The power supply design in consumer electronics products always brings serious challenges to the realization of appearance, cost and efficiency indicators. The TV market is a good example. It is undergoing a tremendous shift from a bloated, CRT-based solution to a flat-screen TV using liquid crystal displays (LCD) and plasma displays. LCD currently dominates the flat-panel TV market, and its expected sales in 2008 will exceed 100 million units. At the same time, content has shifted from analog to digital, and new features have increased dramatically, such as multi-tuners for picture-in-picture, full HD picture quality (1080p), enhanced audio, and even Internet access. Energy-saving standards Historically, energy-saving standards for consumer electronics products, such as the US Energy Star and the EU Ecolabel, have focused primarily on the impact of static power consumption. However, with the transition to flat-panel TVs, people's concerns have expanded to work (active mode) power consumption. Figure 1 Common 32-inch LCD TV switching power supply The actual power topology with the increase in screen area, the power required by the 24V DC voltage rail will continue to increase until the flyback topology can no longer be used to implement switching power supplies. Therefore, many power-intensive topologies, including half-bridge dual inductance plus single capacitor (LLC) topologies, have been considered for high efficiency operation with low electromagnetic interference in a compact space. Figure 2 Simple LLC half-bridge power stage Figure 3 Block diagram of a complete 220W LCD TV power supply
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In addition, the screen size limitations of CRT will no longer exist. Currently, 32-inch widescreen TVs are the most popular, followed by 40-42-inch TVs. Although the adoption of new technologies has promoted the growth of sales and improved the viewing experience for TV users, it has also caused a significant increase in power consumption.
The actual test shows that the power consumption of the 42-inch flat-screen TV is between 180 and 500W, and the specific value is related to the technology (LCD or plasma display), feature set and design choice. In comparison, the power consumption of a 29-inch CRT TV is about 100W.
Obviously, part of the increased power consumption is directly related to the increase in screen size, but this is not the only reason. For example, LCD TV displays require a backlight subsystem; this part of the power consumption cannot be ignored and is directly related to the screen size. The increase in average power, coupled with the increase in hours of use caused by activities such as games, watching music programs, web browsing, and home theater, has increased household electricity consumption.
Norms and government agencies are addressing this shift. The ENERGY STAR standard for TVs has just been revised and version 3.0 has been launched (since November 2008), which will include operating power limits. These limits are based on screen area and resolution (high-definition or standard-definition), regardless of display technology (LCD, plasma, or rear projection).
Calculating power limit According to the screen size and resolution, there are various algorithms to calculate the power limit. For example, for a high-definition TV with a screen area in the range of 680 to 1068 square inches (4387 to 6890 cm2), the formula is as follows:
Pmax = (0.240 × Area + 27) W
Note: The unit of area is square inches.
Therefore, the operating power limit of a 32-inch HDTV is 120W, while the limit of a 42-inch HDTV is 208W. These limits are based on actual testing of multiple products from multiple vendors. In the test sample, 27.4% passed the target working and static power requirements. The static power consumption requirement remains the same: it has been the maximum limit of 1W since July 2005.
A typical power supply for a 32-inch LCD TV (see Figure 1) generates several voltage rails to power various system modules, such as audio, backlight, and signal processing modules. The main power supply does not provide all the required voltage values. Instead, the various low-voltage rails are provided by local linear and DC / DC converters. 
There may be five or more linear or low-dropout voltage regulators on the signal processing board, and there are several buck converters used to generate low-voltage power rails for deep sub-micron digital signal processing modules. As shown in the figure, it is quite common for manufacturers to use a unified power supply that supports 90 to 265V AC voltage. This approach makes it possible to use a single voltage design for a series of modules in different regions according to the size of the TV, simplifying logistics and reducing development costs.
If the LCD TV is for the global market and the power exceeds 75W, then this TV must comply with the European standard IEC61000-3-2 for harmonic reduction. In this case, the working power factor control stage needs to be used.
Backlight power For TVs larger than 26 inches, backlight is the largest part of power consumption. The 24V voltage rail powers the inverter stage, which drives the backlight cold cathode fluorescent lamp (CCFL). The inverter converts the 24V DC voltage into a high-voltage, low-current AC signal, which is used to start and drive these fluorescent lamps.
The structure of the two main inverter units is very common: one is used to power the backlight inverter, and the other is used to control audio / video and signal processing. Historically, the power levels of switching power supply stages have been based on single-switch quasi-resonant (QR) or fixed-frequency pulse-width modulation (PWM) flyback topologies. Depending on the TV's feature set and the different power requirements for the 12V and 5V voltage rails, there may be a dedicated switching power supply in the circuit to provide static power and meet the 1W static power limit.
The half-bridge LLC topology is regarded as a series resonant converter. As shown in Figure 2, LLC refers to the inductance-inductance-capacitance structure. The first inductor is connected in series in the circuit, the transformer represents the second inductor, and the capacitor is connected to the output of the transformer. 
The basic idea of ​​this method is: the half-bridge field effect transistor (FET) is driven by a 50% duty cycle waveform, and the power is adjusted by changing the frequency. Usually, the purpose of this design is to make the switching frequency exceed the resonance frequency of the circuit.
In this area, the current delays the voltage through the switch, so the switch is turned on in the zero-voltage switching area, which almost eliminates capacitive switching losses. Because this is a resonant topology, its efficiency is very high over a wide voltage range.
Figure 3 shows an example of a complete power supply based on a half-bridge LLC structure. In this example, the HB-LLC stage produces multiple outputs. This design is part of a series of Greenpoint reference designs developed by ON Semiconductor and demonstrates high-efficiency power supply topologies. In this example, for a 115 or 230V AC main power supply, the overall efficiency is greater than 88% in the range of 90 to 220W. 
In addition to achieving high overall efficiency, the shape of this power supply is also small, only 25mm high. For flat-panel TVs, the height of the power supply is very important, because it will affect the total thickness of the TV. There is growing interest in designing ultra-thin flat-screen TVs that can be easily hung on the wall. This trend poses a further challenge to the power supply because the volume needs to be reduced, and the airflow through the power supply may be more restricted.
The challenge of designing high-density, high-efficiency power supplies is driving power designers to adopt innovative power architectures to support these rapidly evolving consumer applications. Half-bridge resonant LLC can meet the efficiency and volume targets required by flat-panel TVs, while still providing low-cost solutions for consumer electronics.
With the introduction of new working power standards and consumers gradually realizing the energy consumption brought about by switching to large-screen digital TVs, attention to efficient energy-saving solutions will increase day by day. This not only requires switching to an efficient power supply scheme, but also requires the creation of a new system architecture to reduce the power consumption of signal processing and LCD panels.
