By Adnaan Lokhandwala, Product Manager – Texas Instruments
LED Backlighting in LCD TV - The liquid crystal display (LCD) TV industry is seeing a rapid shift in backlighting technology from CCFL to LED. There are several attributes in a TV that justify this transition: improved contrast ratio, mercury-free and lower power consumption.
The TV or TV-panel manufacturer has a number of different options available with LED backlighting – type (white LED or WLED versus RGB), location (edge versus direct) and dimming (group versus local). Each offers a different set of implementation challenges, performance and price points. LED backlighting also offers improvements in image quality and is being used by TV original equipment manufacturers (OEMs) as a differentiator. Another selling point has been the cool and sleek factor with LED lit TVs, which are capable of achieving very thin designs as compared to their cold cathode fluorescent lamps (CCFL) counterparts. With internal power supplies used in most LCD TVs, the challenge of designing slim profile, high efficiency power supplies at the cost points for this consumer market continues to present a formidable challenge to the power electronics industry.
Another relevant point for discussion is the push towards lower active power consumption for TVs from worldwide energy regulation agencies. A typical LCD TV today consumes one and a half to double the power as compared to an older cathode ray tube (CRT) TV with backlight power accounting for 70 to 80 percent of the total power budget. Energy Star 5.0 is most stringent and expects to limit the total on mode power consumption to 108 W, regardless of the screen size. TV OEMs are quickly responding to such standards, employing more LED backlit TV models for lower run time energy demand from the panel backlight. While energy star is voluntary, some regions however are moving to enact laws to make such limits compulsory for retail sale, e.g., California Energy Commission (CEC). A summary of some of these standards is shown in Figure 1.
To uniformly maintain backlight in a large LCD display, currents in a large number of LED devices must be balanced with high accuracy. An obvious way to maintain the same current is to connect them in series, where each string has a large number of LEDs. However, the number of LEDs in each string is usually limited and dictated by safety and isolation requirements. Using multiple LED strings typically requires dedicated current regulators for each string. Selecting the optimum LED BLU and power supply drive architecture becomes extremely important from a cost and implementation perspective. The edge lit backlight configuration with group dimming is capable of allowing the most cost-effective solution to drive multiple LED strings in a LCD TV backlight.
A typical front-end AC/DC power supply for a CCFL LCD TV is shown in Figure 2. It has three outputs: – 24 V for backlighting power; 12 to 16 V for the audio/T-con; and a 5 V rail for system power. ~90 percent conversion efficiency from AC to the 24 V output can be expected. The 24 V rail powers the inverter stage on the panel which then boosts this to ~1 k VAC to drive the CCFL lamps. Hence, three stages of power processing are needed to drive the lamps; thereby, further reducing the AC to b/l efficiency. Powering the inverter directly from the power factor correction (PFC) output (aka, LIPS – LCD and inverter power supply) eliminates the need for the intermediate step-down stage; thereby, improving the overall system efficiency and reducing bill of material (BOM) cost.
During the initial transition to LED backlighting in TVs, the power supply architecture used by most TV OEMs was simply inherited from their CCFL platforms, i.e., the 24 V output from the AC/DC power supply was used as the input to the LED backlighting driver as shown in Figure 3. Two additional stages of power processing are needed before driving the LED strings, i.e., a DC/DC boost stage to step up to the LED string voltage and a LED current controller to regulate the string current; thereby, penalizing the overall system efficiency.
LED Current Regulation
For the two-stage LED driver architecture shown in Figure 3, either a boost or a buck stage could be used. With both configurations, each LED string requires a dedicated switching current regulator. Boost drivers as shown in Figure 4 are more popular because of the readily available 24 V rail in existing TV platforms. Typical efficiencies for this boost stage is in the low 90s; hence, dropping the overall efficiency from AC to b/l to ~80 percent. The buck implementation requires a higher input voltage and a non-conventional AC/DC front-end power supply. Both architectures are used in the industry today and choosing between the two is dependent on the LED string voltage, dimming scheme and cost considerations.
Linear LED drivers represent the simplest scheme for regulating constant current through LEDs. However, this simplicity comes at the expense of efficiency when compared to switching regulators. Not only is the linear driver inefficient, but since the voltage remains constant across all the LED strings, any variation in the LED string voltage degrades this efficiency further (LEDs from even the same production lot have poorly matched I-V characteristics due to statistical distribution). Linear drivers are usually paired with boost converters as shown in Figure 5. Overall efficiency including boost and linear components varies depending on the range of variation in the LED forward voltages, but overall efficiencies of 80 percent are possible, i.e., a 75 to 80 percent AC to b/l efficiency. Eliminating the intermediate boost stage is being considered today and requires redesigning the AC/DC power supply to directly provide the LED string voltage. Though being somewhat less efficient than switching regulators, this scheme of driving the TV backlight can be cost effective in certain situations when compared to having dedicated controllers for each LED string.
