The configuration of LEDs in an application depends on a number of factors, including application requirements, LED parameters and quantities, input voltage, efficiency, thermal management, size and layout constraints, and optics. The simplest configuration is a single LED. There are many applications in this design, such as car interior ceiling lights (map lights, reading lights).
Since the power and brightness of LEDs are not very high at present, and the actual use occasions usually require planar illumination, it is necessary to arrange multiple LEDs as needed to meet the requirements of larger range, higher brightness, dynamic display, color conversion, etc. Requirements and matching requirements between LEDs and associated drivers.
First, the common form of connection
1, the overall serial form
(1) Simple series connection In the generally simple series connection form, LED1~LEDn are connected end to end, and the current flowing through the LED is equal. For the same-size and batch LEDs, although the voltage on a single LED may have a slight difference, since the LEDs are current-type devices, it is possible to ensure that the respective luminous intensities are consistent, which is a simple series connection. The LED has the characteristics of simple circuit and convenient connection. However, due to the series connection, when one of the LEDs has an open circuit failure, the entire LED string will be extinguished, which affects the reliability of use.
(2) Series form with parallel Zener diodes
An improved series connection of a Zener diode in parallel with each LED is shown in Figure 2. In this connection, the breakdown voltage of each Zener diode is higher than the operating voltage of the LED. When the LED is working normally, because the Zener diodes VD1~VDn are not conducting, the current mainly flows through the LED1~LEDn. When the LED string is damaged by the LED, the VD1~VDn is turned on, except In addition to the faulty LED, other LEDs still have current passing through it. This connection is compared with the simple series form shown in Figure 1 and is greatly improved in terms of reliability.
2, the overall parallel form
(1) Simple parallel connection The LED1~LEDn in the simple parallel form are connected in parallel at the beginning and the end, and each LED is subjected to the same voltage during operation. It can be seen from the characteristics of the 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 limitation of LED manufacturing technology, even the same batch of LEDs, the difference in performance is inherent, so when LED1~LEDn is working, the current of 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. This connection is simpler. However, the reliability is not high, especially for applications where the number of LEDs is large, which is more likely to cause malfunctions.
(2) Independently matched parallel form
For the reliability problem existing in simple parallel, the independent matching parallel form is a good way. Each LED in this mode has its own current adjustability (driver V+ output is L1~Ln, respectively), ensuring that the current flowing through each LED is within its required range, with a good driving effect, a single LED When the protection is complete and the fault does not affect other LEDs, it can match the characteristics of 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 occupied volume is too large, which is not suitable for a large number of LED circuits.
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