[LEDinside] Abstract: As an optical designer, I often encounter problems in the calculation of photoelectric parameters in my work. The previous 100lm/W lamp is a good product, but with the development of LED, the demand is also rising. Now many engineering cases In order to save energy, the light efficiency has risen from 120 to 150 or even 180 lm/W, which is very headache.
Let's talk about how to design a luminaire that meets the requirements of photoelectricity.
The nominal value generally refers to the test data after the product is stable.
You must first know the standard of luminaire testing. Most luminaires can be directly tested by the integrating sphere. According to the method provided by IES LM79, it needs to be tested after the luminaire is stable. As for some parameters, the imaginary standard can be ignored.
Figure 1. Requirements for luminaire stability in the IES LM79
Why must it be stable data? Most LED products have a decay from transient to steady state, and these attenuations are very large and cannot be ignored.
By testing these attenuation sizes, you can wait for a relative thermal attenuation coefficient, see the red part.
Table 2 Test parameters of 8-9W bulbs on the market
LED lamp bead selection and testing
When designing, the first is LED selection, LED specifications are many pages, let you dazzle. Mainly have rated power, luminous flux, voltage, color temperature, color rendering index, color tolerance and so on. If you continue to go deeper, there are several kinds of brackets, such as ppa, pct, and emc. There are many kinds of chip sizes. Phosphor, silica gel, gold wire, and bracket metal are all very greasy, which have a great impact on the life of the light source.
For LEDs, the most important thing is the luminous flux at rated current. For example, the most commonly used 2835 particles, the rated luminous flux of 60mA is 24-26lm. Is that I will solder 100pcs of this LED on the light bar, the luminous flux of the 60mA test is 240-260lm?
The answer is no, the following are some sources of error. The final test report must be based on your own instrument test, so you need to figure out these coefficients.
However, these coefficients are sometimes cumbersome to calculate, and there are many one-to-one tests. So my idea is to directly solder the manufacturer's standard LED lamp to the lamp board, test with a large integrating sphere, DC power supply, and test the data under multiple currents.
If you design a conventional product, there is no requirement for light efficiency, and the test can be done at rated current. But if you need a product with higher efficacy, those methods will not work, either choose a brighter bead, or reduce the current use, more often the two need to be combined.
LED lamp bead number calculation
After doing some of the above work, you still lack two important parameters, one is the power conversion efficiency of the lamp, and the other is the optical efficiency of the lamp. It can be calculated by the following formula. Sometimes it is impossible to start with a new lamp. Experience to make some estimates.
Power efficiency = LED voltage × current through the LED ÷ input power optical efficiency = luminaire luminous flux ÷ bare light source test luminous flux
Note. All of the above are transient tests.
Let's start the formal calculation steps:
♦ 1. Determine the design light efficiency;
Assuming the customer request is 13W 1800lm, then the light efficiency requirement is 139lm/W.
♦ 2. Determine the current using the particles:
The main effects of light efficiency are light transmittance, lamp itself, thermal attenuation coefficient (light flux attenuation, VF attenuation), and the formula is as follows:
LED lamp pearl light effect × transmittance × attenuation coefficient × power efficiency = lamp efficacy
For example, the lamp that needs to be done now has a light transmittance of 94%, a transient to steady state light effect attenuation of 1.5%, a power supply efficiency of 90%, and a lamp bead light effect of 139 lm/W ÷ 0.94 ÷ 0.985 ÷ 0.9 = 167 lm / W. By looking up the table, get a suitable current of 55mA
♦ 3. Determine the number of beads
Single lamp power × quantity = lamp power × power efficiency × attenuation coefficient
After the current is determined, the meter can obtain a single power of 0.153W. The number = 13 × 0.9 × 0.975 ÷ 0.153 = 75, but this number can not be used directly, because the parameters of the power supply should also be considered, with the appropriate series and parallel connection, to get the final accurate value: 40 string 2 and.
Note. It is possible to calculate the number of particles required, but it will be noticeable grainy, so this design is also unreasonable.
Advanced articles - calculation tool design
Through the above process, you can calculate various LED lamp bead solutions. But sometimes it is often necessary to calculate different lamp beads from different manufacturers, so that which solution is better, and repeated calculations are also very troublesome, so it is convenient to hand over to excel.
First, the original data is sorted out, and the following two curves can be obtained. If the requirement is not high, a linear fit can be used, that is, y=ax+b. To be more precise, I used the binomial fit here, y=ax2 +bx+c.
Figure 5 Relationship between current and forward voltage and light efficiency
Through the current, all the parameters of the luminaire are linked, and the table design makes it easier to calculate the light effect. Subsequent parameters such as brightness BIN, voltage BIN, power supply constant current accuracy, etc. can also be improved to calculate the parameter range of the entire batch.
Conclusion: High-efficiency LED is the future trend
In the process of using LED, the ratio of electricity to light is about 40%, while other energy is used for heating, and there is still a lot of room to increase this value. In theory, the luminous efficiency limit of LED is more than 300. As the efficacy of light increases year by year, LEDs will become more and more energy efficient, and the heat generation will be greatly reduced. Even a separate radiator will not be needed in the future.
In the design of high-efficiency products, the performance and cost are always contradictory, and only the various parts can be integrated to achieve a balance. Some personal experiences below can be used for your reference. If there are any deficiencies, please indicate:
1. Use a better light source and reduce the current usage of the light source. To some extent, the target can be achieved, but the cost will be multiplied.
2, improve optical efficiency, such as PCB using highly reflective white oil, metal reflector cup surface treatment, reduce PC haze, use better diffusion particles, through these optimizations, the optical efficiency of bulbs and tubes can reach 97%
3, optimize power efficiency, now non-isolated efficiency is generally 90%, through a certain point of optimization, can also increase 2%
4. Create better heat dissipation conditions. From the beginning of the article, we know that the luminous flux will be instantaneous from the transient to the steady state. If the heat dissipation structure is increased, the aluminum substrate will be used and the copper area will be increased. Steady-state luminous flux. (Author: Wang Li)
Source: LEDinside
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