Large-size LCD TV ACDC power supply architecture and the latest LED backlight solution

The LCD TV market has been growing rapidly in recent years. Market research firm DisplaySearch predicts a compound annual growth rate (CAGR) of 16% between 2008 and 2012, with total shipments expected to reach 140 million units in 2009 and to exceed 200 million by 2012.

As the market scale expands rapidly, under the pressure of standards and green marketing, the working and standby energy consumption of LCD TVs is also getting lower and lower. For example, the Energy Star 4.0 and 5.0 specifications will take effect in May 2010 and May 2012 respectively. The 5.0 version requires that the average working energy consumption of flat-panel TVs with visible screen diagonal sizes of 32 inches, 42 inches and 60 inches be reduced from no more than 78 W, 115 W and 210 W in the 4.0 version to no more than 55 W, 81 W and 108 W, which is equivalent to reducing energy consumption by about 50% in two years.

Statistics show that the proportion of electricity consumed by backlight in the total energy consumption of LCD TVs is as high as 2/3. Therefore, another important trend of LCD TVs is to use new technologies to improve the energy efficiency of backlight and panel and reduce energy consumption. In terms of LCD TV backlight, cold cathode fluorescent lamp (CCFL) backlight currently occupies a dominant position, but this technology has high energy consumption, contains highly toxic mercury, which is not conducive to environmental protection, and the lamp tube is strip-shaped or U-shaped, with a short service life; in comparison, the emerging light-emitting diode (LED) backlight consumes less power, does not contain mercury, is easier to configure for a more uniform backlight, and has a longer service life, so LED backlight has great application potential in LCD TVs. According to statistics, the proportion of LCD TVs using LED backlight in 2009 was 3%, and it is expected that this proportion will increase rapidly in the next few years, reaching 50% by 2014, which can completely compete with CCFL backlight and further replace CCFL backlight thereafter.

LCD TV power supply includes multiple components such as power factor correction (PFC), signal power supply, backlight power supply and ECO standby. Different parts often have different architectures to choose from, see Figure 1. Taking the PFC section as an example, different PFC controllers such as critical conduction mode (CrM), continuous conduction mode (CCM) or interleaved frequency clamped critical conduction mode (FCCrM) can be selected. The choice of power supply architecture needs to be combined with factors such as the screen size, power level and backlight source of the application. In addition, the slim design, which is an important selling point, also affects the choice of power supply solution.

Figure 1: LCD TV power supply architectures vary

1) Power architecture for 32-inch LCD TVs using CCFL backlight

For example, the 32-inch LCD TV has the largest sales volume in the market. Among these LCD TVs, the solution using the standard 24 V inverter power supply still occupies the majority in the market, but the application of the emerging high-voltage LCD TV integrated power supply (HV-LIPS) is also increasing, and the overall situation is coexisting. After receiving the 110/220 Vac mains input, the traditional LCD TV using the standard 24 V inverter is converted into a DC high voltage of 200/400 Vdc through rectification, PFC and filtering. Since the input voltage requirement of the traditional high-voltage inverter is 24 Vdc, the output voltage of the PFC 200/400 Vdc will be buck-converted to generate multiple output voltages, one of which is 24 Vdc voltage provided to the high-voltage inverter, and then converted to an AC high voltage of more than 1,000 V or even 2,000 V through DC-AC to drive the CCFL array of the LCD panel. ON Semiconductor has launched a 220 W LCD TV power supply GreenPoint® reference design for this type of traditional LCD TV. The key components used include the NCP1606 CrM PFC controller, the NCP1396 resonant half-bridge controller and the NCP1027 standby controller.

Unlike traditional power supplies that use a standard 24 V inverter architecture, the high-voltage LIPS solution combines AC-DC, DC-DC and inverter on the same circuit board. After obtaining a 200/400 Vdc PFC output, this output voltage is directly used as the input of the inverter, and then converted through DC-AC to the high voltage of more than 1,000 Vac or even 2,000 Vac required to drive the CCFL array. This eliminates the 24 V conversion stage, reduces a lot of power loss and chassis heat, improves the overall system energy efficiency, reduces system costs, and also complies with relevant energy efficiency specifications such as "Energy Star".

Figure 2: Functional block diagram of ON Semiconductor's 32-inch high-voltage LIPS LCD TV power GreenPoint® reference design

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In response to the demand for LCD TVs with high-voltage LIPS architecture, ON Semiconductor has cooperated with Microsemi to launch a high-voltage LIPS reference design for 32-inch LCD TVs. This reference design uses a simple and direct CrM PFC stage without an additional standby switch power supply, which is extremely cost-effective. This reference design uses key ICs such as ON Semiconductor's NCP1607 PFC controller, NCP1351 or NCP1219 flyback converter controller with low standby energy consumption mode, and Microsemi's LX6503 backlight controller. The functional block diagram of this complete LCD TV LIPS power supply reference design is shown in Figure 2.

It is worth mentioning that this reference design architecture is highly flexible and can support multiple voltage/current configurations with minimal changes to the schematic and components used. And because of the use of an advanced backlight controller with a zero-voltage switching full-bridge topology, the inverter power supply can be easily expanded to support a variety of LCD TV sizes from 26 inches to 42 inches. Customers can use this energy-efficient advanced reference design to shorten the development cycle and speed up the product launch process.

2) 46-inch LCD TV power architecture

ON Semiconductor has also launched a 46-inch LCD TV power reference for higher power LCD TVs. This power reference is suitable for any type of backlight solution and can be expanded to support 40/42-inch or 52/55-inch LCD TVs, see Figure 3.