With the trend in LCD TV’s moving towards slimmer designs, end-to-end high efficiency is paramount. Adding additional power processing stages limits the overall performance and adds overhead to the drive architecture. Efficient and cost-effective LED driver circuits that eliminate the intermediate stages (i.e., run directly from PFC output) have gained the much needed attention of the industry. Additionally, with continuous advancements in LED technology and performance, the TV b/l architecture (# of strings, LED string voltage and current) are continuously changing; hence, having a modular power supply solution that can be easily adapted to these changes can significantly reduce design cycle time.
The fundamental concept described here is to feed a regulated current into a single/series of transformers and the rectified secondary-side current directly driving the LED strings. Since the primary currents of the transformers are equal, the secondary currents are also virtually equal (assuming same transformer turns ratio) – implying current balancing between the multiple strings. Pulse-width modulation (PWM) dimming for all LED strings can be accomplished by summing the total LED currents through a single dimming switch. A simplified schematic of the proposed scheme is shown in Figure 6. Note that either one or total LED string current can be sensed to control the LLC converter since the string currents are matched; hence, one control loop can be used to control all the LED string currents to further simplify the design. An additional benefit with this proposed architecture is that it can be readily modified for a different number of LED strings/combinations. The control concept described here is also applicable to general LED lighting applications (street, commercial).
The half bridge LLC converter has become the preferred choice of topology in medium- and high-power digital TV (DTV) applications. Some of its key benefits include lower stress on output rectifiers, low electromagnetic interference (EMI), narrow frequency variations to cover entire load range, wide input operation range without penalizing normal operation efficiency and extended hold-up time. Probably of utmost importance is the high efficiency with this topology that allows slim power supply designs. LED string current control is achieved here using a LLC resonant half bridge topology in current mode control. Typical efficiency for the LLC in this application is shown in Figure 7, making overall efficiencies >90 percent from AC to b/l achievable with this architecture.
The LLC controller function includes a voltage controlled oscillator (VCO) with programmable FMIN and FMAX, the LLC half-bridge gate drivers with a fixed dead time and a PWM drive output (PWMO) for the series LED switch. The LLC power delivery is modulated by the controller’s VCO frequency. PWM Dimming is used to control an external LED series switch as well as to gate the LLC power stage on and off. The LLC edges are ramped in and out of dimming to control audible noise and are compensated with an extended on-time to maintain closed loop control of the LED current. The number of transformers that can be put in series with this architecture is quite flexible as the winding ratios can be selected to support many LED strings, depending on the size of the panel.
Test results on a 100 W LED backlight design for eight strings (80 V/120 mA) at 50 percent PWM dimming are shown in Figure 8 (DIM signal – Blue, LED current – Pink, LLC resonant tank current – Green). You can see that very fast LED rise and fall currents can be achieved with this control scheme that allows excellent linearity performance under any dimming condition. Also, by controlling the rise and fall time for the LLC tank current, smooth transitions in and out of dimming are achieved to avoid potential audible noise issues. The LED string current matching performance is shown in Figure 9. The current for all eight LED strings are tightly matched (<1 percent) for the entire PWM dimming range of 1 to 100 percent.
LCD TV technology and market is evolving rapidly. With a clear market trend towards thinner and lighter TVs, there is less available space within the chassis; hence driving higher power densities and the need for higher efficiency end-to-end power solutions. New regulatory changes and energy standards for more eco-friendly TVs with lower run time power consumption have been a catalyst for the growth of LED backlit LCD TVs. In this paper, we discussed the different AC/DC power supply architectures used in the LED backlit LCD TV market today. A key figure of merit – AC to b/l efficiency was highlighted for each solution. Finally, we presented an overview on the single stage LLC current regulator that can be driven directly from the PFC output. The proposed architecture is simple, achieves excellent current matching between multiple LED strings and the highest AC to b/l efficiency.
For more information about LED technology, visit: www.ti.com/led-ca.
Foot note: The # of LED strings varies with panel manufacturer & screen size. Typical LCD TV panels today are edge or direct lit with 1 to 32 LED strings. The market is moving towards less number of higher voltage LED strings.
About the Author
Adnaan Lokhandwala is product manager for Consumer Isolated Power at Texas Instruments, where he is responsible for understanding the market and customer needs for new IC products and driving new product roadmaps for AC/DC consumer power space. He received the M.S. degree in Electrical & Computer Engineering from the University of Illinois, Chicago, Illinois. Adnaan can be reached at: email@example.com.