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Brief Analysis on Connection Mode of Multiple LED Used Together

2019/10/12

LED is a solid semiconductor light source based on the principle of electroluminescence. It has many advantages, such as rich color, compact size, high brightness, long life, low working voltage, safe use, fast response, 0-100% adjustable light output, shock and vibration resistance, no ultraviolet and infrared radiation. Therefore, the application scope is gradually expanding and has a good application prospect.

Single packaged high-power and high-brightness LED has high cost and is mainly a point light source, so it is still a long time before it can be widely used. At present, the power of a single high-brightness LED is very small, but in most practical applications, it needs planar light. Therefore, it is necessary to arrange and combine multiple LEDs according to the requirements. On the one hand, it can meet the requirements of large-scale, high brightness, dynamic display, color change and other applications, on the other hand, it can meet the driving matching requirements of the driver matching with the LED.

Common Connection Forms

In the application, there are four kinds of common connection forms, which are arranged and combined by several LED according to certain rules. They are introduced as follows.

1. Whole Series Form

In the simple series connection mode, the LED 1-N and the end are connected, and the current flowing through the LED is equal when it works. For the same specifications and batches of LED, although the voltage on a single LED may vary slightly, because the LED is a current-mode device, it can ensure that their luminous intensity is consistent. Therefore, the simple series LED has the characteristics of simple circuit and convenient connection. However, because of the series mode of LED 1-n2, when one of the LEDs breaks down, it will cause the whole LED lamp string to be extinguished, which will affect the reliability of the use.

The improved series connection with bypass is 1.1. In this way, each LED is connected in parallel with a Zener diode D1-n whose breakdown voltage is slightly higher than the working voltage of the LED. When the LED is working normally, because the D1-n is not conducting, the current mainly flows through the LED 1-N series, and is equal, the LED 1-N series normally emits light; when the LED 1-N series is damaged and causes the lamp string to open, because the D1-n conduction guarantees the current flows through the entire LED string, only the faulty LED fails, and the entire lamp string does not go out. Compared with the previous connection mode, it greatly improves the reliability of use.

2. Overall Parallel Connection Form

2.1 Simple Parallel Form

In the simple parallel mode, the first and last of the LED 1-N are connected in parallel, and the voltage on each LED is equal when working. From the characteristics of LED, it can be seen that it belongs to current-mode devices. The slight change of voltage on the LED will cause great change of current. In addition, due to the limitation of LED manufacturing technology, even for the same batch of LEDs, their performance differences are inherent, so when the LED 1-N works, the current flowing through each LED is unequal. It can be seen that the uneven current distribution of each LED may lead to a sharp decrease in the lifetime of the LED with excessive current, or even burn out. Although this connection method is relatively simple, but the reliability is not high, especially in the case of a large number of LED applications, it is easier to cause use failure.

2.2 Parallel Form of Independent Matching

 

For the reliability problem in 2.1, the parallel form of independent matching is a good way. In this way, each LED has its own current adjustability (driver V + output terminal L1-n respectively), which ensures that the current flowing through each LED is within its required range. It has the characteristics of good driving effect, complete protection of a single LED, no influence on other LED work when the fault occurs, and can match the LED with great differences. The main problems are: the structure of the whole drive circuit is more complex, the cost of the device is high, the occupied volume is large, which is not suitable for a large number of LED circuits.

3. Concatenated Form

Mixed connection is proposed by combining the advantages of series connection and parallel connection. There are two main forms.

3.1 Series-to-Parallel Hybrid Mode

When the number of LED 1-N is large, simple series connection or simple parallel connection is unrealistic, because the former requires the driver to output a high voltage (n times the voltage of a single LED VF), and the latter requires the driver to output a large current (n times the current of a single LED IF). This brings difficulties to the design and manufacture of the driver, and also involves the structure of the driver circuit and the overall efficiency. The output voltage of the driver is determined by the product of the number of series LEDs N and the working voltage VF of a single LED; the product of the number of parallel LEDs m and the working current IF of a single LED mIF determines the output current of the driver, and the value of mIF * nVF determines the output power of the driver. Therefore, the series-to-parallel hybrid mode is mainly used to ensure a certain degree of reliability (the failure of the LED in each series only affects the normal luminescence of the series at most), and to ensure the matching with the driving circuit (the driver outputs the appropriate voltage), which improves the reliability compared with the simple series mode. The whole circuit has the characteristics of simple structure, convenient connection and high efficiency, which is suitable for the application occasion of large number of LED.

3.2 Parallel-Series Hybrid Mode

The parallel and serial connection mode of LED is another kind of hybrid mode different from 3.1. Because of the parallel connection of LED 1-n~LED 1-n, the working reliability of each group of LED faults is improved. But the current sharing problem of each group of parallel LED is very important. It can be solved by pair selection, selecting the LED with the same working voltage and current as possible as a group of parallel connection, or connecting each LED with a small current sharing resistance in series. As for the other characteristics and problems of this connection mode, we can refer to the corresponding expression in 3.1. Limited to length, no more details.

4. Cross Array Form

Cross array is proposed to improve the reliability of LED and reduce the failure rate. The main form is: each series of three LED as a group, respectively access to the driver output of Va, Vb, Vc output. When three LEDs in a series are normal, three LEDs emit light at the same time; once one or two of them fail to open, at least one LED can be guaranteed to work properly. In this way, the reliability of each group of LED can be greatly improved, and the overall reliability of the whole LED can also be improved.

Yiwu Fuwei e-commerce firm (IACS Co., Limited) is a very professional Led manufacturer who produces high quality outdoor led light. Our popular products are solar led lights, UFO lights, underwater lights and so on. If you have any question about the led product, please feel free to contact us.