A typical LED luminaire design includes electrical, thermal and optical simulation calculations. The high-power LED light source determines the final luminous flux of the high-power LED luminaire. Therefore, the selection of high-quality adaptive light sources is the basis of lamps. The research and development of LED chips is important, but the design and research and development of lamps and lanterns cannot be ignored either. The structure of the main lighting optical system of large lighting fixtures includes three designs:
(1) The primary optical design of the chip plus the optical system is suitable for application, which is completed by the manufacturer of the LED device.
(2) Put a single LED of the primary optical design into a tailor-made lens to obtain the required directional beam, which is the so-called secondary optical design, so as to obtain the LED component that meets the single optical requirements.
(3) Combine several LED components completed by secondary optical design into a lamp with a considerable scale and meet the light distribution curve of lighting requirements, which is the third optical design of LED.
In LED light source products, most products, such as street lamps, light beams, lawn lamps, underground lamps, decorative lamps, etc., require DC/DC constant current drive circuits to be set in these lamps to adapt to the characteristics of LED current drive. This power supply must not only provide the constant current output required by the LED, but also have a high conversion efficiency, otherwise the advantages of LED energy saving will be lost, and the low cost is also very important. , according to different requirements of current stability, transient overshoot, safety and reliability.
Another important consideration for LED light source products is the reliability of the LED device. As a semiconductor device, the study of the failure mode of the LED is very important to evaluate the life of the LED light source. LED devices also conform to the failure rule of the bathtub curve, so it is particularly important to study how to screen early failure devices, that is, the accelerated life test method. 1. Improving the luminous efficiency of ED and improving the heat dissipation characteristics is one of the problems that must be solved in the development of LED light sources. At the same time, LED reliability research is carried out under the cooperation of the industry, and the aging screening standards of devices are standardized, which is the solution to the application of LED light sources. One of the effective ways to encounter some problems.
- Determine lighting needs and design goals
Design goals are based on the performance of existing luminaires, or based on the lighting requirements of the application. LED lighting must meet or exceed the lighting requirements of the target application. Therefore, lighting requirements must be determined before design goals are established. For some applications, there are existing lighting standards that can directly determine the requirements. For applications without lighting standards, the existing lighting characteristics can be determined first, and then the lighting requirements of the application can be determined. The light output and power characteristics of lighting fixtures are the key to determining the existing lighting characteristics. According to the technical parameters provided by the lighting fixtures, the key characteristics of various lighting fixtures can be obtained, thereby determining the existing lighting characteristics.
After the lighting requirements are determined, the design goals of LED lighting can be determined. Design goals should be based on lighting application needs and should list all other goals that affect the design, such as special light requirements, high temperature requirements, etc. As with defining lighting requirements, key design goals relate to light output and power consumption. Design objectives should include operating environment, bill of materials (BOM), cost and service life.
- Estimating the efficiency of optical, thermal and electrical systems
Design goals impose constraints on the optical, thermal, and electrical systems. Based on these constraints, the efficiency of each system is estimated. Combining the lighting goals and system efficiency can determine the number of LEDs required for lighting. One of the most important parameters in the design process is how many LEDs are required to meet the design goals. Other design decisions revolve around the number of LEDs, because the number of LEDs directly affects light output, power consumption, and lighting costs.
Look at the typical luminous flux listed in the LED data sheet and divide that number by the design target lumens. This design approach will not meet the lighting application requirements. Because the luminous flux of LEDs depends on many factors, including drive current and junction temperature. To accurately calculate the number of LEDs required, one must first estimate the efficiencies of the optical, thermal, and electrical systems.
(1) Optical system efficiency. Estimate the efficiency of an optical system by analyzing light losses. The two main light losses to analyze are:
1) Secondary optics. Secondary optics are all optical systems that are not part of the LED itself, such as lenses or diffusers on the LED. The losses associated with secondary optics vary depending on the specific components used. The typical optical efficiency of each secondary optical element is between 85% and 90%. If secondary optics are required for illumination, there is secondary light loss.
2) Light loss within the luminaire. Luminaire light loss occurs when the light hits the luminaire cover before reaching the target. Some light is absorbed by the luminaire cover, and some is reflected back to the luminaire. The efficiency of the fixture is determined by the layout of the lighting source, the shape of the lamp housing and the material of the lamp cover. LED light emission is directional, and the achievable efficiency is much higher than that possible with omnidirectional lighting sources.
