Driving LED Backlights in Large LCD Televisions

May 22, 2007 4:29 PM
By Ahmed Masood, Vice President of Marketing, Supertex, Sunnyvale, Calif.

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There is an ongoing effort to replace conventional CCFL backlighting in large-area (40-in. or greater) LCD TVs with LED backlighting. The benefits LEDs provide include improved color gamut, adjustable color temperature, backlight blinking to compensate for motion artifacts of the LCD, longer reliability, and mercury-free components. Additionally, since LEDs are inherently point sources of light, a matrix of direct-emitting LEDs can use a local dimming driving scheme across the panel to greatly improve contrast ratio. This scheme dims, or turns off, LEDs in darker areas in the image, and turns on LEDs more fully in brighter areas of the image. This is a major benefit compared to global dimming, where the entire backlight is dimmed uniformly in response to the displayed image.
An important consideration for any LED backlight unit (BLU) is the driving scheme used for powering the LEDs. The architecture chosen for this scheme can have a dramatic impact on the TV’s overall cost and performance including it’s power consumption (energy efficiency), contrast ratio, motion artifacts, white point balance, and audible noise.

To maintain backlight uniformity in a large-area LCD display, currents in a large number of LED devices must be balanced with high accuracy. The most obvious way of balancing LED current is to connect them in series strings, where each string includes a large number of LEDs. Parallel connecting LEDs is usually complicated by a significant variation of their forward voltage, VF, at a given current, IF. For example, typical VF of a blue LED can vary between 3 V and 4 V even at room temperature. The problem is further aggravated by a significant temperature dependence of VF. However, the number of LEDs in each string is usually limited by a certain total voltage dictated by some practical considerations of power conversion or by safety requirements. Hence, multiple LED strings must be used. Current in each string must be controlled by a dedicated regulator, either linear or switching, that forces the LED current to meet a reference level common for all LED strings of a given color.

LED Driver Architectures

Switching LED drivers achieve the highest efficiency. Each series LED string requires a dedicated switching current regulator. The power architecture of the switching regulators depends on the number of LEDs in each string and on the supply voltage available in the TV. Two basic types of regulators are generally used: a stepup (boost) regulator and a stepdown (buck) regulator. Linear regulators are not very practical in most cases, since they need to dissipate a large amount of power to balance the LED string currents. However, mixed solutions exist that combine a single switching regulator with multiple linear current sinks. In this case, the switching regulator is used to regulate the dropout voltage across the linear sinks, thereby minimizing the power dissipation in them. Though being somewhat less efficient than switching regulators alone, this second class of BLU drivers can be cost effective in certain situations.

Linear drivers are usually paired with boost converters since the output LED string voltage is generally higher than the input voltage. This boost converter may be integrated or discrete, and has a typical efficiency in the low 90% range. Usually, output LED voltages are less than 50 V. Control circuitry monitors and selects the channel with the lowest headroom for regulation to maximize efficiency. Overall efficiency including boost and linear components varies depending on the range of variation in the LED forward voltages, but efficiencies of 80% are feasible (Fig.1).

Boost drivers such as that shown in Fig. 2 are popular because regulated 12-V and 24-V supplies are commonly available in existing TV platforms. To minimize the number of boost converters, and therefore system cost, the number of LEDs connected in series can be made quite large. Output LED string voltages are typically up to 200 V, with efficiencies in the low 90% range. Typical boost drivers can increase the input voltage up to ten times while achieving high efficiency.

Buck drivers may alternatively be used instead of boost drivers (Fig. 3). While lower supply voltages are common for smaller size TVs, they can cause a significant thermal problem in larger ones. High currents up to tens of amperes would need to be distributed across the panel to power such a low-voltage backlight unit. However, a supply voltage of 60 V or greater can be used to minimize the power loss in the wiring. In this case, a buck regulator can power each LED string.

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