A brief discussion on the ten key points of LED fluorescent lamp power supply design

At present, the LED fluorescent lighting market is relatively active, and LED driver power supply manufacturers are mainly divided into three types:

　　The first category is to develop factories that make LED chips or LED lights, and then penetrate downstream;

　　The second category is factories that originally made general lighting;

　　The third category is completely newly opened factories, which used to make power supplies or other products or are new businesses.

　　The LED fluorescent lamp power supply is a very important component of the LED fluorescent lamp. If the LED fluorescent lamp is not properly selected, the LED fluorescent lamp will not be able to perform as it should, and may even not be used for normal lighting. Here are some small suggestions on LED fluorescent lamp lighting for your reference.

　　1. Why must the LED fluorescent lamp power supply have a constant current?

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

　　2. How can the LED fluorescent lamp power supply be matched with the light board?

　　Some customers design the light board first and then look for a power supply, but they find it difficult to find a suitable power supply. Either the current is too large and the voltage is too small (such as I>350mA, V<40V); or the current is too small and the voltage is too high (such as I<40mA, V>180V). The result is 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.

　　3. What is the most suitable working current of LED fluorescent lamp power supply?

　　Generally, the rated working current of LED is 20 mA. Some factories use it to the maximum from the beginning and design it for 20 mA. In fact, the working current under this current generates serious heat. After many comparative tests, it is found that the design of 17-19 mA is more ideal, and the recommended design is 18 mA.

　　4. What is the working voltage of the LED fluorescent lamp power supply?

　　The recommended operating voltage of general LED is 3.0-3.5V. After testing, most of them work at 3.125V, so it is more reasonable to calculate according to the 3.125V formula. The total voltage of M lamp beads in series = 3.125*M

　　5. How wide should the series and parallel connection and wide voltage of the LED light panel be?

　　To make LED fluorescent lamps work in a relatively wide input voltage range (full voltage) AC85~265V, the LED series-parallel connection mode of the lamp board is very important. Since the current power supply is generally a non-isolated step-down power supply, when a wide voltage is required, the output voltage should not exceed 72V, and the input voltage range can reach 85~265V. In other words, the number of series connections should not exceed 23. The number of parallel connections should not be too large, otherwise the working current will be too large and the heat will be serious. It is recommended to use 6 parallel/8 parallel/12 parallel. The total current should not exceed 240 mA. There is also a wide voltage solution, which is to first use L6561/7527 to raise the voltage to 400V, and then step down the voltage, which is equivalent to two switching power supplies. The cost is twice as expensive. This solution is not cost-effective and has no market.

　　6. What is the relationship between the series and parallel connection of LEDs and the PFC power factor and wide voltage?

　　There are three types of PFC in the power supply on the market: one is without PFC dedicated circuit, and its PFC is generally around 0.65; one is with passive PFC circuit, the lamp board is well matched, and the PFC is generally around 0.92; another is made with active 7527/6561 circuit, and the PFC can reach 0.99, but the cost of this solution is twice as expensive as the second solution. So the second solution is more common. For passive PFC circuit: also called valley-filling PFC circuit, its operating voltage range is half of the peak value of the AC input voltage. If the input is 180V, its peak value is 180*1.414=254V, and half of the peak voltage is 127V, minus the voltage difference of the buck type 30V, its maximum output is 90V, so the maximum number of LED lamp beads in series is 28. Therefore, in order to obtain a relatively large power factor, the number of lamp beads in series cannot be too many, otherwise, the low voltage requirement cannot be met.

　　7. What is the most appropriate constant current accuracy of LED fluorescent lamp power supply?

　　The constant current accuracy of some power supplies on the market is too poor. For example, the constant current solutions such as the popular PT4107/HV9910/BP2808/SMD802 solutions on the market have an error of ±8% or ±10%, which is too large. Generally, ±3% is enough. According to the 3% error, 6 channels are connected in parallel, and the error of each channel is about ±0.5%. If 12 channels are connected in parallel, the error of each channel is about ±0.25%, which is enough. If the accuracy is too high, the cost will increase greatly. Moreover, for LEDs, 17 mA and 17.5 mA have little effect.

　　8. Isolation/non-isolation

　　Generally, if the isolated power supply is made into 15W, and placed in the LED tube, its transformer is very large and difficult to put in. Especially for T6/T8 tubes, it is almost impossible, so the isolation can 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, and generally non-isolation is 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.

　　9. LED fluorescent lamp power efficiency

　　Output power (output LED voltage * output current / input power). This parameter is particularly important. If the efficiency is low, it means that a large part of the input power is converted into heat and dissipated; if it is installed in a lamp tube, a very high temperature will be generated, and the heat dissipated by the light efficiency ratio of our LED will be superimposed to 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 in the power supply will be too great. Generally, it is enough to be above 80%, but the efficiency is related to the matching connection method of the lamp board.

　　10. LED fluorescent lamp power supply size

　　Height is the main limiting factor. Generally, the dimensions for T6/T8 tubes require that the height cannot be too high ≤9 mm. The height of T10 tubes is ≤15 mm. The length can be longer to facilitate heat dissipation.