LCDs are, by far, the most widely produced and sold television display type. The energy efficient LCD screen is available in a wider range of screen sizes than CRT and plasma displays. Every functional block in the LCD TV such as audio, backlighting, and video and audio signal processors require a particular power solution. This requires the generation of several voltage rails to power them. The backlight is the largest power consuming sub-system within the flat TV. From Plasma technology to Cold Cathode Fluorescents Lamps (CCFL), now the trend is towards an LED backlight solution for LCD TVs to allow SLIM cabinets.
TV Manufacturers are challenged to select an architecture which allows design optimization without increasing cost and circuit complexity. Commonly, manufacturers use a universal power supply that supports 90-265 Vac. This can allow one power supply design for a specific TV size to be used for a series of models, leading to reduced development cost. Manufacturers are focusing their attention on methods to optimize active and standby power consumption. These requirements are dictated by the regulatory standards. The ON mode power consumption increases with the screen size of direct view flat TVs. Standby power consumption in consumer devices is becoming more and more critical because these devices are always connected to the AC mains and constantly consume power, even in off mode. Thus, key players are targeting to reduce standby power consumption to meet regulatory standards. Further, an OFF mode allows for < 50 mW or even 0.0 W. An LCD TV for global use also has to meet the EU standard for harmonic reduction, so an active power factor control (PFC) stage is used.
Modern Integrated Circuit controllers (IC) are used to implement the active PFC front end, Half Bridge Resonant LLC converters and standby power supply in LCD TVs. The half-bridge LLC resonant converter has gained popularity in flat panel TVs, due to its high-efficiency, low level of EMI emissions, and ability to achieve high power density. Furthermore, the nonlinear circuit elements lead to a poor power factor in electronic power conversion equipments. So, the use of active power factor correction (PFC) in the front-end allows system optimization because the PFC output voltage is well regulated.
If you are keen to understand the design of power supplies for LCD TVs in detail, here are some reference designs with complete schematics, layout, BOM, and application notes:-
- 300 W SLIM LCD TV Power solution: This reference design describes an LCD-TV power solution that achieves optimum performance while simplifying overall BOM. The design allows advanced Slim & low depth LCD TV by limiting the height to 8 mm. The design uses a 290 mm x 310 mm PCB designed to fit into the chassis of a Flat-TV. This is done by utilizing a low power mode standby integrated switcher, a high efficiency interleaved Frequency PFC, and a Flyback controller topology that provides the power to the overall TV signal processing and audio amplifiers. The converter supports remote control, IR function, peripheral control to start the TV, and a standby current consumption below 8 mA to keep the overall TV consumption under 90 mW. More on this Reference Design
- 300-W Universal Input Power Supply for LCD TV: This reference design explains a power supply for powering LCD TVs or other flat screen applications. The power supply accepts a universal AC line-input voltage (85 VRMS to 265 VRMS), and produces an output voltage of 24-VDC for loads up to 12 A (288 W). To design a highly efficient and thin power supply (to fit behind the screen inside the TV case), a two-stage power converter topology is utilized. three converters that make up the complete power system. Three blocks that make up complete power supply include the boost PFC pre-regulator, the LLC series-resonant DC-DC converter, and the 5-W bias supply. These stages are controlled by three integrated circuit (IC) power controllers. The PFC pre-regulator stage is controlled by the UCC28061 PFC controller, the resonant LLC converter uses the low-cost controller UCC25600 and the third stage is controlled by UCC2813D-4. More on this Reference Design
- 60W Power Supply For LCD Monitor/TV: This reference design describes a 60W Switch Mode Power Supply (SMPS) for LCD TVs or monitors. The design accepts full range input voltage (90 to 265Vrms) and delivers 5V for the scaler and microprocessor, and a 12V for the backlight and audio. The design has a very good standby performance with standby power consumption of 0.8W at 230Vac. The converter topology of this SMPS is the standard fly back, working in discontinuous and continuous current mode. The design meets good efficiency along with providing protection against faults like output short circuit or over voltage. More on this Reference Design