Figure 3: ON Semiconductor 46-inch LCD TV power supply reference functional block diagram

The key ON Semiconductor ICs used in this power reference include the new interleaved FCCrM PFC device NCP1631, the new NCP1379 quasi-resonant flyback converter controller with valley lock function or the NCP1252 fixed frequency pulse width modulation (PWM) controller, and the NCP1053A high-efficiency, low standby power consumption switching power supply for performance-oriented projects that only focus on the power supply segment. In the first phase, the circuit board height of this power reference is less than 13 mm, and the total height of the LCD TV is less than 17.5 mm; the subsequent second phase will also reduce the circuit board height to less than 8 mm and the total height to less than 12.5 mm.

Today, when slim design has become an important selling point for LCD TVs, power supply design faces more stringent challenges, such as the need to use low-height transformers, coils or heat sinks, and to connect multiple components in series or install them horizontally. As for PFC controllers, although ON Semiconductor's NCP1606 or NCP1654 can already reduce the height of the circuit board to a very low level, to support LCD TV designs with lower thickness, ON Semiconductor's new single-chip interleaved PFC controller NCP1631 can be used for power applications up to 300 W. It also provides a narrow voltage range version of 140 to 264 Vac for applications with power greater than 200 W and slim design.

In terms of signal power supply, as mentioned above, the new NCP1379 quasi-resonant flyback converter controller or NCP1252 fixed frequency flyback controller is used. Among them, the maximum power of NCP1379 is 70 W (50 W for 32-inch design), and it provides up to 3 output voltage capabilities (equivalent to 32 inches), with a maximum output current of 4 A. NCP1379 provides a variable frequency mode for ultra-low standby energy consumption mode, maintaining high energy efficiency over a wide power range. NCP1252 provides a fixed frequency PWM of 50 kHz, supports natural CCM, and improves the current factor and transformer coupling/cross regulation for multi-output solutions.

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As for the standby power supply, there are two options. One is a dedicated ECO standby switching power supply, which uses an integrated high-voltage switching regulator NCP1053A, which is very suitable for small-scale standby TV microprocessors with a maximum power of less than 5 W, and provides an extremely low standby energy consumption of less than 90 mW. This device has a hysteresis mode to improve ECO performance, lower switching frequency and IC power consumption; the low-frequency mode supports DCM mode to reduce switching losses; the current is limited to reduce possible noise problems. Another option is to use a standby relay, which is directly controlled by the TV microprocessor (standby/conduction), and provides an optional ECO "conduction/off switch", with a no-load input energy consumption of less than 20 mW under 230 Vac conditions.

3) 46-inch LCD TV reference backlight solution

The 46" LCD TV reference power supply is suitable for any type of backlight solution, including standalone and dedicated backlight solutions. For example, a high-voltage LIPS is provided for CCFL/EEFL backlight, using two high-side drivers NCP5111 and a Microsemi backlight controller. The new NCP1397 quasi-resonant flyback converter or the CAT4026 edge-lit LED driver (see below) can also be used to provide a high DC output voltage half-bridge LLC for LED backlight drivers. The market-proven NCP1392/NCP1392 controller can also be used for traditional 24 V inverter solutions to power CCFL backlights. To adapt to plasma TV (PDP) applications, the NCP1397 controller can also be used and a buck or boost converter can be added.

Among them, in the 46-inch LIPS inverter of CCFL/EEFL backlight, the ON Semiconductor NCP5111 high-end driver used can improve energy efficiency and reduce the size of the full-bridge 500 V power MOSFET. Under the condition of modifying the high-voltage transformer design, the height on the circuit board is less than 13 mm, thus supporting low-thickness design.

Figure 4: ON Semiconductor's 46-inch LCD TV backlight solution (see yellow background)

As for LED backlight, the common backlight units in the market include edge-lit and direct-lit types, each with its own advantages and disadvantages; but relatively speaking, edge-lit LED backlight provides energy efficiency above 90%, which is very popular in the market. As for the edge-lit LED backlight unit with 46-inch power reference, the key ICs that can be used by ON Semiconductor include the NCP1397 primary side controller with high-end driver and the new 6-channel linear LED driver controller CAT4026, which are suitable for ultra-thin LCD TV design (height below 8 mm on the circuit board and total height below 12.5 mm), see Figure 4.

Among them, NCP1397, as LLC segment controller, provides many advantages in backlight applications, such as no need for driver transformer, bringing cost and installation advantages; simple application of skip cycle mode, suitable for dimming needs; and simple application of transition protection, helping to save costs, etc. Therefore, NCP1397 is a cost-effective and high-safety solution.

On the other hand, in the existing 6-channel side-lit LED driver backlight solutions on the market, each channel contains a dedicated driver IC, an inductive DC-DC boost converter and a switch; in comparison, ON Semiconductor's CAT4026, as a multi-channel LED linear driver, supports 6 channels with a single IC and is easily classified into up to 12 or 18 channels (using 2 or 3 controllers accordingly), which is a cost-effective solution with a target energy efficiency of more than 90% and a typical energy efficiency of 94%. In addition, this driver also provides a forward voltage monitoring function to limit the overall power dissipation; it can also apply different LED string faults such as LED open circuit and excessive LED short circuit.

Summarize

This article analyzes the market and technology development trends of LCD TVs, and introduces ON Semiconductor's complete LCD TV power supply solutions, including power supply reference designs for 32-inch traditional and high-voltage LIPS LCD TVs that can be expanded to support 26 to 42 inches, power supply reference designs suitable for any backlight type 46 inches and expandable to 40 to 55 size applications, and 46-inch edge-lit LED backlight power supply solutions. These reference designs have the advantages of high energy efficiency and high flexibility, suitable for slim and ultra-thin LCD TV designs, and provide great synergy advantages, facilitating reuse of solutions, helping customers accelerate the design process and speed up product launch.