This article explains the latest steps taken by major notebook makers and LCD panel makers to make the 'green' benefits of LED backlight (LED BLU: Light Emitting Diode BackLight Unit) technology obvious to final customers. The article will analyze both technical details of the LED BLU that give real improvements to energy saving, and the way international standards for power measurements of the complete application (LED BLU + LCD panel + software) are going to be set to make these benefits evident for the final market.
Why the change from CCFL to LED?
The major benefits of replacing Cold Cathode Fluorescent Lamp (CCFL) with LED BLU are lower power consumption, thinner and lighter panels, a wider color gamut, longer BLU life and the possibility to be RoHS compliant.
LED backlight for notebook PC LCD panels is a technology that enables energy saving compared to traditional backlighting techniques such as CCFL. This is true both for the decrease of power consumption and for the elimination of chemicals used in CCFLs.
The CCFL bulb contains hazardous chemical substances, such as mercury, that need proper and complicated disposal once the notebook with CCFL inside requires dismantling. As LEDs are based on a simple silicon chip with plastic package and metal pads, they do not have this kind of problem and they represent a 'green' solution to the issue.
Power Consumption Comparison
The CCFL needs a high voltage power supply to work: the driving voltage is 1400-2000 V when the bulb strikes, then it settles to 600-800 V. As a consequence, the required application components have to be rated for high voltage operating range, and a High Voltage Transformer is also needed to provide the required power to the CCFL.
Compared to the LED BLU, the CCFL component's size, in particular the transformer, is considerably larger, the cost is high and the design phase more difficult.
Moreover, the CCFL power consumption is much greater than the LED BLU. Any improvement achieved on the LCD panel backlight power consumption is reflected in the notebook battery life.
Considering a numerical example of a 14.1 inch notebook with CCFL backlight, the LCD Display power consumption PLCD represents more than 30 percent of the entire notebook power consumption PNB, as shown in Figure 1. Reducing PLCD = 5 W by a factor of 30 percent means saving 1.5 W of power. This represents 10 percent of the PNB.
The typical battery life of a 14.1 inch notebook is around three hours. The energy savings due to the utilization of an LED BLU instead of a CCFL enable increased battery life of 20 minutes.
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Figure 1: Power consumption breakdown of an entire notebook
LED BLU Configuration
The reduction of the display size (mainly the thickness) and the decrease in the boards' assembly and mounting phase timings are due to the mechanical differences between the CCFL and the LEDs board. The CCFL required space in the panel is bigger, so it is the panel optical part which guides the light from the CCFL to the display. The LEDs board is more robust and the optical light guides can be smaller.
The required application components, such as inductors and capacitors, for the LED BLU are 'standard' ones, not high voltage rated (such as the High Voltage Transformer of the CCFL). This allows the cost of the application BOM (Bill Of Materials) to be reduced while also achieving a compact area in the PCB (Printed Circuit Board). The design of an LED BLU is also easier than in the case of CCFL BLUs.
The greater benefit of LED BLU to the environment from an electrical point of view is the reduction in the notebook power consumption. This is related to the intrinsic LED driving technology, and the fact that the use of WLED (White LEDs) versus CCFL provide higher electrical efficiency in the silicon driver IC side.
The LEDs are electrically disposed in a 'matrix' mode. For this article we are taking into consideration a typical LED driver IC configuration, using the ST PM6600 and PM6602 devices as reference. They are designed as a monolithic boost converter with integrated n current generators (n=6 for the PM6600 and n=8 for the PM6602) that sink a set current from the LEDs strings.
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Figure 2: A typical LED driver IC configuration
