The move from CRT to flat-panel TVs means an increase in energy costs and will shift the focus to more energy-efficient solutions
BY BERNIE WEIR
ON Semiconductor
Phoenix, AZ
http://www.onsemi.com
Power supply design for consumer electronics has always posed difficult challenges in achieving form factor, cost, and efficiency targets. The television market is a case in point. It is experiencing a dramatic shift away from bulky, CRT-based solutions to flat-panel TVs that use LCD and plasma displays. LCDs now dominate the flat-panel TV segment, with sales forecasts of more than 100 million units in 2008. At the same time, content has shifted from analog to digital and new features have proliferated, such as multiple tuners for picture-in-picture, full high definition (1080p), enhanced audio, even Internet access.
Moreover, the screen-size limits imposed by CRTs have disappeared. The 32-in. wide-screen TV is now the most popular, with the 40 to 42-in. range running a close second. While the adoption of new technologies has fueled growth and improved the TV user’s viewing experience, it also has resulted in a dramatic increase in power consumption.
Energy standards
Historically, energy-saving standards for consumer electronics, such as the U.S. Energy Star and European Union Eco-label, have focused on the impact of standby power. With the shift to flat-panel TVs, however, attention has expanded to operating (active mode) consumption.
Real-world power consumption testing of 42-in. flat-panel TVs shows power ranging from 180 to 500 W, depending on the technology (LCD or plasma), feature set and design choices. This compares with approximately 100 W for a 29-in. CRT TV.
Clearly, some of this increase relates directly to increased screen area, but that does not tell the whole story. For example, LCD TV panels require a backlighting subsystem; the power consumption is significant and relates directly to screen size. This increase in average power, combined with more hours of usage for activities like gaming, music viewing, Web browsing and home theater, is driving up home energy use.
Regulatory and government agencies are working to address this shift. The Energy Star standard for TVs has just been revised to Version 3.0 (effective November 2008), and will now include active power limits. These limits are display technology neutral (LCD, plasma or rear projection), and are based on screen area and resolution (high or standard definition).
Calculating power limits
There are several algorithms to calculate the limit, depending on screen size and resolution. For example, the equation for high-definition TVs in the range of 680 to 1,068 in.2 – (4387 to 6,890 cm2 ) is as follows:
Pmax = (0.240* Area + 27) W Note: Area is in units of in.2
Therefore, a 32-in. HDTV would have an active power limit of 120 W, while a 42-in. HDTV would have a limit of 208 W. These limits are based on real-world testing of a variety of products from multiple manufacturers. For samples tested, 27.4% passed the target active and standby requirements. The standby power requirement has not changed; a maximum limit of 1 W has been in place since July 2005.
A typical 32-in. LCD-TV power supply for LCD-TV (see Fig. 1 ) generates several voltage rails to power the various system blocks such as audio, backlighting and signal processing. The main power supply does not generate all the voltages required within the set; instead, local linear and dc/dc converters are used to provide various low-voltage rails.
There may be five or more linear or low dropout regulators on the signal processing board and several buck converters to generate the low-voltage power rails for the deep submicron digital signal processing block. As illustrated, it is fairly typical for manufacturers to use a universal power supply that supports 90 to 265 Vac. This allows use of a single power supply design based on TV size for a series of models for different regions, which simplifies logistics and reduces development cost.
If the LCD TV is for global use and the power exceeds 75 W, the TV must comply with IEC 61000-3-2, the European standard for harmonic reduction. In this case, an active power-factor-control stage is used.
Backlight power
Backlighting is the largest consumer of power for TVs greater than 26 in. The 24-V rail powers the inverter stage that drives the backlight cold-cathode fluorescent lamps (CCFL). The inverter function transforms the 24 Vdc into a high-voltage, low-current ac signal used to strike and drive the lamps.
It is common to have two main converter blocks: one to power the backlight inverter and the other for control audio/video and signal processing. Historically, the power level of the SMPS stages is based on a single switch quasi-resonant (QR) or fixed-frequency PWM flyback topology. Depending on the feature set of the TV and power required for the 12- and 5-V rails, there may be a dedicated SMPS that provides standby power as well to meet the 1-W limit standby requirement.
Fig. 1. Typical 32-in. LCD-TV SMPS power supply.
Practical power topologies
As the screen area increases, the power required for the 24-Vdc rail continues to rise until it is no longer practical to implement the SMPS using a flyback topology. As a result, a variety of higher-power topologies, including the half bridge LLC topology, has been considered to achieve high efficiency in a compact space with low-EMI generation.
The half-bridge LLC topology is considered a series resonant converter. As Fig. 2 illustrates, the LLC refers to an inductor- inductor-capacitor configuration. The first inductor is in a series configuration, with the transformer representing the second inductor, and the capacitor is at the output of the transformer.
The basic concept for this approach is that the half-bridge FETs are driven by a 50% duty-cycle waveform, and power is adjusted by varying the frequency. Normally, this is designed so that the switching frequency is above the resonant frequency of the circuit.
In that region, the current lags the voltage through the switch, so the switches are turned on in the zero-voltage switching region, which practically eliminates capacitive switching losses. Because this is a resonant mode topology, it is highly efficient over a broad power range.
Fig.2. Basic LLC half-bridge power stage.
Figure 3 shows an example of a complete power supply based on the half bridge LLC. In this example, multiple outputs are generated from the HB-LLC stage This design is part of a series of Greenpoint reference designs developed by ON Semiconductor that demonstrates efficient power supply topologies. In this example, the overall efficiency is greater than 88% from 90 to 220 W for both 115- and 230-Vac mains.
Fig. 3. Block diagram of complete 220-W LCD-TV power supply.
In addition to achieving high overall efficiency, this supply is designed to have a low profile of 25 mm. For flat-panel TVs, power supply height is important because it contributes to the overall TV thickness. There is growing interest in the design of very thin flat-panel TVs that can easily be mounted on a wall. This trend poses further challenges to the power supply as cabinet volume is reduced and air flow across the power supply may be further limited.
The challenge of designing high-density, high-efficiency power supplies is driving power designers to adopt innovative power architectures to support these fast-growing consumer applications. The half-bridge resonant LLC can achieve the efficiency and space targets required for flat-panel TVs, while still offering a cost-effective solution necessary for consumer electronics.
As new active-power standards come on board and consumers become aware of the ongoing energy cost to transition to large-screen digital TVs, the focus on energy-efficient solutions will increase. This involves not only migrating to more efficient power solutions, but creating new system architectures that reduce the power of the signal processing and the LCD panels. ■
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