Electronic Design - Common problems of LED fluorescent lamp PCB design

The following is an introduction to the common problems of LED fluorescent lamp PCB design:

At present, the LED fluorescent lamp market is very active, and manufacturers are mainly divided into three categories: the first type is the factory that used to make LED chips, which penetrates downstream, and has little understanding of circuit knowledge and LED fluorescent lamp power; the second type is the original general lighting The factory enters a new field and knows a little about the circuit knowledge; the third category is completely new to the factory, they used to do other products or new ventures, some understanding of LED power supply, some do not. The LED fluorescent lamp power supply is the most important part of the LED fluorescent lamp. If the LED fluorescent lamp is not selected properly, the LED fluorescent lamp will not be able to exert its performance or even be used normally.

1. Why constant current is necessary:

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

2. The matching of LED fluorescent lamp power supply and PCB lamp board:

Some customers first design the circuit board light board and then look for the power supply. They find that it is difficult to have 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 heat generation, low efficiency, or insufficient input voltage range. In fact, choose the most excellent series-parallel connection method, the voltage and current applied to each LED are the same, but the power supply effect can exert the best performance. The best way is to communicate with the power supply manufacturer first, and tailor it.

3. LED working current:

Generally, the rated working current of LED is 20 mA, and some factories use it up at the beginning. The design is 20 mA. In fact, the working current under this current is very serious. After many comparison tests, it is ideal to design 17 mA. . The total current of N parallel connections=17*N;

Fourth, the working voltage of LED:

The recommended working voltage of general LEDs is 3.0-3.5V. After testing, most of them work at 3.125V, so the calculation formula of 3.125V is more reasonable. The total voltage of M lamp beads in series = 3.125*M

5. Series-parallel connection and wide voltage of LED PCB light boards:

To make the LED fluorescent lamp work in a relatively wide range of input voltage range AC85-265V, the LED series-parallel connection of the circuit board light 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 does not exceed 23 series. Do not connect too many in parallel, otherwise the working current will be too large and the heat will be 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 solution, which is to use L6561/7527 to increase the voltage to 400V first, and then step down, 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 the series-parallel connection of LEDs and the PFC power factor and wide voltage:

There are currently three types of power supply PFCs on the market: one is without a dedicated PFC circuit, and the PFC is generally around 0.65; the other is with a passive PFC circuit, the circuit board is well prepared, and the PFC is generally around 0.92. ; Another one is made with active active 7527/6561 circuit, PFC can reach 0.99, but the cost of this scheme is twice as expensive as the second scheme. So the second scheme is more. For passive PFC circuits: also called valley-filled PFC circuits, the 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 subtracting the step-down voltage difference of 30V, its maximum output is 90V, so the number of LED lamp beads in series is at most 28 strings. 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 will not be met.

7. Constant current accuracy:

The constant current accuracy of some power supplies on the market is too poor. For constant current solutions such as the popular PT4107/HV9910/BP2808/SMD802 solution on the market, the error can reach ±8% or ±10%, and the constant current error is too large. The general requirement is within ±3%. According to the error of 3%, 6 circuits are connected in parallel, and the error of each circuit is about ±0.5%. If 12 circuits are connected in parallel, the error of each circuit is about ±0.25%. This accuracy 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. Isolation/non-isolation:

Generally, the isolated power supply is made of 15W and placed in the LED tube. The transformer is very large and difficult to put in. Especially for T6/T8 lamps, it is almost impossible, so the isolation can generally only be 15W, and there are few more than 15W, and the price is very expensive. Therefore, isolation is not cost-effective. Generally, non-isolated products dominate the mainstream, and the volume can be made smaller, with a minimum height of 8 mm. In fact, non-isolated security measures are in place and there is 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 to be emitted; if it is installed in the tube, it will produce a very high temperature, plus a light efficiency ratio of our LED. Dissipating heat, it will superimpose to produce 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 that determines the life of the power supply. The efficiency cannot be too low, otherwise the heat consumed on the power supply is too large. Generally, more than 80% is fine, but the efficiency is related to the matching connection method of the circuit board light board.

10. Size design requirements:

Height is the main factor that restricts the design of LED fluorescent lamp circuit board. Generally, the size of T6 tube/T8 requires that the height should not be too high ≤ 9 mm. The height of the T10 tube is less than or equal to 15 mm. The length can be longer, which makes it easier to dissipate heat.