Life of electronic devices
It is difficult to indicate the exact lifetime value of a particular electronic device before it fails, however, after the failure rate of a batch of electronic device products is defined, a number of life characteristics characterizing its reliability can be obtained, such as average life, reliable life, median life characteristic life, etc.
(1) Average life μ: refers to the average life of a batch of electronic device products.
(2) Reliable life T: refers to the work time experienced when the reliability R (t) of a batch of electronic device products drops to y.
(3) Median life: refers to the life of the product when the reliability R (t) will be 50%.
(4) Characteristic life: refers to the reliability of the product R (t) reduced to
1 / e hour of life.
4.2, LED life
If you do not consider the failure of the power supply and drive, the life of the LED is reflected in its light decay, that is, as time goes on, the brightness becomes darker and darker until finally extinguished. It is usually defined to decay 30% of the time as its life.
4.2.1 Light Decay of LED
Most white LED is obtained from yellow phosphor irradiated by blue LED. There are two main reasons for LED light decay, one is the light decay of the blue LED itself, the light decay of the blue LED is far faster than the red, yellow, green LED. Another is the light decay of phosphors, and the attenuation of phosphors at high temperatures is very serious.
Various brands of LED its light decay is different. Usually LED manufacturers can give a standard light decay curve. For example, the light decay curve of Cree in the United States is shown in Figure 1.
As can be seen from the figure, the light decay of the LED is 100
And its junction temperature, the so-called junction temperature is half 90
Temperature of the conductor PN junction, the higher the junction temperature the earlier
There is light decay, that is, the shorter the life. From Fig. 80
As can be seen, if the junction temperature is 105 degrees, the brightness drops to 70% of the life of only ten thousand 70 Junction Tenpeature (C) 105 185 175 55 45
Hours, there are 20,000 hours at 95 degrees, and the junction temperature
Reduced to 75 degrees, the life expectancy is 50,000 hours, 50
Figure 1. Light decay curve of Cree's LELED
When the junction temperature is increased from 115 ° C to 135 ° C, the life is reduced from 50,000 hours to 20,000 hours. The decay curves of other companies should be available from the original factory.
O4.2.2 The key to extending life: reducing its junction temperature
The key to reducing the junction temperature is to have a good heat sink. The heat generated by the LED can be released in a timely manner.
Usually the LED is welded to the aluminum substrate, and the aluminum substrate is installed on the heat exchanger, if you can only measure the temperature of the heat exchanger shell, then you must know the value of a lot of thermal resistance to calculate the junction temperature. Including Rjc (junction to housing), Rcm (housing to the aluminum substrate, in fact, which should also include the thermal resistance of the film printed version), Rms (aluminum substrate to the radiator), Rsa (radiator to air), which as long as there is a data inaccuracy will affect the accuracy of the test.
Figure 3 shows a schematic diagram of each thermal resistance from LED to the radiator, in which a lot of thermal resistance is combined, making its accuracy more limited. That is to say, the accuracy of inferring the junction temperature from the measured heat sink surface temperature is even worse.
Temperature coefficient of volt-ampere characteristics of O LED
O We know that an LED is a semiconductor diodes, which, like all diodes
Has a volt-ampere characteristic, which has a temperature characteristic. Its characteristic is that when the temperature rises, the volt-ampere characteristic shifts to the left. Figure 4 shows the temperature characteristics of the LED's volt-ampere characteristics.
Assuming that the LED is supplied with constant current lo, the voltage is V1 when the junction temperature is T1, and when the junction temperature is increased to T2, the entire volt-ampere characteristic shifts to the left, the current lo is unchanged, and the voltage becomes V2. These two voltage differences are removed by temperature to obtain the temperature coefficient, expressed in mvic. For ordinary silicon diodes this temperature coefficient is -2 mvic.
How to measure the junction temperature of LED?
