A brief analysis of the connection method when multiple LEDs are used together

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

　　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 for it to be widely used. The power of a single high-brightness LED that has entered practical use is very small, and most of the actual use occasions require plane light emission, so multiple LEDs must be arranged and combined according to the requirements. On the one hand, it can meet the requirements of a larger range, higher brightness, dynamic display, color change, etc., and on the other hand, it can meet the driving matching requirements of the driver that matches the LED.

　　
　　Common connection forms

　　In applications, there are four common connection forms in which multiple LEDs are arranged and combined according to certain rules. They are introduced as follows.
　　
　　1. Overall series connection

　　Generally, LED1-n are connected end to end in a simple series connection, 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 be slightly different, since LEDs are current-type devices, the luminous intensity of each can be guaranteed to be consistent. Therefore, simple series LEDs 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 with bypass is an improvement of 1.1. In this method, each LED is connected in parallel with a Zener diode D1-n whose breakdown voltage is slightly higher than the LED operating voltage. 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.

　　2. Overall parallel form

        2.1 Simple parallel form

        In the simple parallel connection mode, LED1-n are connected in parallel end to end, and the voltage on each LED is equal when working. As can be seen from the characteristics of LED, it is a current-type device. 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 from 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.

　　2.2 Independent matching parallel form

　　In view of the reliability problem in 2.1, the independent matching 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 the 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 driving 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.

　　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.

　　3.1 Series-first then parallel hybrid mode

　　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 method of series connection first and parallel connection later 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 form. 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.

　　3.2 Parallel-first then series hybrid mode

　　The LED connection method of connecting in parallel first and then in series is another mixed connection method different from 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. This can be solved by using a paired selection method to select LEDs with the same working voltage and current as possible as a parallel group, or by 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 3.1. Due to space limitations, I will not go into details.

　　4. Cross array form

　　The cross array shape 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 the circuit, it can ensure that at least one LED works normally. In this way, the reliability of each group of LED light emission can be greatly improved, and the overall reliability of the entire LED light emission can also be improved.