At present, the LED fluorescent lamp market is very active, and the manufacturers are mainly divided into three categories: one is the factory that originally made LED chips, which infiltrates downstream, and has little understanding of circuit knowledge and LED fluorescent lamp power supply; The second is the factory that used to do general lighting, entering a new field and understanding some circuit knowledge; The third category is completely new factories. They used to do other products or start new businesses, and some know or don’t know about LED power supply. LED fluorescent lamp power supply is the most important component of LED fluorescent lamp. If it is improperly selected, LED fluorescent lamp can not play its performance or even use normally.
1. Why constant current is necessary:
The characteristics of LED semiconductor determine that it is greatly affected by the environment. For example, when the temperature changes, the LED current increases and the voltage increases, the LED current will also increase. Working over the rated current for a long time will greatly shorten the service life of LED. LED constant current is to ensure that its working current remains unchanged when environmental factors such as temperature and voltage change.
2. Matching of LED fluorescent lamp power supply and lamp board:
Some customers first design the lamp board and then find the power supply. They find it difficult to have a suitable power supply. Either the current is too large and the voltage is too small (such as I > 350mA, V < > < 40mA, V > 180V), resulting in serious heating, low efficiency, or insufficient input voltage range. In fact, choose the best series parallel connection mode, the voltage and current added to each LED are the same, but the effect of the power supply can play the best performance. The best way is to communicate with the power supply manufacturer and customize it.
3. Working current of LED:
Generally, the rated working current of LED is 20 mA. Some factories use it all at the beginning. The design is 20 mA. In fact, the working heat under this current is very serious. After many comparative tests, it is ideal to design it as 17 ma. Total current of n-way parallel connection = 17 * n;
4. Operating voltage of LED:
The recommended working voltage of general LED is 3.0-3.5v. After testing, most of them work at 3.125v, so the calculation formula of 3.125v is more reasonable. Total voltage of M lamp beads in series = 3.125 * m
5. Series parallel connection and wide voltage of LED lamp board:
In order to make the LED fluorescent lamp work in a wide range of input voltage AC85-265V, the LED series parallel 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 shall not exceed 72V, and the input voltage range can reach 85-265V. That is, the number of strings in series does not exceed 23. The number of parallels should not be too large, otherwise the working current is too large and the heating is serious. It is recommended to be 6 parallel / 8 parallel / 12 parallel. The total current should not exceed 240 ma. There is also a wide voltage scheme, that is, first raise the voltage to 400V with L6561 / 7527, and then reduce the voltage. It is equivalent to two switching power supplies, and the cost is twice as expensive. This scheme is not cost-effective and has no market.
6. Relationship between series parallel connection of LED and PFC power factor and wide voltage:
At present, there are three situations of power PFC in the market: one is without PFC special circuit, and its PFC is generally about 0.65; One is with passive PFC circuit, the lamp board is well prepared, and the PFC is generally about 0.92; Another one is made of active 7527 / 6561 circuit. The PFC can reach 0.99, but the cost of this scheme is twice as expensive as the second scheme. Therefore, the second scheme is more. For passive PFC circuit: also known as valley filling PFC circuit, its working voltage range is half of the peak value of AC input voltage. If the input is 180V, the peak value is 180 * 1.414 = 254v, half of the peak voltage is 127V, and then minus the step-down differential voltage of 30V, the maximum output is 90V, so the number of LED beads in series is up to 28. Therefore, in order to obtain a relatively large power factor, the number of lamp beads in series cannot be too much, otherwise, it will not meet the requirements of low voltage.
7. Constant current accuracy:
The constant current accuracy of some power supplies in the market is too poor. For constant current schemes such as pt4107 / HV9910 / bp2808 / smd802, the error reaches ± 8% or ± 10%, and the constant current error is too large. The general requirement is ± 3%. According to the error of 3%, 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 sufficient. If the accuracy is too high, the cost will increase greatly. And for LEDs, 17 Ma and 17.5 ma have little effect.
8. Isolated / non isolated:
Generally, if the isolated power supply is made of 15W and placed in the LED lamp tube, the transformer has a large volume and is difficult to put in. Especially for T6 / T8 tubes, it is almost impossible, so the isolation can only be 15W, few more than 15W, and the price is very expensive. Therefore, the cost performance of isolation is not high. Generally, non isolation accounts for more mainstream, and the volume can be made smaller, with a minimum height of 8mm. In fact, if the non isolation safety measures are well taken, there is no problem.
9. Power efficiency:
Output power (voltage of output LED * 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 emitted; If it is installed in the lamp tube, it will produce a very high temperature. Coupled with the heat emitted by a light efficiency ratio of our LED, it will superimpose to produce a higher temperature. The life of all electronic parts in our power supply will shorten with the rise of temperature. Therefore, efficiency is the most fundamental factor to determine the life of the power supply. Efficiency cannot be too low, otherwise the heat consumed in the power supply is too large. Generally more than 80%, but the efficiency is related to the matching connection of the lamp board.
Height is the main limiting factor. It is generally used for the size of T6 pipe / T8. The height shall not be too high ≤ 9mm. T10 tube height ≤ 15mm. The length can be longer, which is easier to dissipate heat.
Responsible editor; zl