The LED is installed in the heat exchanger and the constant current drive is used as the power supply. At the same time, the two wires connected to the LED are drawn out. Connect the voltage meter to the output (the positive and negative poles of the LED) before the power is on, Then turn on the power supply, while the LED has not yet heated up, immediately read the reading of the voltmeter, which is equivalent to the value of V1, and then wait for at least 1 hour, so it has reached thermal equilibrium, and then measure again, the voltage at both ends of the LED is equivalent to V2. Subtract these two values to find the difference. Remove it by 4mV and you can get the junction temperature. In fact, LED is mostly a lot of series and then parallel, it does not matter, then the voltage difference is made up of a lot of series LED common contribution, so to divide the voltage difference by the number of series LED to divide by 4mV, you can get its junction temperature.
4.3, LED lamp life dependence
LED life can reach 1000000 hours?
This is only a higher level of LED theoretical data, is omitted some boundary conditions (that is, ideal conditions) under the data, and LED in the real use of many factors affecting its life,
there are the following four factors:
1, chip
2, package
3, lighting design
4.3.1. Chip
In the course of LED manufacturing, the life of LED will be affected by the pollution of other impurities and the imperfection of crystal lattice. O4.3.2. Packaging
Whether the post-process packaging of LED is reasonable is also one of the important factors affecting the life of LED lamps. At present, the world's major companies such as cree, lumilends, nichia and other high level of LED packaging have patent protection, these companies after the process of packaging requirements are relatively high level, LED life and therefore guaranteed.
At present, most enterprises have more imitation of LED after process packaging, which can be seen from the appearance, but the process structure and process quality are poor, which seriously affects the life of LED;
Heat dissipation design
The shortest heat transfer path, reducing heat conduction resistance; Increase the mutual conduction area and increase the heat transfer speed; Reasonable calculation and design heat dissipation area; Effective use of heat capacity effect.
4.3.3. Luminaire design
Whether the lighting design is reasonable is also a key issue affecting the life of LED lamps. Reasonable lamp design in addition to meeting other indicators of the lamp, a key requirement is to emit the heat generated when the LED is lit, that is, to use the high-quality LED original products of Cree and other companies, used in different lamps, LED life can vary by several times or even dozens of times. For example, there are sales of integrated light source lamps on the market (single 30W, 50W, 100W), and the heat dissipation of these products is not smooth. As a result, some products in the light of 1 to 3 months on the light failure of more than 50%, some products use about 0.07W of small power tube, because there is no reasonable heat dissipation mechanism, leading to light decay very fast, and even some urban policy promotion, the results make some jokes. These products have low technical content, low cost and short life;
4.4.4. Power supply
Whether the power supply of the lamp is reasonable. LED is a current driving device, if the power current fluctuation is large, or the frequency of the power tip pulse is high, it will affect the life of the LED light source. The life of the power supply itself depends mainly on whether the power supply design is reasonable, and under the premise of reasonable power supply design, the life of the power supply depends on the life of the components.
At present, LEDs are mainly used in three major areas:
1) Display: such as indicator lights, lights, warning lights, display screen, etc.
Lighting: flashlight, miner's lamp, directional lighting, auxiliary lighting, etc.
3) Functional radiation: such as biological analysis, phototherapy, light curing, plant lighting, etc.
The main parameters to measure the photoelectric performance of LED are shown in Table 1.
|
Radiation \ Function |
Performance Display Lighting Function Radiation |
distribution |
Functional radiation |
|
|
Luminance or luminous intensity of optical properties, beam angle and light intensity |
color standard, color purity and main wavelength luminous flux (effective luminous flux), luminous efficiency (lm/W), central light intensity, beam angle, light intensity distribution, color coordinates, color temperature, color index effective radiation power, effective radiance, radiation intensity distribution, central wavelength, peak wavelength, bandwidth |
current, unidirectional breakdown voltage, reverse leakage current Photobiosafety retinal blue light exposure value, eye near ultraviolet hazard exposure value |
What is luminous flux?
The total amount of emitted by the light source in unit time is called luminous flux, expressed by Φ
Units are lumens (lm)
1w (wavelength 555 nm) =683lumens
The luminous flux of some common light sources:
Bicycle headlights: 3W 30lm
White light: 75W 900lm
Fluorescent lamp “TL”D 58W 5200lm
The character of light required by LED illumination
Four basic measurements of lighting
What is illumination?
