Power design challenges for Digital Flat TVs

Introduction
The growth trajectory of Digital Flat TVs puts them on a pace to gain >50% of the worldwide TV market in the next year. The power design challenges for the flat TVs (Plasma and LCD) differ significantly from the CRT TV applications. The primary challenge is in fitting the power supply into the thin panel format. Along with this come the tasks of managing the heat and EMI in close proximity to the video circuits. The power levels required are higher compared to CRT requirements and the required voltages are different, thus complicating the problem further. By one estimate, power supply constitutes 12% of the BOM cost of LCD TV if the display cost is excluded. Physically also, it occupies a significant part of the real estate behind the screen as shown in Figure 1.

Figure 1. Picture of a 46" LCDTV back panel. Source iSuppli

Figure 2 provides the architectural overview of an LCD TV system with the power system blocks highlighted. The front-end is an ac-dc converter that takes universal ac input (90-265 Vrms) and converts it to isolated dc voltage(s) after applying power factor correction (PFC) upfront. For smaller screen sizes, if the power input is below 75 W, PFC may not be required. One of the primary voltages (24 V typical) generated by the ac-dc converter primarily feeds the backlight inverter and its current/power requirement depends on the number of CCFL lamps used in the display (larger the screen size, higher the power requirement). Backlighting can consume more than 80% of the total power in a large LCD-TV. The second key voltage out of the ac-dc converter is the 12 V output used for the audio subsystem and downstream system power after a dc-dc conversion stage for the low voltage signal processing and processor power. Small TVs may use 12 V for backlighting and powering the audio amplifiers. There is an additional requirement for a 5 V standby power with up to 2 A current capability.

Figure 2. Typical LCD TV block diagram with power processing blocks highlighted

All other voltages required in the system are derived by additional processing using LDO or dc-dc converters. The physical location and choice for this processing depends on current requirements, physical constraints, etc.