New LED driving techniques reduce power in LCD TVs
Peter Rust, Werner Schögler, Manfred Pauritsch, Herbert Truppe, ams - February 12, 2013
According to the SEAD (Super-efficient Equipment and Appliance Deployment) initiative, televisions are responsible for approximately 3 to 8% of global residential electricity consumption. An analysis conducted by Lawrence Berkeley National Laboratory suggests that advances such as more efficient LED driving can yield major reductions in television electricity consumption in the coming years.
There seems little doubt that LCD technology with LED backlighting is the only viable way to reach the efficiency targets that authorities are proposing. Plasma has the disadvantage that each pixel is an active light emitter, so power consumption is directly proportional to the number of pixels. As a result, HD plasma televisions consume around two to three times the power of an LCD display for the same resolution & brightness.Highly touted OLED technology - as recently reported - may not come any time soon, if ever. The investment required for this “bleeding edge” large panel technology is prohibitive. However, large display panels with current state-of-the-art TFT-LCD technology and “smart” direct LED backlighting with local dimming is far less expensive than OLED but compares well for both power consumption and picture quality.
But today's LCD TVs, even those with LED backlighting, are still some distance from achieving the efficiency targets they will face in the coming years. However, new design techniques in LED driver circuits promise to deliver significant energy savings that will go a long way to helping TV manufacturers meet the tough requirements for power consumption.
Changing requirements of TV power standards
Standards for TV power consumption, such as Energy Star, came out in 2008 and each year the specification reduces the amount of power the TV can draw. The design challenge gets even tougher for large screen TVs since the current maximum for any size screen is 85 Watts.
Energy Star is voluntary but highly influential but it's not the only form of regulation. For instance, the State of California's Energy Commission introduced its own standard. This regulation is a bit tougher than Energy Star and also has real teeth – it prohibits the sale of TVs in California that do not meet its efficiency specifications.
In Europe, regulations have for many years allowed direct comparison of the energy consumption of white goods (EU Energy Label) and customers use it as a basis for purchasing decisions. These regulations are now mandatory for TVs, cars etc.
The operation of LED backlighting
Since LED backlighting power ranges from 30% to 70% of overall system power in LCD TVs, improvements in the efficiency of the backlighting power circuit can make a considerable contribution to system efficiency. As is often the case in power system design, a number of small improvements in efficiency can deliver a large combined saving.
There are two ways to implement LED backlighting (see figure 1). In indirect or edge-lit backlighting, the LEDs are arranged at the edges of the screen. A light guide distributes the light uniformly across the display. This arrangement can be deployed with good optical uniformity in screen sizes up to 60”, and enables backlighting units with thickness of just 5-10 mm.
In direct backlit systems, the LEDs are located directly behind the LCD, enabling low power, good thermal design and excellent scalability with practically no limit to the screen size. These panels tend to be thicker than edge-lit versions, but with the latest technologies for light distribution, displays as thin as 8 mm can now be found. An important advantage of direct backlighting is that it enables sophisticated local dimming, which lowers power consumption and increases the dynamic contrast ratio, allowing the latest TV designs to compare favorably with OLED.