The luminous flux incident on the unit area of the illuminated object is the illuminance.
Denoted by E. ln lux (lx=lm/m2)
Illuminance is independent of the direction in which the luminous flux is incident on the surface
Usually indoor and outdoor illumination levels
Different positions in the sun at noon
How to measure light? By what are they measured?
1. Light source
2. Opaquescreen
3. Photocell
4. Light rays (reflected once)
5. Light rays (reflected twice)
Luminous intensity: direction finding photometer(as the image)
Illuminance: illuminometer ( image)
Brightness: luminance meter ( image)
5.2, the color temperature and color rendering of the light source
I. Color Temperature
A standard black body is heated (such as a tungsten filament in an incandescent lamp), and the color of the black body begins to change gradually along the dark red - light red - orange - yellow - white - blue as the temperature rises. When the color of light emitted by a light source is the same as the color of a standard blackbody at a certain temperature, we call the absolute temperature of the blackbody at that time the color temperature of the light source.
The temperature K is expressed. Basic color
As shown in the table:
Color temperature common sense:
|
Color temperature |
photochron |
Atmosphere effect |
Tricolor fluorescence |
|
Greater than 5000k |
Cool bluish white |
The cold feeling |
Mercury lamp |
|
3300-5000k abut |
Middle close to natural light |
No obvious visual psychological effects |
Eternal color fluorescence |
|
3300k less than |
Warm white with orange flowers |
A warm feeling |
Incandescent lamp quartz halogen |
Color rendering
The degree of light source to the color of the object itself is called color rendering, that is, the degree of color lifelike, the light source with high color rendering is better to the color, the color we see is close to the natural color, the light source with low color rendering is poor in color reproduction, and the color deviation we see is also large, represented by color rendering index (Ra).
The International Lighting Committee CIE sets the color index of the sun at 100. the color index of all kinds of light sources is the same.
For example, the color index of high pressure sodium lamp is Ra=23, and the color index of fluorescent lamp is Ra=60-90. The closer the color index is to 100, the better the color rendering is.
As shown below: the effects of objects with different color indices:
Color rendering and illumination
The color rendering index of the light source together with the illuminance determines the visual clarity of the environment. Studies have shown that there is a balance between illumination and color rendering index: lighting the office with a lamp with a color rendering index Ra > 90 is better than lighting the office with a lamp with a low color rendering index (Ra < 60) in terms of satisfaction with its appearance.
The degree value can be reduced by more than 25%.
The light source with the best color rendering index and high luminous efficiency should be selected as much as possible, and the appropriate illumination should be used to obtain good vision with minimum energy cost.
Appearance effect.
For example the wonled LED Rechargeable table lamp
This cutting-edge lamp is equipped with USB Type-C technology to provide a seamless and rapid charging experience. One of the standout features of this lamp is its powerful 3600mAh battery, ensuring long-lasting illumination. With a working time of 8-16 hours, you can confidently rely on this lamp to accompany you throughout the day and night. And thanks to the touch switch, adjusting the brightness to suit your preference is as simple as a swipe of your finger.What sets our LED rechargeable table lamp apart is its IP44 waterproof function. The charging time is a breeze, taking just 4-6 hours to fully charge. Utilizing the convenience of USB Type-C, you can easily charge this lamp with various devices, ensuring versatility and hassle-free use. With an input of 110-200V and output of 5V 1A, this lamp is both efficient and reliable.
|
Product Name: |
restaurant table lamp |
|
Material: |
Metal+Aluminium |
|
Usage： |
cordless rechargeable |
|
Light source： |
3W |
|
Switch： |
Dimmable touch |
|
Battery: |
3600MAH(2*1800) |
|
Color: |
Black,White |
|
Style： |
modern |
|
Working time： |
8-16hours |
|
Waterproof： |
IP44 |
Features:
lamp Size:100*380MM
Battery: 3600mAh
2700K 3W
IP44
Charging time :4-6 hours
Working time :8-16 hours
Switch: touch switch
lnput 110-200V and output 5V 1A
