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TV upgrade
Since the British Broadcasting Corporation (BBC) opened the world's first public television broadcast in London in 1936, television technology has made great strides: from the BBC in 1953, the first color TV broadcast, to the Japanese NHK in 1981 The first HDTV system demonstrations, and so on. Currently, countries around the world have taken action to upgrade TV signals from analog transmission to digital systems with higher quality. In the United States, for example, by February 2009, the United States will stop analog TV signal transmission.
However, "the way to sit in front of a large old box (watching TV shows) in the living room has become obsolete. For the TV industry, the development of new technologies is giving birth to endless opportunities." This is a scene described by Newsweek in June 2005. One of the technologies that drive these opportunities is flat panel display (FPD). The technology has the following two distinguishing features:
â— Support HDTV (HDTV) up to 1080p
â— The screen size is larger, but the overall shape is smaller
Power conversion chain for different size flat-panel TVs
A big difference between flat-panel TVs and traditional TVs is that the cathode ray tube (CRT) used in traditional TVs has been replaced by LCD or plasma screens, which in turn is a significant reduction in the thickness of the TV and the size of the body. However, we should pay attention to:
â— Flat-panel TVs consume a relatively high amount of power, and flat-panel TVs of different sizes and combinations of functions consume different amounts of power. Compared to CRT TVs, flat-panel TVs consume much more power per cubic centimeter of size.
â— Traditionally, consumers will place TVs in the living room, and the noise of the TV body will spread, which may cause a problem. If a cooling fan is added to the TV design, it may not be popular with consumers.
â— In the field of Consumer Electronics, the competition is very fierce, and the cost issue is very important. At present, the price of flat-panel TVs is relatively high.
In this case, flat-panel TV manufacturers use different power conversion chains depending on the size of the panel to optimize the design of each TV set.
Small size: up to 21 inches
Flat-panel TVs of this size typically consume less than 70W. This value is lower than the power consumption requirements of most harmonic content standards, so there is no need to use power factor correction (PFC) technology. In this case, a switching power supply (SMPS) is usually used. In normal mode, the switching power supply must output rated power, while in standby mode, the switching power supply must have high energy efficiency.
There are also different ways of dealing with the market: if an external power supply is used, the adapter complies with various standards and codes of conduct. Of course, as an alternative, the power supply can also be embedded inside the TV as an open source. This type of power supply must meet standby energy requirements and be more energy efficient in active mode operation, reducing heat generation in the panel.
In both the internal power supply unit and the external power supply unit, a flyback topology is usually used. The converter can operate at both fixed and variable frequencies (especially in quasi-resonant mode).
To achieve high energy efficiency at both rated load and light load conditions, the key is to use an intelligent switching power supply controller that can adjust the operating mode according to the load conditions.
In response to this situation, a number of semiconductor companies have developed a number of alternatives, taking ON Semiconductor as an example:
â— Skip redundant cycle schemes: fixed frequency such as NCP1200/1216/1271, variable frequency such as NCP1207/1337. The scheme is shown in Figure 1.
Figure 1 Periodic transformation when the current is at a minimum in fixed frequency mode
â— Frequency anti-going scheme: NCP1351
Medium size flat screen TV: between 26" and 32"
For flat-panel TVs of this size panel, the power consumption is greatly increased, up to 180W. Since the input power is higher than 75W, this application should comply with the European Union IEC1000-3-2 Class D standard or similar regional harmonic content standards. Here, power factor correction (PFC) technology is also beginning to be applied; moreover, the main power supply must be optimized to achieve higher energy efficiency and smaller size. Therefore, the active PFC can play a prominent role in limiting the change in the input voltage of the main power supply unit. At this power level, Critical Conduction Mode (CRM) PFC is the most widely used topology. In this regard, ON Semiconductor's NCP1606 offers a cost-effective and reliable solution.
Figure 2 NCP1351 - When the output power is reduced, the switching frequency is also reduced
In flat panel televisions of this size range panel, there are two power conversion chains commonly used.
The first method consists of two power supplies (as shown in Figure 3). One of the switching power supplies uses a flyback topology, which is dedicated to the backlight and provides 24 V@5A output power to the panel. The other switching power supply also uses a flyback topology, which is responsible for controlling the audio and video input and output signals. (CAVIO) board power supply, can provide 40W@12V power (5V under certain conditions). The latter is also used in standby mode, where a variety of rigorous light-load energy efficiency standards can be applied.
Figure 3 26 to 32 inch panel with 2 flyback power supplies
Figure 4 Power structure in a flat-panel TV with a panel size between 26 and 32 inches
The second power conversion chain contains only one main switching power supply, which can supply 24V to the panel and 12V for the CAVIO board. The required power here will be 170W. In addition, it includes another device dedicated to standby mode that provides 10W of power in normal mode and consumes only 500mA in standby.
In order to accommodate higher output power, the topology of the main switching power supply should not be a single-switch flyback, but a two-switch flyback should be used, although a half-bridge resonant LLC is also used in this area. This topology shares a common topology with panels with larger screen sizes.
A significant benefit of this approach is the optimization of standby power consumption, because in this mode, the main switching power supply and PFC functions are turned off.
Of the two methods, the approach of using a dual switching power supply has many advantages:
• Power is better balanced to allow the use of a single-switch flyback converter.
â— Eliminate the worry of interactive voltage regulation during the digital dimming process of the backlight, avoiding the problem of excessive load change in this process.
â— It is easier to remove the PFC level for US/North American product models that do not require IEC harmonic compatibility specifications.
