New trends on LCD backlighting will change the power-consumption tradeoffs
The energy consumed annually by LCD TVs worldwide is expected to exceed 200 terawatt-hours (TWh) in 2012, increasing to 1200 TWh by 2020, as forecast by Digital CEnergy of Australia. This is almost a ten-fold increase from 2008.
The upside of the LCD TV revolution (lower power consumption, better form factor) have also had some ambiguous downsides. Not only was a CRT TV almost always replaced by a larger LCD screen, but also over time, consumers bought more of them. Today’s household has, on average, about 2.4 LCD TVs as compared to one CRT TV back in 1990. So even though LCD screens consume less power on a unit-area basis, their popularity has actually increased household power consumption. LCDTVs now rank right behind refrigerators as the biggest power guzzlers amongst household appliances.
A slew of new standards are now attempting to curb LCD TV power consumption worldwide. Perhaps the most stringent is EPA’s Energy Star 5.1 rating, which goes into effect May 1, 2012. It is particularly unforgiving on LCD TV screens larger than 50 inches, mandating that they only consume 108 Watts of active mode power regardless of their size. For the first time, Energy Star is moving away from efficiency-based guidelines towards absolute consumption directives.
So how can we keep energy consumption in check? Besides buying small-screen LCDs (something consumers won’t like), the only solution is a sustained, comprehensive overhaul of the LCDTV power system. Incremental power-efficiency improvements seen thus far have been offset by increased functionality – HD, 1080i/p, 240Hz, 3D etc. – which has spawned non-standard power configurations that require additional computation and consume more power.
Over the past decade, we saw dramatic improvements on the image processing side of LCD TVs where innovation and integration of the scaler, de-interlacer, video decoder, analog front end (AFE), demodulator, and even tuners led to unprecedented video-quality and contrast-ratio improvements. To close the gap between video performance and power efficiency, similar innovations are required on the power chain.
However, power-chain advancements have begun. AC/DC power supplies have made considerable progress in recent years, where innovative system designs have shrunk them from over 25 mm to under 10 mm in height. These smaller components fit inside the thinnest LCD TVs. Further improvements in isolation, system efficiency and EMI can help close the gap further.
Transition to LED backlights: edge vs. direct (local dimming)
The other major innovation is in LCD backlighting. Conventional CCFL (Cold Cathode Fluorescent Lighting) backlights are being replaced by thinner, lighter, more energy-efficient light-emitting diodes (LEDs). CCFLs have been the biggest power guzzlers inside LCDTVs, often consuming more than 60 percent of their total power. Besides being mercury-free and consuming less power, LEDs can actually improve video quality.
Initially, the industry adopted edge-lit LCD TVs, where LEDs are placed along the edge of the screen, much like notebooks. Alternatively, direct backlighting (LEDs placed right behind the TFT screen) allows local dimming; meaning individual zones of LEDs can be dimmed or brightened independently. Direct backlighting is gaining traction because it significantly improves black levels, uniformity and viewing angles, and reduces motion blur.
Disadvantages of direct backlighting include the cost of the many LEDs, the power they consume, and the system complexity including the varying forward voltages (Vf ) of the LEDs needing to be driven together, causing significant power loss as heat. Traditional methods of driving direct LEDs have not solved this thermal problem.
Companies such as iWatt have pioneered innovative schemes to overcome the thermal and cost problems, enabling the direct LED backlight market. iWatt’s adaptive switching technology senses the Vf mismatch and makes appropriate adjustments per channel.
Figure 1: LED driver with adaptive switching technology enables cost/power savings in segment and local dimming (Click on image to enlarge)
This reduces wasted power by up to 60 percent, minimizes the heat generated, and enables many more channels to be driven together. The company’s DC-DC LED backlight driver IC, the iW7040, is capable of driving up to 64 channels of LEDs, resulting in bill-of-material (BOM) and power savings over prevailing solutions.
Given that approximately 175M LED backlit LCD TVs could be sold in 2012 alone, the energy savings resulting from their use would be somewhere around 30 TWh. This could power all households in the city of New York for up to 10 months.
About the author
Gyan Tiwary is the Senior Vice President and General
Manager of the DC/DC group at iWatt,
Inc., Los Gatos, CA.