The primary purpose of the secondary optics is to change the light output of the LED. Figure 1 compares the beam angle of the CreeXlampXR-E LED to the light output of the target luminaire. The beam angle of the bare LED is very similar to that of the target luminaire, so no secondary optics are required. Therefore, there is no light loss due to secondary optics. Just calculate the lamp loss, assuming that the reflector of the lamp reflector is 85%, 60% of the light will hit the reflector. Therefore, the optical efficiency is ƞ=(100%?0%)+(85%×60%)=91%
(2) Heat loss. The relative luminous flux output of LEDs decreases with increasing junction temperature, most LED data sheets list typical luminous flux values at 25°C, while most LED applications use higher junction temperatures There is a curve in the LED data sheet , the relationship between relative light output and junction temperature is given, and the corresponding luminous flux reduction curve of XLampXR-E white LED junction temperature is shown in Figure 2. This curve gives other characteristic values by choosing a specific relative light output or a specific junction temperature.
XLampXR-E LEDs provide an average lumen maintenance rate of 70% after 50 000 hours of operation under rated conditions, and the junction temperature remains below 80°C. Therefore, the maximum suitable junction temperature is 80°C. The corresponding minimum relative luminous flux is 85%, as shown in Figure 2, this 85% relative luminous flux is an estimate of the thermal efficacy of the lighting.
(3) Electrical loss. LED drivers convert an available power source into a regulated current source, a process that, like all power sources, does not reach 100% efficiency. Electrical losses in the driver reduce the overall lighting efficiency, as part of the input power is wasted heating up rather than emitting light. Electrical losses should be considered when starting to design an LED system.
Typical LED drivers are between 80% and 90% efficient, and drivers with efficiencies higher than 90% are much more expensive. Driver efficiency can vary with output load, as shown in Figure 3. The driver should be specified to operate at greater than 50% output load to maximize efficiency and minimize cost. For indoor applications, a driver efficiency of 87% is a good estimate. Drives for outdoor use or requiring a very long life may be less efficient.
- Calculate the number of LEDs needed
(1) The actual amount of lumens required. Based on the design goals and the estimated loss, the number of LEDs that meet the design goals can be calculated. Once all system efficiencies have been estimated, the actual number of LED lumens required to achieve the design goals can be calculated. Because electrical efficiency only affects total power consumption and luminaire efficiency, not the light output of lighting.
(2) Working current. The operating current of the LED is critical to determine the efficiency and lifetime of the LED lighting. As the operating current increases, the light output of each LED will increase, thus reducing the number of LEDs required, and increasing the operating current also brings several disadvantages:
1) Reduced efficacy. Increasing the operating current will reduce the efficacy of the power LED. Generally, the size of the power supply will increase with the increase of the operating current, because more power is required to generate the same number of lumens.
2) The maximum ambient temperature is reduced or the service life is shortened. The increase of the current will increase the temperature difference between the LED junction and the LED hot channel. In fact, since the maximum junction temperature has been determined, this can reduce the maximum ambient temperature of the lighting. Conversely, if the maximum ambient temperature is not decreased, but increased, Then, during the lifetime of the LED, the output of the light source will drop faster.
Depending on the application, these drawbacks are acceptable considering the higher lumen output per LED. Longevity and power efficiency are design goals that should be prioritized, and a minimum operating current of 350mA for a 1w LED listed in the XLampXR-E data sheet maximizes LED efficacy and extends lifespan.
(3) Number of LEDs. After the working current is determined, the lumen output of each LED can be calculated. Since the heat loss of the LED has been considered in the calculation of the actual required lumens, the technical parameters provided by the LED supplier can be used directly.
- Choose the best design
Once the number of LEDs required has been calculated, consider all design possibilities that meet the design goals. Since each LED is a light source and has a much longer lifespan than traditional lighting, LEDs can be integrated with new and unconventional design components. In lighting systems, the directivity of LED light emission and the large number of available secondary optics can be fully utilized in the design to optimize the preliminary design.
(1) Optical system options. The optical secondary development of LED lamps plays a very important role in the entire lamp design process, and should be a priority in the overall design process of the entire lamp. Commonly used reflective LED secondary optical designs are:
1) Reflective secondary optical design. The reflective secondary optical design mainly uses various quadratic curves independently or combined into reflectors, and coats the reflective surface with materials such as silver, aluminum or chromium with high reflectivity. In this structure, nearly 50% of the light is directly emitted, and the light reflected by the reflective surface can also reduce the absorption loss through the control of the reflectivity by the film layer, so the reflective design can achieve high lighting efficiency.
However, LED is a light source that emits light in half space. Generally, the reflector has a limited envelope angle to the light source, and it is not easy to achieve wide-angle light distribution of the bat wing. It requires multiple columns of reflectors to be arranged and matched at a certain angle, or to change the reflector and the light source. The relative position of , increases the reflector envelope angle. The former solution needs to increase the degree of cooperation between the lamp base structure and the LED light-emitting array, while the latter method requires that the design of the reflector’s curved shape tends to be finer, and sometimes a free-form reflector should be considered to meet the light distribution requirements.
