LED is a solid semiconductor light source based on the principle of electroluminescence. It has rich color, small size, high brightness, long life, low working voltage, safe use, fast response, 0~100% dimmable output, shock and vibration resistance. There are many advantages such as ultraviolet and infrared radiation. Therefore, the application range is gradually expanding and has a good application prospect.
The high-power, high-brightness LEDs in a single package are expensive, and are mainly point light sources, so the distance is practical for a long time. At present, the power of a single high-brightness LED that is put into practical use is small, and most of the occasions require planar illumination in actual use. Therefore, it is necessary to arrange a plurality of LEDs according to requirements, on the one hand, it can satisfy a larger range and The requirements of high brightness, dynamic display, color change and other applications, on the other hand, can meet the drive matching requirements of the driver matched with the LED.
1 Common connection forms In the application, there are four main types of connection by multiple LEDs according to certain rules, which are described below.
1.1 Overall series form 1.1.1 Simple series form In the simple serial connection mode, LED1-n is connected end to end, and the current flowing through the LED is equal. For LEDs of the same specification and batch, although the voltage on a single LED may vary slightly, since the LEDs are current-type devices, it is possible to ensure that the respective luminous intensities are consistent. Therefore, the simple series-connected LED has the characteristics of simple circuit and convenient connection. However, since LED1-n 2 is in series, when one of the LEDs has an open circuit failure, the entire LED string will be extinguished, which affects the reliability of use.
The series connection with bypass is an improved version of 1.1.1. Each of the LEDs in this manner is connected in parallel with a Zener diode D1-n having a breakdown voltage slightly higher than the operating voltage of the LED. When the LED is working normally, since D1-n is not conducting, the LED1-n string through which the current mainly flows is equal, and the LED1-n string is normally illuminated; when there is damage in the LED1-n string, the string is opened, due to D1 The conduction of -n ensures that current flows through the entire LED string, so only the failed LED fails and the entire 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 The LED1-n in the simple parallel mode is connected in parallel with each other, and the voltage on each LED is equal during operation. It can be seen from the characteristics of the LED that it belongs to a current-type device, and a small change in the voltage applied to the LED will cause a large change in the current. In addition, due to the limitation of LED manufacturing technology, even in the same batch of LEDs, the difference in performance is inherently present. Therefore, when LED1-n is operating, the current flowing through each LED is not equal. It can be seen that the insufficiency of the current distribution of each LED can cause the life of the LED with excessive current to be sharply reduced or even burned out. Although this connection method is relatively simple, the reliability is not high, especially for applications where the number of LEDs is large, which is more likely to cause malfunctions.
1.2.2 Parallel form of independent matching For the reliability problem existing in 1.2.1, the parallel form of independent matching is a good way. Each of the LEDs in this mode has a current tunability (driver V+ output is L1-n, respectively), ensuring that the current flowing through each LED is within its required range. It has a good driving effect, complete protection of a single LED, does not affect other LED operations when faults, and can match LEDs with large differences. The main problem is that the structure of the whole driving circuit is complicated, the cost of the device is high, and the volume occupied is large, which 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 Mixing mode after stringing first When the number of LED1-n applied is large, simple series or simple parallel is not realistic, because the former requires the driver to output a very high voltage (n times the single LED voltage VF) The latter requires the driver to output a large current (n times the IF of a single LED current). This poses difficulties for the design and manufacture of the driver, and also involves structural problems of the drive circuit and overall efficiency issues. The product n of the number of LEDs connected in series and the operating voltage VF of a single LED determines the output voltage of the driver; the product mIF of the number m of parallel LED strings and the operating current IF of a single LED determines the output current of the driver, and the value of mIF×nVF is determined. The output power of the drive. Therefore, the hybrid method of using the first string and the back is mainly to ensure a certain reliability (the LED fault in each string only affects the normal illumination of the string at most), and also ensures the matching with the driving circuit (the driver outputs a suitable voltage) It improves reliability than a simple series connection. The whole circuit has the characteristics of simple structure, convenient connection and high efficiency, and is suitable for applications with a large number of LEDs.
1.3.2 First and last series of hybrid mode The first and last string of LED connection is another hybrid method different from 1.3.1. Since the LED1-n~LED1-n is connected in parallel, the reliability of each group of LEDs is improved, but the current sharing problem of each group of parallel LEDs is crucial. In the way of selecting, select the LEDs with the same working voltage and current as the parallel group, or solve the problem by connecting each LED with a small current sharing resistor. As for the other characteristics of the connection method and the existing problems, the corresponding expressions in 1.3.1 can be referred to. Due to space limitations, we will not repeat them.
1.3 Cross-array form The cross-array shape is mainly proposed to improve the reliability of LED operation and reduce the failure rate. The main form is: Each string is made up of 3 LEDs, which are respectively connected to the Va, Vb, and Vc outputs of the driver output. When three LEDs in a string are normal, three LEDs emit light at the same time; once one or two of the LEDs fail open, at least one LED can be guaranteed to work normally. In this way, the reliability of each group of LED illumination can be greatly improved, and the overall reliability of the entire LED illumination can be improved.
2 Different types of connections have different characteristics and different requirements for the drive. Especially in the case of a single LED failure, the operation of the circuit, the reliability of the overall illumination, ensuring that the overall LED can continue to work, and reducing the overall LED failure rate are particularly important. Table 1 gives a comparison of the relevant feelings under different connection methods, which can have an intuitive understanding.
3 Conclusion In summary, the group application of LED is an important way of practical application of LED. Different LED connections are critical to the use of a wide range of LEDs and to the design requirements of the drive circuit. Therefore, in the combination of actual circuits, the correct selection of the appropriate LED linkage mode has positive significance for improving the luminous effect, the reliability of the work, the difficulty of designing and manufacturing the driver, and the efficiency of the entire circuit.
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