Several common problems with LED fluorescent lamp power supply

At present, the LED fluorescent lamp market is very active, and manufacturers are mainly divided into three categories: the first category is the factory that originally made LED chips, and then penetrated downstream, and has little knowledge of circuit knowledge and LED fluorescent lamp power supply; the second category is the factory that originally made general lighting, and entered a new field, and has some knowledge of circuit knowledge; the third category is the factory that has completely entered, they used to make other products or start a new business, and some of them know about LED power supply, and some do not. LED fluorescent lamp power supply is the most important component of LED fluorescent lamp. If it is not selected properly, the LED fluorescent lamp cannot play its performance, or even cannot be used normally.

　　1. Why must constant current be used?

　　The characteristics of LED semiconductors determine that they are greatly affected by the environment. For example, when the temperature changes, the LED current increases, and when the voltage increases, the LED current also increases. 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. Matching of LED fluorescent lamp power supply and light board:

　　Some customers design the light board first, then look for the power supply, and 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), 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.

　　3. LED operating current:

　　Generally, the rated working current of LED is 20 mA. Some factories use it to the maximum from the beginning and design it to 20 mA. In fact, the working current under this current is very hot. After many comparative tests, it is more ideal to design it to 17 mA. The total current of N parallel circuits = 17*N;

　　4. LED operating voltage:

　　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. Series and parallel connection and wide voltage of LED light panels:

　　To make the LED fluorescent lamp work in a relatively wide input voltage range of 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 use L6561/7527 to raise the voltage to 400V first, and then step down the voltage, which is equivalent to two switching power supplies, and the cost is twice as expensive. This solution is not cost-effective and has no market.

　　6. The relationship between LED series and parallel connection, 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, half of the peak voltage is 127V, and then minus the voltage difference of 30V of the buck type, 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. Constant current accuracy:

　　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. 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, 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 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. Dimensions:

　　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.