2) Refractive secondary optical design. In the occasions where there is a high requirement for the uniformity of the contrast, the secondary optical design based on the light distribution lens can be considered. The function of the light distribution lens is to generate a rectangular illumination spot with uniform brightness on the illuminated surface. The light intensity distribution of existing LEDs is mostly a Lambertian distribution that is rotationally symmetric about the main optical axis. To re-distribute the rotationally symmetric light distribution to form the final rectangular optical class, it is determined that the light distribution lens must be an optical lens with a free-form surface. In the secondary optical design, it is necessary to accurately establish the luminous model of the light source, and at the same time determine the light distribution of the target illumination area. Based on this, the transformation equations before and after the light entering and leaving the light distribution lens are listed and the specific form of the light exit surface of the light distribution lens is obtained by solving.
3) For the secondary optical design combining reflection and refraction, the structure of simply using aspheric reflectors will leak some of the light from around the lens due to the limited envelope angle, which cannot be effectively used. Therefore, it is necessary to add a reflector to the structure of the lens to fully and effectively collect and utilize the LED light flux. In addition, an aspheric fly-eye lens is also an effective structure for generating a uniform light spot. The secondary optical device using a fly-eye lens is also composed of a reflector and a lens. The reflector collects light and modulates it into a relatively parallel beam. The fly-eye lens scatters the light at a certain angle, the structure mechanism is clear, and the uniform and comfortable lighting effect is achieved.
In the field of commercial lighting, the optical software commonly used in China is mainly divided into two categories: lighting engineering design software and lighting distribution design software. The function of the lighting engineering design software is to formulate a set of optimal usage plans based on the existing standardized lamps to provide users with the best visual needs. At present, common lighting engineering design software includes DIALux, OxyTech, etc. The function of light distribution design software for lamps is to design corresponding optical devices according to the characteristics of the light source and the desired optical effect. Taking LED lamps as an example, LED reflectors and lenses are used. It is the commonly used optical device to achieve the desired optical effect.
1) Bare LEDs and existing luminaire reflectors. Existing CFL luminaires have very similar angles to LEDs, so no secondary optics are used if chosen. Costs can be minimized, system light loss can be minimized, fewer components are used, and less expense can be used, resulting in simpler and less expensive lighting system assembly. The disadvantage is that there will be multiple lighting shadow effects. If the light distribution of the LED is very different from that of the target lighting, this method cannot be used, and secondary optics should be used.
2) LEDs and existing luminaire reflectors with secondary optics. Secondary optics are additional optical elements in addition to the primary optics of the LED that are used to shape the light output of the LED. Typical secondary optics types are reflective (light is reflected back from a surface) or refraction (light is refracted by Materials are curved, refractive materials are usually glass or plastic), and secondary optics can be custom designed by purchasing standard parts, off-the-shelf parts, or through ray tracing simulations with lighting models.
Using one secondary optic per LED, the beam angle of each LED can be tailored to get the exact light output required, e.g., the beam angle of each LED can be narrowed to optimize spot lighting instead of general lighting, Using secondary optics has the following disadvantages:
, the cost of lighting is higher because of increased components and more complex assembly.
, there may still be multiple lighting shadows due to optics attached to the individual LEDs.
, secondary optics reduce the efficacy of the optical system.
3) Instead of using an optic for each LED, the entire LED array can use a diffuser to spread the light. The advantage of this approach is that the beam angle is wider than that achievable with bare LEDs, and the multi-illumination shadow effect is eliminated. Light distribution, multi-source shadow effects, and aesthetics often also determine the choice of optical system.
Common forms of LED optics include reflectors and lenses, each of which has its own advantages and limitations, and should be selected according to needs. Compared with the reflector, the loss of light by the lens is larger, and the lens also has the problem of dispersion; however, when dealing with the light distribution requirements of small angles (for example, the beam angle is below 20), the reflector will be very clumsy, and the lens will be awkward. is easier.
(2) Thermal system options. The thermal structure design of the lamp body is a problem that cannot be ignored in the manufacture of LED lamps. The lowest cost method in thermal design is to use the existing designed lamp housing as the LED lighting lamp housing, but this method cannot be used in the design. , Because most of the existing lamp housings are made of steel, the thermal conductivity is poor. Generally speaking, choosing a steel cover is not conducive to heat dissipation. Buy off-the-shelf heatsinks in your design because they have a proven design and the manufacturer has full specifications, but their performance, size, and shape may not be optimized for the target application. Using a custom solution provides an optimized heat sink for the application, but the design requires thermal simulation software or thermal design based on the target application and the characteristics of the LED.
