Common connection forms and characteristics of LED

LED is a solid-state semiconductor light source based on the principle of electroluminescence . It has many advantages such as rich colors, small size, high brightness , long life, low operating voltage, safe use, fast response speed, 0~100% adjustable light output, impact resistance and vibration resistance, no ultraviolet and infrared radiation, etc. Therefore, the scope of application is gradually expanding and has good application prospects. The cost of

a single packaged high- power high-brightness LED is high, and it is mainly a point light source, so it will take some time before it can be widely used. At present, the power of a single high-brightness LED that has entered practical use is very small, and most of the actual use occasions require planar light emission. Therefore, multiple LEDs must be arranged and combined according to the requirements. On the one hand, it can meet the requirements of applications such as a larger range, higher brightness, dynamic display , and color changes, and on the other hand, it can meet the driving matching requirements of the driver that matches the LED. 1 Common

connection forms
In applications, there are four common connection forms in which multiple LEDs are arranged and combined according to certain rules, which are introduced as follows.
1.1 Overall series connection form
1.1.1 Simple series connection form
In general, LED1-n are connected end to end in a simple series connection mode, and the current flowing through the LEDs is equal when they are working. For LEDs of the same specification and batch, although the voltage on a single LED may have a slight difference, since LEDs are current-type devices, their luminous intensity can be guaranteed to be consistent. Therefore, LEDs in a simple series connection form have the characteristics of simple circuits and convenient connections. However, since LED1-n are connected in series, when one of the LEDs fails to open the circuit, the entire LED string will be extinguished, affecting the reliability of use.
The series connection mode with bypass is an improved mode of 1.1.1. In this mode, 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, since D1-n is not conducting, the current mainly flows through the LED1-n string and is equal, and the LED1-n string emits light normally; when there is damage in the LED1-n string and the light string is open, the conduction of D1-n ensures that the current flows through the entire LED string, so only the faulty LED fails, and the entire light string does not go out. Compared with the previous connection method, the reliability of use is greatly improved.
1.2 Overall parallel form
1.2.1 Simple parallel form
In the simple parallel method, LED1-n is connected in parallel from head to tail, and the voltage on each LED is equal during operation. It can be seen from the characteristics of LED that it is a current-type device, and a slight change in the voltage applied to the LED will cause a large change in the current. In addition, due to the limitations of LED manufacturing technology, even LEDs of the same batch have inherent differences in performance. Therefore, when LED1-n is working, the current flowing through each LED is not equal. It can be seen that the uneven current distribution of each LED may cause the life of the LED with excessive current to be sharply reduced, or even burn out. Although this connection method is relatively simple, its reliability is not high, especially for applications with a large number of LEDs, it is more likely to cause failures in use.
1.2.2 Independently matched parallel form
For the reliability problem in 1.2.1, the independently matched parallel form is a good way. In this way, each LED has independent current adjustability (the driver V+ output terminal is L1-n respectively), ensuring 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 impact on the operation of other LEDs in case of failure, and can match LEDs with large differences. The main problems are: the structure of the entire drive circuit is relatively complex, the cost of the device is high, the volume occupied is large, and it is not suitable for a large number of LED circuits .
1.3 Hybrid form
The hybrid form is proposed by combining the respective advantages of the series form and the parallel form. There are two main forms.
1.3.1 Hybrid connection of series connection first and then parallel
connection When the number of LED1-n applied is large, simple series connection or simple parallel connection is not realistic, because the former requires the driver to output a very high voltage (n times the voltage VF of a single LED), and the latter requires the driver to output a large current (n times the current IF of a single LED). This brings difficulties to the design and manufacture of the driver, and also involves the structural problems of the drive circuit and the overall efficiency problems. The product nVF of the number of LEDs in series and the working voltage VF of a single LED determines the output voltage of the driver; the product mIF of the number of LED strings in parallel and the working current IF of a single LED determines the output current of the driver, and the value of mIF×nVF determines the output power of the driver. Therefore, the hybrid connection of series connection first and then parallel connection is mainly to ensure a certain reliability (the failure of LEDs in each string will only affect the normal light emission of the string at most), and to ensure matching with the drive circuit (the driver outputs a suitable voltage), which improves reliability compared with the simple series connection. The whole circuit has the characteristics of simple structure, convenient connection, high efficiency, etc., and is suitable for applications with a large number of LEDs.
1.3.2 Parallel-first then series connection method
The parallel-first then series connection method of LEDs is another hybrid connection method different from 1.3.1. Since LED1-n~LED1-n are connected in parallel first, the working reliability of each group of LEDs when they fail is improved. However, the current sharing problem of each group of parallel LEDs is crucial. It can be solved by pairing selection, selecting LEDs with the same working voltage and current as possible as a parallel group, or connecting a small current sharing resistor in series with each LED. As for other characteristics and existing problems of this connection method, please refer to the corresponding description in 1.3.1. Due to space limitations, it will not be repeated.
1.3 Cross array form
The cross array form is mainly
proposed to improve the reliability of LED work and reduce the failure rate. The main form is: each string is a group of 3 LEDs, which are connected to the Va, Vb, and Vc output terminals of the driver output respectively. When the 3 LEDs in a string are normal, the 3 LEDs light up at the same time; once one or two of the LEDs fail and open, it can ensure that at least one LED works normally. In this way, the reliability of each group of LEDs can be greatly improved, and the overall reliability of the entire LED can be improved.
2 Comparison of different forms of connection
Different connection methods have their own characteristics and different requirements for the driver. In particular, when a single LED fails, the circuit operation, the reliability of the overall light emission, the ability to ensure that the overall LED can continue to work as much as possible, and the reduction of the overall LED failure rate are particularly important. Table 1 gives a comparison of the relevant situations under different connection methods, which can give you an intuitive understanding.
3 Conclusion
In a word, the group application of LED is an important way of LED practical application. Different LED connection methods are crucial for the use of a large range of LEDs and the design requirements of the drive circuit. Therefore, in the combination of actual circuits, the correct selection of the appropriate LED connection method has a positive significance for improving its lighting effect, working reliability, the difficulty of driver design and manufacturing, and the efficiency of the entire circuit.