1. The LED lamps connection arrangement directly affect the backlight performance
There are two main ways of LED backlight technology applied in liquid crystal television: bottom LED backlight source and edge LED backlight source. Although each has its own merits, edge LED backlight module is becoming a major in the market because of its relatively low cost.
The design of LED backlight module not only has to meet the requirement of optical index and minimum of the cost, but also emphasizes the light and thin features of edge LED backlight. Therefore, it is very important to choose the right luminous efficiency and color gamut LED lamp for the backlight of liquid crystal television. The connecting arrangement of LED is generally divided into Serial Connection, Parallel Connection and Serial and Parallel Hybrid Connection. They all have their own advantages and disadvantages.
2. LED Serial Connection
Advantage: Constant current and the brightness of all LED lamps are basically the same.
Disadvantage: If one LED lamp is open circuit, all the other LED lamps will stop working.
The advantage of the LED serial connection is that constant current is in the whole circuit. Although there is a batch of differences and individual differences in the productive process, each LED lamp has a slightly different voltage, the brightness of each LED lamp is basically the same because of the same current. But the disadvantage is obvious that if one LED lamp is disconnected, all the LED lamps will stop working. It requires a relatively high driving output voltage for serial connection. Typically, the closer LED driver output voltage to the total forward voltage in serial connection, the more efficient. But it requires that LED driver output voltage is higher than the total forward voltage in serial connection. Thus, it is less efficient in serial connection.
When a short circuit happens in serial connection, all the LED lamps are still working if they adopt constant current control, and they will stop working if they adopt constant voltage control. If a short circuit happens in one LED lamp in constant voltage control, the voltage of the rest LED lamps rise up, and then they are broken. If a short circuit happens in one LED lamp in constant current control, the current of the rest LED lamps remain the same current and are still working. When one LED lamp is in an open circuit state due to poor quality or other reasons, all the other LED lamps will stop working. The way to solve this problem is very simple which adds a Zener diode in parallel connection in each LED. And it will require extra voltage and increase additional power consumption and cost, so it should not be used in a large number of applications.
3. LED Parallel Connection
Advantage: If one LED lamp is open circuit, all the other LED lamps will be still working.
Disadvantage: It requires a larger amount of current in LED driving output.
When you need to adjust each LED lamp current individually, there will be common anode or common cathode in parallel connection. The advantage of parallel connection is that if one LED lamp is open circuit, all the other LED lamps will be still working. Meanwhile, the disadvantage is that it requires a larger amount of current in LED driving output. Since all the LED lamps have the same voltage, so when LED individual difference is large, the current through each LED and the brightness of each LED will have significant differences. The individual difference is inevitable because of the limitation of LED manufacturing technology. Therefore, due to the inherent shortcoming of parallel connection, some LED lamps which have large current have short using time and will even burn out.
If LED lamps adopt constant voltage driving when one LED lamp is open circuit, the driver circuit output current will be reduced, all the remaining LED lamps can still work. If LED lamps adopt constant current driving when one LED lamp is open circuit, the driver circuit output current will remain, and it will increase the current in the remaining LED lamps, which may damage them. The solution to solve this problem is to increase the LED lamps as much as possible. When one LED lamp is open circuit, the remaining LED lamps may still work despite there is a little bit of incremental current. Therefore, it is better not to choose constant current driving in parallel connection. There is only one special occasion is that when there is a large amount of LED lamps, and current is large, short circuit LED lamps burn out into open circuits, so the remaining LED lamps can be still properly functioning.
4. LED Serial and Parallel Hybrid Connection
Advantage: It has advantages of both serial connection and parallel connection.
Disadvantage: It has a complex design.
If it only adopts serial connection in LED large amount of application, it will require higher output voltage in driving circuit, or if it only adopts parallel connection in LED large amount of application, it will require larger current in the driving circuit. Thus, if it only adopts serial connection or parallel connection, it not only limits the application of LED, but also complicates the design and increases the cost. It usually adopts serial and parallel hybrid connection which has advantages of both serial connection and parallel connection and has two mainly form to solve this problem: first serial connection then parallel connection and first parallel connection then serial connection.
If LED lamps are evenly distributed in serial connection and parallel connection, the voltage, current and illumination of each LED lamp are basically the same. At the same time, every one serial LED lamps have basically the same current. If one LED lamp has a short current, the remaining serial LED lamps will get more current, and it is very easy to burn all out, no matter they adopt constant voltage driving or constant current driving. It is lucky that if they adopt constant voltage driving when one serial LED lamps are burned out and are open circuit, the remaining serial LED lamps are properly functioning. It is unlucky that if they adopt constant current driving, the current of the one serial LED lamps are getting higher, and it is very easy to make all the LED lamps burn out. There is one solution that adding the LED lamps in parallel connection as many as possible. When one LED lamp is open circuit, the current of the remaining LED lamps only gets a little higher, and the remaining LED lamps will be properly functioning.
There is another hybrid connection that all the LED lamps are evenly distributed into every group where LED lamps are parallel connection and then every group connects serially. When one LED lamp is in a short circuit, the whole group LED lamps will stop working, no matter they adopt constant voltage driving or constant current driving. If they adopt constant current driving, since the driver output current is the same in all groups, except one group where one LED lamp is in a short circuit, the remaining LED lamps will be properly functioning. Hypothetically, there are a large number of LED lamps in parallel connection, the driver output current is so large that it burns short current LED lamp to open current. Since there are so many LED lamps, if one LED lamp is open circuit, the remaining LED lamps could still be properly functioning even though the current is getting a little higher. If they adopt constant voltage, there is a great chance that all the remaining LED lamps will burn out when one LED lamp is in a short circuit because of the instantly increasing driving output voltage. But there is a small chance that the one LED lamp which is in a short circuit burn into an open circuit, the remaining LED lamps will still be properly functioning even though the current is getting a little higher.