Tooling and manufacturing expenses can make the unit cost of a custom heatsink higher than the cost of a finished heatsink. The cost of target lighting, available heat sink development time, and target maximum ambient temperature often determine thermal system selection. In general, where cost reduction is more important than maximum ambient temperature, a finished heat sink is a better choice. In situations where the maximum ambient temperature is more important, a custom heatsink is preferable (eg for outdoor lighting or indoor lighting in poor conditions).
If LED lighting uses a finished heat sink with a thermal resistance of 0.47°C/W, the maximum ambient temperature using this heat sink can be calculated by the following formula (4):
(3) Electrical system options. The power supply system of LED lamps is also different from traditional light sources. The requirements of LEDs for driving circuits are to ensure constant current output characteristics. Since the junction voltage changes relatively small when the LEDs work in the forward direction, ensuring the constant LED driving current is the basis for Guaranteed constant LED output power.
In order to make the LED drive circuit have constant current characteristics, looking inward from the output end of the drive circuit, its output internal impedance must be high. When working, the load current also passes through this output internal impedance. If the driving power supply is composed of a linear constant current source circuit or a general switching power supply plus a resistance circuit, it will consume a lot of active power, so these two types of driving circuits Under the condition that the constant current output is basically satisfied, the efficiency cannot be high.
The optimized design is to use an active electronic switch circuit or a high-frequency current to drive the LED. The above two designs can make the drive circuit still have high conversion efficiency under the condition of maintaining good constant current output characteristics.
1) Finished LED driver. Since the off-the-shelf LED driver has a reference circuit design, all devices are tested for electromagnetic interference (EMI) and safety, and generally, the lowest cost per unit is available in batch. Therefore, using off-the-shelf LED drivers will save the design time and cost of LED luminaires. The disadvantage is that the efficiency of the finished LED driver is usually about 80%. Due to the technical equipment of the manufacturer, the performance of the LED driver is quite different, and the LED service life and operating temperature may have problems.
2) Custom designed L.ED driver. With the increasing popularity of LED lighting applications, more LED drivers are optimized for the application, which is more efficient and can be fully approved by the management department, but may prolong the development time of LED lamps. As far as the characteristics of the current high-power LED products are concerned, the improvement of the total lighting efficiency is more affected by the LED itself than by the driver. In the development of general-purpose LED products, using the finished LED driver to complete the product design as soon as possible may be more optimized than custom design. The LED driver should be beneficial.
- Energy efficiency of LED lamps
The research plan formulated by DOE is to increase the light source efficiency of SSL in the field of commercial lighting from the current 301m/W to more than 1501m/w. Based on the current draft ENERGY STAR plan for SSL. lighting fixtures, the overall energy efficiency of SLL lighting fixtures constructed using existing components can be calculated. The minimum brightness is 5001m, and the minimum light source efficiency is 351m/W
A common warm white LED is used as the lighting source. The rated index of the LED is 3000KCCT, 350mA, and the maximum forward voltage drop of 3.8V. Under the test conditions, the LED junction temperature is 25℃, and the current pulse action time is 25ms. In the actual working process, the junction temperature of the LED will rise to a certain stable value according to the thermal design effect of the lighting fixture. For any LED, as the temperature rises, the brightness it produces will decrease, compared with the test value. , the brightness of LEDs operating at 60°C will be reduced by 10%.
This means that the actual luminous brightness of the LED is 541m in a stable working state. If the energy efficiency of the driver is taken into account, the worst-case energy efficiency can be calculated to be 33.2lm/w, which can meet the current Energy Star Program Requirements. The choice of isolated driver also plays an important role in improving the energy efficiency of the overall lighting installation, which is currently a trade-off between price and performance. Linear drives are the cheapest, but their energy efficiency is in the 50% to 60% range. The energy efficiency of switching drives is the highest, generally between 80% and 85%. To achieve higher energy efficiency, more complex circuits are required, and therefore the cost of the driver is higher.
- Sample trial production and performance evaluation
Sample trial production and performance evaluation are the final steps in the design of LED lighting fixtures, which can be completed according to the following steps,
(1) Circuit board layout. Select circuit board materials according to thermal indicators and cost constraints. Screen electrical components and necessary performance tests according to the designed circuit, and conduct PCB layout and wiring design. It should be noted in the design that the lighting light output cannot be affected by the layout of the device. and cooling channels.
(2) Structural design. Structural design includes thermal design and appearance design. When considering heat dissipation and mechanical properties, the appearance and cost should be taken into account.
(3) Test samples. The test sample is to verify the characteristics and functions of the designed product to verify whether the optical performance, thermal performance and electrical system performance of the product reach the design target value.
(4) Modify the design. Based on the information obtained from the sample test, make a decision on whether the sample design needs to be modified.