Trends in flat TV energy efficiency requirements

Over the past few years, there has been dramatic growth in flat TVs. While the majority of the flat TV market has adopted LCD, PDP still represents about 8-10 percent of the flat TV units shipped annually and remains popular for larger screen sizes. As these digital TV technologies took hold, governmental agencies, regulatory bodies, and energy providers became concerned that this shift would drive up home energy consumption as the cost of large TV screens fell considerably. A study within the European Union (EU) estimated that without energy saving improvements, annual TV energy consumption would rise from 60TWh/year in 2007 to 132TWh/year in 2020. Although regulatory bodies had always encouraged energy savings in TVs, the focus had targeted standby (OFF Mode) power. Over the past two years, the spotlight has shifted to reducing active (ON Mode) power consumption.

Effective November 2008, Version 3 of the US ENERGY STAR TV program enacted voluntary active mode energy consumptions based on screen size and resolution. For example a 32in HDTV must consume <120W under a specific set of test conditions. This is only the beginning and over the next two years, new voluntary and mandatory standards are being put in place to drive the reduction in active mode TV power consumption. In a fact in November 2009, California (CEC) became the first state to establish mandatory requirements for new TVs sold within the state. This is significant as unlike ENERGY STAR which is voluntary; this is a mandatory and will be effective in January 2011. This has a big impact as manufacturers would like to be able ship the same product throughout the US market. Within the EU, the EcoDesign Directive (EuP) for TVs has been put in place and will include active mode requirements starting in August 2010. The table below highlights the timelines and key requirements and Figure 1 illustrates how these standards will be phased in over time based on the example of a 42in HDTV.


FIGURE 1

There are a few important things to note about these requirements and the differences in the regulatory approach. While the ENERGY STAR standard is the most aggressive in terms of target, it is also a voluntary requirement that manufacturers strive to achieve. As a result, the EPA who administers this program attempts to set targets that only the top 25 percent of the products on the market will meet, thus giving consumers a clear indication of products that represent significant energy savings. The CEC and EuP requirements are mandatory to the manufacturers. This explains why the CEC requirements are significantly more relaxed than ENERGY STAR. Effectively this established a minimum level of performance that the TV manufacturer must meet to be sold in one of the largest consumer markets within the US.

In looking at the requirements more closely, there is a significant disparity in the EuP requirements, which is related in part to the testing criteria. The EuP does not consider in its test regime the impact of energy savings of automatic brightness control (ABC). LCDs dominate the market for flat digital TVs, and ABC is a feature that can dramatically save power. For large LCD TVs, approximately 75-80 percent of the power budget is for backlighting. The remaining 20-25 percent is used for audio and signal processing, and panel power. By automatically adjusting the backlight to the ambient room lighting conditions, significant energy savings can be achieved. For TVs that have ABC, the ENERGY STAR and CEC average 45 percent of the power consumption of the TV in a darkened room (0lux on the light sensor) with 55 percent of the power consumption in bright room (300lux on the light sensor). Since backlighting is such a significant portion of the power budget, this averaging significantly lowers the power.

Given the important role of backlighting, there has been significant attention focused on improving the energy efficiency along multiple fronts. There are three complementary avenues to address this challenge. First, if the LCD panel and backlight design can be improved to reduce the transmission losses, then more light can pass through the panel and the light generated from the backlight can be reduced. On some panels, less than 5 percent of the light generated from the backlight is emitted from the panel. There has been progress in this area with the development of new brightness enhancing films and innovative LCD structures, and new panels that use these technologies are more efficiency. The second area is improving the efficacy (lumens/W) of the light source. The dominant light sources used now are Cold Cathode Fluorescent Lamps (CCFL) and manufacturers have been improving the efficacy with new lamp designs that generate more light with fewer lamps. Moreover new lamp technologies like Hot Cathode Fluorescent (HCFL) and LED have entered the mainstream. 2009 was the first year that widely available LED backlight TV models entered the market and while they represent a small percentage of the market, as LED efficacy improves and costs are reduced they will capture more market share. For the near term, CCFL continues to dominate. The third area is in the power conversion architecture. In this area, significant progress has been made. CCFL lamps are driven with a high voltage and low current. The historical power conversion process for LCD TVs was to step down the AC line voltage to 24VDC that powered the inverter stage, which would boost this to ~1kVAC to drive the CCFL lamps. This required three power conversion stages. By powering the inverter directly from 400Vdc, one complete conversion stage can be eliminated thus improving overall efficiency and reducing component count and bill-of-material cost. An example of this is illustrated in Figure 2 and illustrates a complete 32in-42in High Voltage LCD and Inverter (HV-LIPS) power supply developed by ON Semiconductor in cooperation with Microsemi.


FIGURE 2

As illustrated, the 85-265VAC is converted by the NCP1606 PFC boost stage to 400VDC. Power factor is required to meet the harmonic content in countries that are covered by IEC61000-3-2, which applies to all TVs above 75W. While power factor is not a strict requirement in the US, TVs over 100W must have a power factor of <0.9 to meet the upcoming CEC requirements. The regulated 400VDC powers a full bridge inverter that drives the lamps. All the backlighting control is handled by the Microsemi LX6503. The NCP1351 flyback stage provides not only the audio and signal processing power, but also acts as a low power standby supply.

In addition to improving the backlighting power conversion efficiency, a critical element that can save overall power is smart backlighting control. To implement this, a sensor is necessary to measure the ambient room light. An example of this type of sensor is the ON Semiconductor NOA1302, which includes an ambient light sensor, A/D converter and control circuitry with I2C interface. With this type of sensor, the system controller can monitor the ambient light and dim the backlight based on the room conditions. One advantage of a smart sensor is that it is designed to be fully calibrated, with a response that is matched closely to the human eye thus providing accurate information to the controller. The addition of ambient light sensing to any TV not only has a big impact on reducing the average active mode power consumption, but also improves the viewing experience since the display can be dynamically changed as the room environment changes.

Beyond backlighting, the other area of power improvement is in the audio and signal processing section. Here the power saving gains are less dramatic, most of the audio amplifiers already use efficient Class D amplifiers. What is changing is the signal processing. With the shift to full HD, more functions are being integrated into deep submicron system-on-a-chip controllers. To achieve this high level of integration, core voltages have dropped while currents have increased. As a result there has been a shift away from linear regulators to power various discrete interface and control blocks to an increased use of high current synchronous rectification buck regulators to deliver high efficiency at low output voltage. Nevertheless as functions are integrated, new functions are added to the TVs to address market needs for connected devices that can access the internet for both data and content so the power budget for the signal processing board does not see all the power reduction benefit of higher levels of integration.

LCD TVs continue to dominate the market and are evolving rapidly. These changes present numerous design challenges based on regulatory changes, new functionality, and new technology advances such as in LED. Moreover, with the trend to thinner and lighter TVs, there is less available space within the chassis, so power densities are increasing, driving the need for higher efficiency power solutions. All these changes will result in the long term with more eco-friendly TVs that meet users’ needs while at the same time saving energy.

Author Information
Bernie Weir is the System Applications and Marketing Manager for ON Semiconductor.

Caption
Figure 1: 42in HDTV active power limits over time.
Figure 2: Complete HV-LIPS Greenpoint LCD TV Reference Design