Electronic Design

Solving The Power Puzzle For Edge-Lit LED TV Backlighting

Thanks to their improved luminous efficiency and longer life, coupled with increased demands for a wide color gamut, light-emitting diodes (LEDs) are steadily replacing the venerable cold-cathode fluorescent lamp (CCFL) to backlight liquid-crystal-display (LCD) panel television sets. LEDs are important for color and can match the long-life product cycles of CCFLs, because they have an RGB wavelength that doesn’t overlap the peak of the actual RGB wavelength. They also help in the miniaturization process, and are able to withstand a tremendous amount of shock.

To generate sufficient brightness for large-scale LCD TV sets, an LED backlight requires many LED arrays. Arrangement of the arrays determines whether an LED backlight unit (BLU) is an edge-lit LED BLU (Fig. 1) or a direct-lit LED BLU (Fig. 2). In edge-lit LED BLUs, high-efficiency and high step-up dc-dc converters are required to operate the serial-connected LED strings. Cascade dc-dc converters can meet these requirements but have challenges, such as extra complexity and higher costs. This article discusses how a voltage-doubled boost converter can help overcome these challenges.

Edge-Lit LED BLU Power Requirement
The BLU in LCD TV sets plays an important role in the overall cost of the application. It can account for 30% to 40% of the total cost, and is integral in the final angular luminance, contrast ratio, and brightness.

For an edge-lit BLU in large-size TV sets, 60 LEDs are placed in serial at the edge of the BLU. This requires the power supply to deliver a high voltage and stable current source. In general, cascade boost converters are mainly used to increase 24 V of switch-mode power-supply (SMPS) output to 130 V.

As previously mentioned, there are drawbacks, including the cost and driving complexity. As a result, many papers introduce a high step-up ratio boost converter as the solution.1 Often times, then, designers turn to a voltage-doubled boost converter. Such a topology typically falls into the hands of semiconductor suppliers, which can provide efficient and cost-saving solutions.

In this article, a voltage-doubled boost dc-dc converter is designed and verified using a 37-in. LED backlight panel. Table 1 spells out the power requirements for an edge-lit LED backlight.

Voltage-Doubler DC-DC Converter
As previously mentioned, a high-efficiency and high step-up dc-dc converter design is required for an edge-lit LED backlight. One popular solution is to use a voltage-doubler boost converter (Fig. 3). To help further explain the circuit’s operation, Figure 4 shows topological stages of the circuits in Figure 1 during a switching cycle, while Figure 5 depicts ideal waveforms.

This type of boost converter operates in two modes. The first is by switching the MOSFET on or off, which occurs during the time interval when MOSFET(Q) is on. The second mode occurs during the time interval T 0 – T1 (this is shown in Figure 3, where the inductor charges the energy). Diode D1, a reverse-biased and blocking capacitor Cblock, is charged from COUT1. During this stage, filter capacitor COUT2 supplies the load current (Fig. 4, again):

When MOSFET(Q) is off, the inductor current

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When MOSFET(Q) is off, the inductor current is commutated from the MOSFET into D1. Then the inductor’s stored energy is released to COUT1 and the blocking capacitor Cblock discharges into filter capacitor COUT2.

The voltage-conversion ratio can be calculated from the volt-second balance on the inductor. From equations 1 and 2, it follows that:

where D is the duty cycle, VOUT is the output voltage, and VIN is the input supply voltage.

The drain-to-source voltage of MOSFET(Q) during off time equates to half that of output voltage. Therefore, the MOSFET with the lower drain-to-source breakdown voltage (BV DSS) can be selected even if it delivers higher output voltage. This means that the boost converter’s overall efficiency increases with a lower BVDSS MOSFET. Since drain-to-source on-resistance (RDS(on)) and gate charge (QG) values are the most important characteristics for MOSFETs, it’s possible to obtain positive BVDSS.

Inductor and capacitor design are key factors toward achieving fast transient performance and slim design in the LED backlight application. Because the inductor is the energy storage device, a low dc-resistance (DCR) and high saturation current inductor is required with a small profile. In addition, to get a lower ripple current, a higher inductor value is recommended. The inductor value can be calculated as follows:

where fS is the switching frequency, VOUT is the output voltage, VIN is the input supply voltage, and ΔIL is the inductor ripple current.

The output capacitor affects the output voltage ripple, which means a smaller output voltage ripple reduces waterfall noise. Generally, a larger output capacitor value keeps the output voltage ripple smaller. The formula of output ripple ΔVOUT is below:

where COUT is the output capacitor, and ESR is the equivalent series resistance of the output.

New Trench MOSFET
Today, the trend in power-supply design focuses on increasing efficiency by reducing losses. To maximize a power system’s efficiency, it’s important to select a switching device with low on-resistance (RDS(on)) and gate charge (QG) characteristics.

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As an example, new mid-voltage MOSFETs (BVDSS:100 ~ 200 V), developed with a new trench technology by Fairchild Semiconductor, possess lower gate-charge characteristics and are balanced with on-resistance rating to achieve lower switching and conduction losses. A MOSFET’s switching loss is the main concern toward improving efficiency in a dc-dc boost converter, because of the high output voltage and low output current of an LED application. The new trench MOSFETs are optimized for a boost converter with lower QG and RDS(on). Test results comparing the FDD86102 with a conventional MOSFET helps clarify rating distinctions (Figs. 6 and 7, and Table 2).

With the reduction of gate-to-drain capacitance (Miller capacitance), total gate charge of the FDD86102 drops by 40% versus a conventional device. Lower gate charge reduces switching loss, which in turn improves efficiency in high-frequency applications.

In addition, high efficiency and low temperature characteristics are critical parameters in LCD displays with respect to size and thickness. Generally, a thermal increase of the main component in display systems, such as a MOSFET and inductor, should not exceed 65°C at 25°C room temperature without some mode of airflow. Figures 8 and 9 show performance results from efficiency and thermal testing.

Table 3 lists the different package MOSFETs according to the various LED TV screen sizes.2

1. Qun Zhao and Fred C. Lee, “High Performance Coupled-Inductor DC-DC Converters,” Applied Power Electronics Conference and Exposition, 2003. APEC ’03 Eighteenth Annual IEE
2. Fairchild Semiconductor website “FDD86102, FDMC86102 and FDT86102 datasheet.”

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