Introduction to common connection forms in LED applications

The configuration of LEDs in an application depends on many factors, including application requirements, LED parameters and quantity, input voltage, efficiency, thermal management, size and layout restrictions, and optics. The simplest configuration is a single LED, and there are many applications using this design, such as automotive interior ceiling lights (map lights, reading lights), etc.

Since the power and brightness of LEDs are not very high at present, and actual use scenarios usually require planar light emission, multiple LEDs need to be arranged and combined as needed to meet application requirements such as larger range, higher brightness, dynamic display, color change, and matching requirements between LEDs and supporting drivers.

1. Common connection forms

1. Overall series connection

(1) Simple series connection

A simple series connection of LEDs is shown in Figure 1.

Generally, LED1~LEDn in a simple series connection form are connected end to end, 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, the luminous intensity of each can be guaranteed to be consistent. Therefore, simple series connection LEDs have the characteristics of simple circuits and convenient connections. However, due to the series connection form, when one of the LEDs has an open circuit failure, the entire LED string will be extinguished, affecting the reliability of use.

(2) Series connection with parallel Zener diode

The improved series connection form in which each LED is connected in parallel with a Zener diode is shown in Figure 2. In this connection mode, the breakdown voltage of each Zener diode is higher than the working voltage of the LED. When the LED is working normally, since the Zener diodes VD1~VDn are not conducting, the current mainly flows through LED1~LEDn. When there is a damaged LED in the LED string causing the light string to be open, since VD1~VDn is conducting, except for the faulty LED, other LEDs still have current passing through and emit light. Compared with the simple series connection form shown in Figure 1, this connection mode has greatly improved reliability.

Figure 1 Simple series connection

Figure 2 Series connection with parallel Zener diode

2. Overall parallel form

(1) Simple parallel form

Figure 3 shows a simple parallel connection of LEDs.

In the simple parallel form, LED1~LEDn are connected in parallel end to end, and the voltage of each LED is equal when working. It can be seen from the characteristics of LED that it is 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 limitations of LED manufacturing technology, even the same batch of LEDs have inherent differences in performance. Therefore, when LED1~LEDn is working, the current of each LED is not equal. It can be seen that the uneven distribution of current for 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) Independent matching parallel form

In view of the reliability problem in Figure 3, the independent matching parallel form is a good way (see Figure 4). In this way, each LED has independent current adjustability (the driver V+ output terminals are L1~Ln, respectively), ensuring that the current flowing through each LED is within the required range, with 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 too large, and it is not suitable for circuits with a large number of LEDs.

Figure 3 Simple parallel form

Figure 4 Independent matching parallel form

3. Hybrid form

The hybrid form is proposed by combining the respective advantages of the series form and the parallel form. The main forms are the following two.

(1) Series-first then parallel hybrid form (see Figure 5)

When the number of LEDs used is large, simple series connection or 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 n VE 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 mixing series and then paralleling is mainly to ensure a certain degree of 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 entire circuit has the characteristics of relatively simple structure, convenient connection, and high efficiency, and is suitable for applications with a large number of LEDs.

(2) Parallel-first then series hybrid

The hybrid form of several LEDs in parallel first and then in series is shown in Figure 6. Since LED1-n~LEDm-n are connected in parallel first, the reliability of each group of LEDs under fault conditions is improved, but the current sharing problem of each group of parallel LEDs is crucial. To this end, it can be solved by pairing and selecting LEDs with the same operating voltage and current as much as possible as a parallel group, or by connecting a small current sharing resistor in series to each LED. The other characteristics and problems of this hybrid form are similar to those of the series-first-parallel connection form.

Figure 5: Hybrid connection mode of series connection first and parallel connection later

Figure 6: Parallel-first then series hybrid mode

(3) Cross array form

Figure 7 shows the LED cross array form

The cross array form is mainly 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 LEDs fail and open the circuit, it can be guaranteed 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.

2. Comparison of different connection forms

Different connection forms have their own characteristics and different requirements for the driver, especially when a single LED fails, the circuit operation, the overall light reliability, 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 when different connection forms are used.

Table 1 Comparison of different ED connection forms

In short, the group application of LED is an important way of LED practical application. Different LED connection forms are crucial for the convenience 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 convenience of driver design and manufacturing, and the efficiency of the entire circuit.