Conventional LED is generally rack , using epoxy resin, less power , less overall luminous flux , high brightness can only be used as some special lighting. With the development of LED chip and packaging technologies , and conform to the high flux LED lighting products demand , power led street lighting Manufacturer into the market . This power type of LED light-emitting chips are generally placed on the cooling heat sink assembly above the optical lens to achieve a certain spatial distribution of the optical lens inside the flexible silicone filling low stress .
To actually enter the power LED lighting, lighting for home daily , and many of their problems to be solved , of which the most important is the luminous efficiency. Currently on the market reported the highest power LED lumen efficiency 50lm / W or so, still far less than the daily household lighting requirements . To improve the power LED luminous efficiency , on the one hand its efficiency needs to be improved light-emitting chips ; hand, power LED packaging technology also need to further improve the structural design, materials technology and process technology, multi-pronged approach to improve product Package the light extraction efficiency.
Affect the light extraction efficiency of packaging elements
1. Cooling technology
For the PN junction consisting of a light emitting diode when forward current is flowing from the PN junction , PN junction with heat loss through the heat bonding adhesive , potting material , heat sink , etc., the radiation into the air , in the process for each part of the material has a thermal resistance of heat flow to stop , that is the thermal resistance , thermal resistance is a fixed value by the size , structure and material of the device of the decision . The thermal resistance of the light emitting diode disposed Rth (℃ / W), the thermal dissipation power is PD (W), PN junction temperature this time due to the current increase caused by heat loss are:
T (℃) = Rth × PD.
PN junction junction temperature :
TJ = TA + Rth × PD
Where TA is the ambient temperature . Since the probability of the junction temperature rise will decrease PN junction light-emitting compound , the brightness of the light emitting diode will drop . Also, due to the temperature rise caused by heat loss increased brightness light-emitting diodes will no longer continue with the current proportional increase shows that the thermal saturation. Further , as the junction temperature rises , the light emitting peak wavelength longer wavelength drift will also be about 0.2-0.3nm / ℃, the blue chip which is coated by the YAG phosphor is obtained by mixing a white LED , blue wavelength drift, and phosphor excitation wavelength will cause a mismatch , thereby reducing the overall luminous efficiency of the white LED and causes the white color temperature changes .
For power light emitting diode , the driving current of a few hundred milliamperes or more generally , the current density PN junction is very large, the temperature of the PN junction is obvious. For packages and applications, how to reduce the thermal resistance of the product , so that the heat generated by the PN junction can be distributed as soon as possible to go out , not only can improve the saturation current products, improve the luminous efficiency of products, but also improve the reliability and longevity . In order to reduce the thermal resistance of the product , first select the packaging material is particularly important , including heat sink , bonding glue , etc., lower thermal resistance of each material , which requires good thermal conductivity. Second, structural design should be reasonable, thermal conductivity material between each successive match , a good connection between the thermal conductivity material , to avoid bottlenecks in the thermal cooling channels to ensure that heat from the inside to the outside layers of distribution. Also, make sure the craft from the heat dissipated in time according to a pre-designed cooling channels .
2 Select . Filled rubber
According to the law of refraction , when light is incident from the optically denser medium to optically thinner medium , when the incident angle reaches a certain value, that is more than equal to the critical angle occurs when the full launch. In GaN blue chips , the refractive index of GaN materials is 2.3 , when the light is fired from the air inside the crystal , according to the law of refraction , critical angle θ0 = sin-1 (n2/n1)
Wherein n2 is equal to 1 , i.e., the refractive index of air , n1 is the refractive index of GaN , thus the critical angle θ0 calculated approximately 25.8 degrees. In this case , the light can be emitted from the light incident angle of ≤ 25.8 ° solid angle of the space, is reported , the external quantum efficiency of GaN chip present in about 30% -40 % , and therefore , the absorption due to the internal on-chip crystal , the light can be emitted to the outside of the proportion of small crystals . Reportedly, the current quantum efficiency of GaN chip about 30% -40 % . Similarly, the light emitted by the chip through the packaging material to be sent to space , but also to consider the impact of materials on the light extraction efficiency .
Therefore, in order to improve light extraction efficiency of the led street lighting Manufacturer package of the product , must increase the value of n2 , i.e., increasing the refractive index of the encapsulating material to increase the critical angle of the product , thereby improving the light emitting efficiency of the product package . Meanwhile, the packaging materials to the absorption of light is small. In order to improve the ratio of the outgoing light , the package is preferably arched or domed shape , so that the light is directed to the air from the packaging material , the interface is almost vertical incident , and thus no longer totally reflected .
3 . Reflective treatment
Reflective treatment mainly two aspects, one internal reflection processing chip , the second is the reflection of light packaging materials , through internal and external aspects of the reflection process to improve the ratio of luminous flux emitted from the chip to reduce the chip absorption, improve the luminous efficiency of the power LED product . From the package , the power LED is usually the power -type chip mounted on a metal frame or substrate with a reflective cavity , the cavity reflection rack plating is generally taken to improve the way the reflection effect, and substrate type is generally polished reflector cavity way , will be conditional plating, but the accuracy of these two approaches and processes affected by mold , reflecting cavity treated a certain reflection effect, but not ideal. At present, the production of substrate type reflector cavity , due to the lack of precision or polished metal oxide coating , poor reflection effect, this has led to a lot of light is absorbed in the reflection area after the incident , the goal can not be expected to reflection smooth , resulting in a final low light extraction efficiency of the encapsulated .
We have gone through extensive research and testing, has developed into a proprietary reflective coating using organic materials processing technology , through this process, so that the reflected light to the carrier sheet to absorb very little cavity , can most reflection of light incident thereon to the light exit surface . Can be increased by 30% -50 % of the product thus treated before the light extraction efficiency as compared with the treatment . We currently 1W white power LED luminous efficiency can reach 40-50lm / W ( test results on the distant PMS-50 spectrometer test equipment ) , get a good package effect.
4 phosphor coating selection and
For the white power LED , the luminous efficiency but also with select and process relevant phosphor . In order to improve the efficiency of phosphor excitation blue chips , the first phosphor selection to the right, including the excitation wavelength granularity size , excitation efficiency , etc. , need a comprehensive assessment , taking into account individual performance. Secondly, the phosphor -coated evenly , preferably a relatively uniform layer thickness of each light-emitting surface of the light emitting chip , so as to avoid uneven thickness resulting in partial light not fired, but also to improve the quality of the spot .
Good thermal design power LED products to improve the luminous efficiency has a significant effect , but also the prerequisite for ensuring the reliability and product life . And good design optical channel , here highlighted that reflector cavity , filled with plastic and other structural design, material selection and process , can effectively improve the power LED light extraction efficiency . White LED for power , the choice of phosphor and process design , to improve efficiency and luminous spot is also crucial.
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