from normal direction, the smaller the relative luminous intensity. The half angle or the angle of view can be obtained from the graph.

(5) the forward operating current If: it refers to the positive current value of the LED when it is normal. In actual use should be based on the need to choose IF below 0.6 ~ IFm.

(6) forward operating voltage VF: the operating voltage given in the parameter table is obtained under a given forward current. Generally measured at IF=20mA. The working voltage of VF is 1.4 ~ 3V. When the outside temperature rises, VF will fall.

(7) V-I characteristics: the relationship between the voltage and current of the light emitting diode is shown in figure 4. When the forward voltage is less than a certain value (called the threshold), the current is very small and does not emit light. When the voltage exceeds a certain value, the forward current increases rapidly with the voltage. The forward voltage, reverse current and reverse voltage can be obtained by V-I curve. Positive light tube reverse leakage current IR<10 A below. Beginners must see: LED science knowledge

(three) classification of LED

1 according to the luminous color

According to the luminous tube color, can be divided into red, orange, green (and subdivided into yellow green, standard green and pure green), Blu ray, etc.. In addition, some LEDs contain two or three color chips. According to the light emitting diode with or without light scattering agent, colored or colorless, the various colors of light-emitting diodes can also be divided into colored transparent, colorless transparent, colored scattering and colorless scattering of four types. A light emitting diode and a light emitting diode.

2 according to the characteristics of light emitting tube

According to the characteristics of the light outlet of the light emitting tube, the utility model is characterized in that a circular lamp, a square lamp, a rectangle, a surface luminous tube, a side pipe, a micro tube for surface mounting, etc.. According to the diameter of the circular lamp is divided into Phi 2mm, Phi 4.4mm, Phi 5mm, Phi 8mm, Phi 10mm and phi 20mm. The diameter of 3mm in foreign countries are usually denoted as T-1 light emitting diode; the diameter of 5mm is denoted by T-1 (3/4); the diameter of 4.4mm is denoted by T-1 (1/4). The angular distribution of circular luminous intensity can be estimated from the angle of half value. From the distribution of luminous intensity angle to divide into three categories:

(1) high directivity. Generally, it is a pointed epoxy package, or a metal reflection cavity package, and no scattering agent. The half value angle is 5 DEG to 20 DEG or less, and has a high directivity, which can be used as a local illumination light source or combined with a light detector to form an automatic detection system.

(2) standard type. The utility model is usually used as an indicating lamp, and the half value angle is 20 DEG to 45 deg.

(3) scattering type. This is a larger viewing angle, half the value of the angle of 45 degrees to 90 degrees or greater, the amount of scattering agent.

3 according to the structure of light-emitting diodes

According to the structure of the light-emitting diode is divided into epoxy encapsulation, metal base epoxy encapsulation, ceramic base epoxy packaging and glass packaging structure.

4 according to the luminous intensity and working current

According to the luminous intensity and working current is divided into ordinary brightness of LED (luminous intensity <10mcd); ultra high brightness LED (luminous intensity >100mcd); the luminous intensity between 10 ~ 100mcd called high brightness led. General LED operating current in more than a dozen mA to tens of mA, while the low current LED operating current below 2mA (brightness and the same as the ordinary light-emitting tube).

In addition to the above classification methods, there are also classified and classified according to the function of the chip material.

(four) application of LED

Because of the color, size, shape, luminous intensity and transparency of light-emitting diodes, etc., so the use of light-emitting diodes should be based on the actual needs of the appropriate choice. Since the LED has the maximum forward current IFm, the maximum reverse voltage VRm limit, when used, should ensure that no more than this value. For safety reasons, the actual current IF should be less than 0.6IFm; the possible reverse voltage VR<0. 6VRm. LED is widely used in a variety of electronic instruments and electronic equipment, can be used as a power indicator, level indicator or micro light source. Infrared light emitting tube is often used in television, video recorders and other remote control.

(1) using high brightness or ultra high brightness light-emitting diode to make a mini flashlight circuit as shown in figure 5. Figure R resistor current limiting resistor, the value should ensure that the maximum power supply voltage should be less than the maximum allowable current LED current IFm.

(2) Figure 6 (a), (b), (c) DC power supply, rectifier power supply and AC power supply circuit.

Figure (a) resistance is in the (E-VF) /IF;

Figure (b) in R ~ (1.4Vi-VF) /IF;

Figure R (c) is Vi/IF type in the effective value of AC voltage Vi--.

(3) single LED level indicating circuit. Output of an amplifier, oscillator, or pulse digital circuit

Semiconductor light-emitting devices, including semiconductor light-emitting diode (LED), digital tube, symbol tube, meter word tube and dot matrix display (matrix). In fact, the digital tube, the symbol tube, the meter tube and the matrix of each light-emitting unit is a light-emitting diode.

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