â— Decouple the panel power from the CAVIO power supply. If you need to use different backlight technologies, such as EEFL, FEL and LED, in the future, CAVIO power supply can simplify the evolution process.
Larger flat screen TV: 37 inches
This size LCD TV consumes up to 220W. In this case, PFC technology must be used and active PFC is highly recommended. At this power level, three alternative topologies can be considered, namely critical conduction mode (CRM), fixed frequency discontinuous conduction mode (FF DCM), and continuous conduction mode (CCM).
A fixed frequency discontinuous conduction mode is used in the NCP1605. This mode combines some of the advantages of a critical conduction mode, such as reducing the peak current from the top of the input sinusoidal voltage. It also combines the strengths of a fixed frequency solution, which clamps the switching frequency when the input voltage passes through zero voltage. Bits for better control of EMI signals.
Figure 5 NCP1605 combines the advantages of fixed frequency discontinuous conduction mode and critical conduction mode
A compact 8-pin PFC controller operating in CCM mode has recently been introduced, such as the NCP1653 and NCP1654 devices from ON Semiconductor.
Similar to panels that range in size from 26 to 32 inches, there are two architectures in the 37-inch panel market:
â— Dual switching power supply architecture: One switching power supply is dedicated to the backlight, and the other power supply device is dedicated to the CAVIO board and supports standby mode.
â— Single main switching power supply architecture: The main switching power supply provides 24V and 12V voltage, plus a dedicated standby switching power supply. In standby mode, the main switching power supply is turned off.
While the dual-switch power architecture has significant advantages, designers must consider that light-load performance is becoming more important at power ranges up to 200W because CAVIO's power capacity has increased. Moreover, the power density that can be achieved by using a conventional topology such as flyback is a problem here. Other topologies that improve energy efficiency, reduce size, and improve cross-regulation must be considered. For example, most designers have chosen a half-bridge resonant LLC solution to achieve these performance improvement goals.
Large flat screen TV: 40" and larger
The power consumption of a 40/42-inch LCD TV can be as high as 300W, and the 46-inch is up to 330W. At this power level, the Continuous Conduction Mode (CCM) topology is most suitable for PFC. In addition, at least two switching power supplies are required to meet the power requirements of backlighting and signal processing, as well as to comply with standby power requirements. At this power level, traditional flyback topologies are no longer applicable and designers must consider new topologies such as single/dual switch forward or half bridge topologies. Both of these topologies need to operate in continuous conduction mode, which can lead to hard switching and EMI signal challenges, as well as electromagnetic problems that are undesirable in compact consumer-oriented applications. For LCD TVs with lower power ratings, they typically use a quasi-resonant mode that increases energy efficiency by reducing switching losses. In large-screen flat-panel TVs with higher power ratings, the advantages of using a resonant topology are prominent. This mode leads designers to adopt a half-bridge resonant LLC, which is a member of the resonant converter family. The advantages of the half-bridge resonant LLC are reflected in:
â— Zero voltage switch (ZVS) based on full load range: Switching is switched at zero drain voltage. The power-on loss is therefore close to zero, the EMI signal quality is better than the half-bridge, and the half-bridge topology is operating under hard-switch conditions.
â— Low shutdown current: The switch is turned off at low current conditions, so the turn-off loss is also lower than the half-bridge topology.
â— The secondary diode can be turned off at zero current: When the converter is operating under full load conditions, the output rectifier will turn off at zero current, thus reducing EMI signal problems.
â— No need to increase the number of components: The number of components is basically equivalent to the traditional half-bridge topology.
â— Good cross-regulation: Despite the fact that a single switching power supply device is used to simultaneously provide 24V to the panel and 12V to the CAVIO board, the digital dimming of the backlight does not interfere with the modulation of the two output voltages.
At this power level, the most common power conversion chain consists of a main switching power supply and a standby dedicated switching power supply. The main switching power supply uses a half-bridge resonant topology that can simultaneously output 24V and 12V.
Figure 6 shows the structure of the resonant converter. A 50% duty cycle half-bridge provides a high voltage square wave that swings from zero to the input voltage VIN to the resonant circuit. By using a voltage controlled oscillator (VCO) to regulate the frequency, the feedback loop can adjust the output level based on power requirements.
Figure 6 Schematic diagram of the resonant converter
Figure 7 is a supplementary diagram of the LLC resonant converter
Figure 8 Half-bridge LLC waveform
The resonant circuit consists of a capacitor Cs and two inductors Ls and Lm connected in series. The Lm inductor represents the transformer magnetizing inductance, which together with Ls and Cs constitutes a resonance point. The reflection from the load on this inductor will either cause the resonance point to disappear from the circuit (in the case of large load currents, Lm will be completely shorted by the reflected resistance load resistor RL), or it will cause it Continue in series with the series inductor Ls under light load conditions. As a result, the resonant frequency varies between minimum and maximum depending on the load conditions.
The operating frequency depends on the power requirements. Under low power conditions, the operating frequency is quite high and is quite far from the resonance point. However, under high power conditions, the control loop reduces the switching frequency and uses one of the resonant frequencies to supply the necessary current to the load.
in conclusion
The design of the flat-panel TV power conversion chain requires consideration of many challenges and conflicting design compromises in order to design a cost-effective, energy-efficient, compact and slim solution that does not require active cooling. In addition, in order to meet the needs of different consumers, flat panel TV manufacturers need to provide a large number of different functional combinations, while requiring no redesign of each power supply. System design and IC manufacturers have collaborated to find the best combination of design compromises. Today, we need to focus on developing the next generation of LCD TVs, enabling the panel backlight subsystem to be powered directly from the power factor conversion section.
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