Ten little-known secrets of LED street light technology

　The lighting engineering of LED lighting industry is not only affected by the global environment, but also has its own industry characteristics. The power supply of LED street lamp is the top priority of the current LED development. There are many solutions and unique design methods for the related design of LED technology. Let's learn about them one by one.

　　1. Why must the LED street light power supply have a constant current?

　　The characteristics of LED lighting materials determine that they are greatly affected by the environment. For example, when the temperature changes, the LED current will increase, and when the voltage increases, the LED current will also increase. Long-term operation exceeding the rated current will greatly shorten the life of the LED lamp beads. The LED constant current is to ensure that its operating current value remains unchanged when environmental factors such as temperature and voltage change.

　　2. LED street light power supply constant current accuracy

　　Some power supplies on the market have poor constant current accuracy. For example, the popular recommended constant current solution has an error of ±8%, which is too large. Generally, ±3% is sufficient. According to the 3% design solution, the production power supply needs to be fine-tuned to achieve a ±3% error.

　　3. Working voltage of LED street light power supply

　　The recommended operating voltage of general LED is 3.0-3.5V. After testing, most of them work at 3.2V, so it is more reasonable to calculate according to the 3.2V formula. The total voltage of N lamp beads in series = 3.2* N

　　4. What is the most suitable working current of LED street light power supply?

　　For example, the rated working current of LED is 350 mA. Some factories use it to the maximum from the beginning and design it to 350 mA. In fact, the working current under this current is very hot. After many comparative tests, it is more ideal to design it to 320 mA. Try to reduce the heat as much as possible and convert more electrical energy into visible light energy.

　　5. How wide should the series and parallel connection and wide voltage be for LED street light power supply boards?

　　To make the LED street light power supply work in a relatively wide input voltage range of AC85-265V, the LED string parallel connection mode of the light board is very important. Try not to use wide voltage, and classify it as AC220V and AC110V as much as possible to ensure the reliability of the power supply. Since the current power supply is generally a non-isolated step-down constant current power supply, when the voltage is required to be 110V, the output voltage should not exceed 70V, and the number of series connections should not exceed 23. When the input voltage is 220V, the output voltage can reach 156V. In other words, the number of series connections should not exceed 45. The number of parallel connections should not be too many, otherwise the working current will be too large and the power supply will heat up seriously. There is also a wide voltage solution, A PFC active power compensation, which is to use L6561/7527 to raise the voltage to 400V first, and then step down the voltage, which is equivalent to two switching power supplies. This solution is only used under specific conditions.

　　6. Isolation/non-isolation

　　Generally, if the isolated power supply is made into 15W and placed in the power tube of the LED street lamp, its transformer is very large and difficult to put in. It mainly depends on the space structure and the specific situation. The isolation can generally only reach 15W, and there are few that exceed 15W, and the price is very expensive. Therefore, the cost performance of isolation is not high. Generally, non-isolated ones are more mainstream, and the volume can be made smaller, and the minimum height can be 8 mm. In fact, if the safety measures of non-isolation are well taken, there will be no problem. If the space allows, an isolated power supply can also be made.

　　7. How can the LED street light power supply be matched with the lamp bead board?

　　Some customers design the light board first, and then look for a power supply, but find it difficult to find a suitable power supply. Either the current is too large and the voltage is too small (such as 7X1WI>350mA, or V<20V); or the current is too small and the voltage is too high (such as I<200mA or V>25V), resulting in severe heating, low efficiency, or insufficient input voltage range. In fact, if you choose the best series-parallel connection method, the voltage and current applied to each LED are the same, and the power supply can perform at its best. The best way is to communicate with the power supply manufacturer first and tailor it to your needs. Or produce the power supply yourself.

　　8. LED series and parallel connection and PFC power factor

　　The capacity of electrolytic capacitor at high voltage end of isolated input AC220V is generally 1W=1UF for input power, and 1W=2UF for AC110V. There are three types of PFC in the power supply on the market : one is without PFC and power factor compensation circuit, and its PF value is generally around 0.65; the second is passive power factor compensation PFC circuit, that is, passive power factor compensation lamp, also called flow-following circuit board, which is currently the most widely used and has the best reliability, and the PF value is generally around 0.92; the other three are made of active 7527/6561 circuit, that is, active power factor compensation, called APFC circuit. In AC220V and AC110V, electrolytic capacitors of the same capacity can be used, and 1W=1.5UF is selected. The PF value can reach 0.99, but the cost of this solution is twice as expensive as the second solution and the reliability is slightly worse. Therefore, the second solution is used more. For passive PFC circuit: also called valley-filling PFC circuit, its DC working voltage range is half of the peak value of AC input voltage. If the input is 220V, its peak value is 220*1.414=312V, half of the peak voltage is 156V, and on a non-isolated basis the output wave is the upper half wave without the lower half wave.

　　Therefore, the maximum number of LED street lamp power beads in series is less than 45. Therefore, in order to obtain a relatively large power factor, the number of lamp beads in series cannot be too small, otherwise it will not reach the best working state. The number of series connections on the isolated power supply is related to the number of turns of the secondary winding, and the power supply power must meet the output power. The smaller the working current of the electronic component within the rated voltage working range, the lower the heat and the longer the life, otherwise the life will be shorter. LED street lamp power beads are very sensitive to the AC component. The higher the AC component, the worse the light comfort. Generally, electrolytic capacitors are used to maintain the voltage, and the AC component of the output voltage is reduced as much as possible. The capacity of the low-voltage electrolytic capacitor cannot be too small, and the ratio of capacity to output current is 1UF<1.5MA, otherwise the LED will flicker. The selection of high-voltage electrolytic capacitors at the input end of the non-isolated type is the same as that of the isolated type, and the output end capacitor selection is 1UF<6MA. The electrolytic capacitor at the output end of the dimming LED power supply must meet 1UF<0.5MA.

　　9. LED Street Light Power Supply Efficiency

　　The input power minus the output power value is particularly important. The larger the value, the lower the efficiency, which means that a large part of the input power is converted into heat and dissipated. If it is installed in a lamp, it will generate a very high temperature, plus the heat dissipated by the light efficiency ratio of our LED, which will generate a higher temperature. The life of all electronic parts inside our power supply will be shortened as the temperature rises. Therefore, efficiency is the most fundamental factor in determining the life of the power supply. The efficiency cannot be too low, otherwise the heat consumed by the power supply will be too great. The efficiency of non-isolated type is higher than that of isolated type, generally above 80%, but the efficiency is related to the matching connection method of the lamp board.

　　10. LED street light power supply heat dissipation

　　The main factor of the heat dissipation solution is that the LED street lamp power bead can be used under the condition of not overheating to greatly extend its life. Generally, aluminum alloy is used, which is easier to dissipate heat. That is, the LED street lamp power bead is attached to the aluminum substrate, and the heat dissipation area is expanded as much as possible.

　　Such a vast LED market is full of vitality. As long as it can be developed scientifically and a leading position can be achieved in certain technical fields, I believe that my country's LED companies will become stronger and stronger.