Industry challenges
â— Energy saving problems of the electrical appliances themselves
In the next few years, the number of global CRT TVs is expected to be flat, while the digital TV (DTV) market segment is expected to grow at a compound annual growth rate of 30%, reaching 94 million units in 2009. Flat-panel TVs are based on two technologies : LCD TVs (about 58 million units in 2009) and plasma TVs (about 12 million units in 2009). This article will focus on LCD TVs that account for the largest share of the flat-panel TV market.
Today's digital TV screens are getting bigger and bigger. The current highest output is the 32-inch LCD TV, and the largest size is 61 inches. As the large screen of the TV requires more power, the power consumption of the TV product has soared.
Current plans for TV power consumption specifications such as "Energy Star" focus on standby energy consumption, that is, standby power consumption must be less than 1W from July 1, 2005. Looking ahead, the focus will shift to a higher goal of saving energy in normal operation. E-Star has been finalized to limit the power consumption of external power supplies (such as AC adapters) under normal operating conditions, as well as the initial efficiency of the computer. In addition, the E-Star program has also begun to address the power consumption limitations of digital TV during normal operation.
â— Effective use of the power of the AC grid
Another aspect of saving energy is power factor correction (PFC). The specification of PFC is related to the quality of utilization of AC grid power. The best condition for power transmission from the AC grid is when the current drawn by the electrical load (such as a TV) is the same as the phase of the input voltage, and the current has no waveform distortion (sinusoidal waveform). In this regard, IEC 6100-3-2 version 2.2 is currently a European standard that specifies harmonic distortion specifications for different levels of equipment. In particular, the use of personal computers, monitors, and televisions (Class D equipment) of 75 W or more, the harmonic current regulations must be lower than the limits shown in Figure 1. This means that a 40-inch LCD TV that easily consumes 250W can meet this standard if it is sold in Europe. After the standard is adopted in Europe, the rest of the world will follow suit.
In the European specification, the higher the order of harmonics, the stricter the tolerance requirements; however, the energy storage of higher harmonics is usually lower (intensity), so it is easier to filter out. According to this specification, the maximum allowable harmonic current exceeds 600A, making it more difficult to achieve this requirement at high power.
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solution
There was a major TV manufacturer that asked Fairchild to help design a 40-inch digital TV power system. The design challenge is due to the extremely thin size of the TV set. The power supply system must be small in size but high in efficiency. In normal operation, the efficiency needs to be 85%. The standby power consumption must meet the requirements of “Energy Star†less than 1W. .
A typical 40-inch LCD TV typically requires several AC/DC isolated outputs to convert 90V to 254V AC to DC. Typically, output 1 (24V/8A, 192W) is used to drive the backlight inverter module of the LCD display, output 2 (5V/0.5A) is used to drive the logic control module, and output 3 (5V/2A) is used to drive the tuning module. Output 4 (12V/3A) is used to drive the audio amplifier module. In this way, the total power consumption of the device is 243W and the standby power consumption is less than 1W.
In order to meet the requirements of normal operation and standby power consumption, our strategy is to divide the entire power system into two subsystems: one is the main power subsystem that supports normal operation, and the other is the auxiliary power subsystem that supports standby (see Figure 1). ).
â— Standby status
In standby mode, the main power supply is disconnected from the AC grid. We used a relay and a power supply built only by Fairchild's FSDM311 SMPS power switch to power the logic module. The FSDM311 uses high-voltage start-up switching technology and advanced burst mode operation to reduce switching losses, resulting in low power consumption.
â— Normal operation status
Under normal operating conditions, the device's main power supply is built by Fairchild's FAN4800 PFC/PWM single-chip combination control IC, which uses ZVS (Zero Voltage Switching) and ASHB (Asymmetric Half-Bridge) circuit structures and uses FAN7382 as High-side MOSFET driver circuit for high efficiency, patented LEM (Pulse Front Edge Modulation) PFC/TEM (Pulse Edge Modulation) PWM synchronization technology to reduce the ripple current of the storage capacitor between the PFC and PWM sections. As a result, the device is more efficient and smaller. Thanks to these technologies, the device's overall efficiency at 85Vac is over 85% (including the PFC part), so it can meet customer requirements.
â— PFC
The FAN4800 features a high performance average current mode PFC control module that meets PFC specifications. The device also has an additional feature that its current gain modulator has excellent noise immunity.
in conclusion
The development of large-size flat-panel TVs has brought new challenges to power management and energy conservation. As of now, many regulatory agencies have focused on energy-saving specifications in standby mode, or have just begun to deal with energy-saving issues in normal operating mode. From an energy-saving point of view, for LCD TVs and plasma TVs, due to the limited size and heat generated, the market is forced to require efficient solutions rather than being driven by any existing specifications. The application of advanced architecture and technology, such as the combination of ZVS and pulse front/edge modulation techniques in normal operation mode and standby burst mode, is an integral part of meeting the requirements of modern electrical appliances for energy conservation.
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