Effect of LED connection arrangement on backlight performance

LED backlight technology LCD TV from the technical point of view, there are mainly two ways of side-entry and direct-down. Although the two technologies have their own advantages, from the perspective of cost and other aspects, the side-guide LED backlight module is becoming a mainstream trend.

The design of the side-guided LED backlight module should consider reducing the cost on the basis of satisfying the optical index, and at the same time, it also reflects the light and thin characteristics of the side-guide LED backlight. Therefore, the luminous efficiency and color gamut of LEDs have an important influence on the selection of LEDs for LCD TV backlights. The connection arrangement of LEDs is generally divided into three main structures, namely, series, parallel, and serial and parallel hybrid methods, each of which has advantages and disadvantages.

LED series

Advantages: the current is everywhere, the brightness of the LED is basically the same

Disadvantages: one LED is open and all LEDs are not working

The advantage of the LED series connection is that the LED currents are equal everywhere in the series circuit, because there are batch differences and individual differences in the LED production process. When the LEDs have different consistency, although they are added in different LEDs, The terminal voltage is different, but since the current through each LED is the same, the brightness of the LED is basically the same. However, the series connection of LEDs also has significant drawbacks. If one of the LEDs is open, then all of the LEDs will not work. The use of LEDs in series requires the LED driver to output a relatively high voltage. Generally, the closer the LED driver output voltage is to the total forward voltage in the series, the higher the efficiency of LED driving, but the LED of the series mode requires that the LED voltage output by the driving circuit must be greater than the total LED forward voltage in the series circuit. A slight impact on system efficiency.

It is worth noting that there is no problem with constant current control when a short circuit occurs in series, and constant voltage control will stop working. When an open circuit occurs in series, both constant current control and constant voltage control will cause problems, which is a problem that must be paid attention to when designing the drive circuit. When a certain LED is short-circuited due to poor quality or other reasons, if the constant voltage source is used, the voltage across the remaining LEDs will increase and the output current of the driver will increase due to the constant output voltage of the driver. Large, this can cause damage to all remaining LEDs. If a constant current LED driver is used, when a certain LED is poor in quality, all the remaining LEDs in the series circuit will work normally because the output current of the driver remains unchanged. When an LED is open due to poor quality or other faults, the LEDs connected in series will not light up. An easy way to solve this problem is to connect a Zener diode in parallel with each LED. Of course, the parallel Zener diode breakdown voltage needs to be higher than the LED's turn-on voltage, otherwise the LED will not light up. This solution will add additional power consumption and cost, so it is not suitable for a large number of LED applications.

LED parallel

Advantages: an LED has an open state that does not affect other LEDs.

Disadvantages: require LED driver to output large current

When it is necessary to separately adjust the current of each LED, a common anode or a common cathode connection method is used. The advantage of the parallel mode is that when an LED is in an open state, it does not affect the operation of other LEDs. The disadvantage is that the LED driver is required to output a large current. Since the voltages across all LEDs are the same in parallel, when the consistency of the LEDs is large, the current through each LED will be inconsistent, and the brightness of the LEDs will be significantly different. This difference is inevitable due to limitations in LED manufacturing technology. Therefore, when the LED is in operation, the inaccuracy of each LED current distribution due to the inherent shortcoming of the parallel mode can cause the LED life of the excessive current to be sharply reduced or even burned out.

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